Electrical ineering 3tl|ata, New ^otk A.W....^..^i±.k- Cornell University Library TK 7.L23 Electrical engineering papers, 3 1924 003 990 276 PEED s 'EED " ^ ^ ^ — - — /( /o / 4' /i c- — ^ JLU J 1 A" t>5 ^^ k / —at- f^' k^ / A g^ ^ / ^ >^ ^ r ^ ^ 1 " -^Oe 300 400 500 u Pounds ' orqut » 700 S(K Lit I Foot K 1 yoo KKW 11 clius. K> 1-^00 PIG. 16— PERFORMANCE CURVE OF 75 H. P. TYPE C MOTOR two very important advantages, one of which is mechanical ani the other electrical. With this method of control there are n regulating appliances on the motor and, in consequence, it may b of the simplest possible form. The electrical advantage is tha the motor may be started and controlled from a distance. Thu it may be placed entirely out of reach of the operator. On travel ing cranes, for example, this is of special advantage, for in this cas only the primary wires need be run from the operator's cage to th motor. If there are several motors on the crane, there may be on wire common to all the motors and but two additional wires ue THE POLYPHASE MOTOR 29 motor are required. Thus for the three motors, a minimum of eleven trolley wires may be used. If the variable electro-motive forces are obtained from trans- formers, the switches for operating several motors may be wired to one set of transformers and the motors may be started and regulated independently. For traveling cranes, only one set of transformers is used for the hoisting, bridge and traveling motors, > ■ WESTINGHOUSE 400 H.P. TYPE n MOTOR i e E 3C00 ALTS., 8 POLES 1000 Ml. — H)9 \ 1 1 9U0- 800 109 eoo soo 400 300 800 -^^ —^ EFF'C!(ENC> J 0^90 a: ftWBO / v /, P -f 4- SPEE D .^ C Itf 300 60 — «0 !00 40 — 30 100*0 i 1 n \ / ^kd y^' / o' *->■ x^ ^ '^ ^ V y lOOO 2000, 30 W M^ 5(^ G(^ 7(^ SOiX) 9000 100 Poiunds Torque ;ii i Foot Riidius. wnod 18000 FIG. 17— PERFORMANCE CURVES OF 400 H.P. TYPE C MOTOR and this set may supply currents at different electro-motive forces to all the motors at the same time. A further advantage possessed by this motor lies in the high pulling-out torque. If a heavy over- load, or a load having great inertia, is suddenly thrown on a motor that has a speed-torque curve Hke "a" in Fig. 6, the point of maximum torque may be passed for an instant, and the motor will be stopped tinless the load is quickly removed. A Type C motor in this case would have its speed pulled down for a moment, but this reduction in speed gives an increased torque, thus enabling the motor to carry the overload. 30 ELECTRICAL ENGINEERING PAPERS If the electro-motive force of the system is suddenly lowered, the puUing-out torque of the motors is. lowered very materially. A reduction of twenty percent in the electro-motive force will lower the pulhng-out torque to about two-thirds of its former value, Even with a temporary drop in the electro-motive force, such as would be caused by a momentary short-circuit on the lines, this may be sufficient to stop the motor. But a motor that has a ptilling-out point several times as large as its normal running torque is very rarely in danger of being shut down from this cause. This type of motor has a starting torque from two to four times as large as the fidl-load running torque and it is thus able to start any kind of load. In practice the starting torque is adjusted to the load to be started by applying a smtable electro-motive force, as will be explained below. A last, but not least, advantage of the Type C motor is its adaptability for large sizes. The larger the motor of this type, the lower in proportion can be its magnetic leakage and its magnet- izing current. In consequence, the power factors are very high. The efficiencies are also very good over a wide range of load. The curves for a seventy-five horse-power, six-pole, 3,000-alternation motor are given in Fig. 16; also the curves for a 400-horse-power, 2,300-volt, eight-pole, 3,000-alternation motor in Fig. 17. The power factors of these motors are good examples of what can be obtained on large motors of this type. Speed Variation with Polyphase Motors There are six methods of varying the speed of polyphase motors, but some of them are applicable only in special cases. These methods are : (1) — Varying the number of poles. (2) — ^Varying the alternations applied. (3) — Motors in tandem, or series-parallel. (4) — Secondary run as single-phase. (5) — Varying the resistance of the secondary. (6) — ^Varying the electro-motive force of the primary, with constant secondary resistance. Some of these methods are efficient, while some are very in- efficient if the speed is to be varied over a wide range. Varying the Number of Poles The first method, varying the ntimber of poles, is efficient to a certain extent, but is limited in the number of combinations of THE POLYPHASE MOTOR 31 poles obtainable.' But if combined with some of the other methods it may be made fairly effective over a wide range. It consists in varying the arrangement of the primary coils in such a way that the number of resulting poles is varied. This may be accom- plished by having two or more separate windings on the primary ; or one winding may be used, it being rearranged for different speed. With this method of varying the speed, a secondary of the "cage" type is the only practical one. With a "grouped" or "polar" winding on the secondary, this would need rearranging for the dif- ferent speeds, just 'as in the case of the primary. But the cage winding, being short-circuited on itself at all points, is adapted to any ntunber of poles. In general, this method of regulation will allow for only two speeds without great complications, and the ratio of the two speeds is preferably two to one, although three to one may be obtained. The simplest arrangement of winding con- sists of two separate primary windings; one for one number of poles, and the second for the other. In combination with a var- iable primary electro-motive force, the speed-torque curves being of such shape that this method may be used, the variable-pole method of regulation may be made fairly efficient over a wide range of speed. But the two windings considerably increase the size of the motor, while the one-winding arrangements are rather complicated. Consequently, we may consider that this method of speed variation will be used only in special cases. Varying the Number of Alternations The second method, variable alternations, is theoretically the ideal method; but it is practically limited to a few special applica- tions, for we have as yet no commercial alternation transformer. In a few cases, where but one motor is operated, the generator speed may be varied. If the generator is driven by a water-wheel, its speed may be varied over a wide range, and the motor speed will also vary. If the generator field be held at practically constant strength, then the motor speed may be varied from zero to a maximum at constant torque with a practically constant current. This is a convenient method of operating a motor at a distance from the generator. The speed of the motor may be completely controlled by an attendant at the generating station. Fig. 18 shows the speed-torque and other curves of a motor when operated at 7200, 3600, 1800 and 720 alternations per minute, or at 100, 50, 25 and 10 percent of the normal alternations. The speed-torque curves, corresponding to the above alternations are, 32 ELECTRICAL ENGINEERING PAPERS "a," "b," "c" and "d." The current curves are "a," "b," "c" and "d." This figure shows that for the rated torque "t," the current is practically constant for all speeds, but the electro-motive force varies with the alternations. Consequently, the apparent power supphed, represented by the product of the ciurent by ct a: u)0 n / / •*. / 100 / / ..^^ *J-ir ■^ ■"^ 90_^^^ a. / ^x':. ^\ —T—^^^i^-^za^^^, \, ^-^a,\ 80 // \ )■ ^^^nS — / — ->= / &? J / ^^-^ V // ' B J^>5^ a'/ i/ /^ ^^/^ 1 5 / S/ y v i/ s- — j^. 1 -y / _^-^ ■"'^ y ^^7*=- -^riOJOVOLTs/'^ ^^ ;^^ /'^ - 4-/ ■' "P'^ — !;:^'"^' / X f^"'' ^<^*^^^^ / ~~f- ,.;; \ ;V^1^ ^^'^^^^^'■^SS-ioo., ^S^^"^^ ^SOO VOLTS N. / / f FIG. 18— PERFORMANCE CURVES OP POLYPHASE MOTOR WITH DIF- FERENT ALTERNATIONS AND ELECTRO-MOTIVE FORCES. electro-motive force, varies with the speed of the motor, and is practically proportionate to the power developed. Motors in Tandem or Series-Parallel The third method is to run motors in tandem or series-parallel. In this arrangement, the secondary of one motor is wound with a grouped or polar winding to give approximately the same electro- motive force and number of phases as the primary. The secondary is connected to the primary of a second motor. The secondary of the second motor may be closed on itself, with or without a resistance, or may be connected to the primary of a third motor, etc. The arrangement with two motors is shown in Fig. 19. At start, motor No. 1 receives the full number of alternations on its primary, and its secondary delivers the same number to the prim- ary of motor No. 2. Both motors will start. As motor No. 1 speeds up, its secondary alternations fall. At about one-half speed, its secondary alternations are about one-half its primary, and motor No. 2 receives one-half the alternations of motor No. 1 ; it also tends to run at half-speed. Therefore, if both motors are THE POLYPHASE MOTOR 33 coupled to the same load, this half speed is a position where the two motors tend to operate together. By connecting both primaries across the line, both motors \vill be run at full speed. Thus, with two motors, two working speeds may be obtained. This method always requires at least two motors. Its application is limited to a few special cases. Secondary With Only a Single Circuit Closed The fourth method — the secondary run with a single circuit closed — will give a half-speed, and with two or more circuits closed, will give full speed. But the power factor at the half-speed is very low, and the efficiency is not nearly so good as when run at full speed. This may have a few special applications. Fig. 20 shows this arrangement. fig. 19— diagrammatic arrangement of the two polyphase - motors connected in tandem or series parallel Varying the Resistance of the Secondary The fifth arrangement is by varying the resistance" in the secondary. This method was considered before when the speed- torque characteristics were shown. This will not ^ve constant speed except with constant load, as the speed-torque curve, with a relatively large resistance is a falling curve. At heavy torques, the motor will run at very low speeds, while with light loads it will run at almost full speed. The speed regulation will be similar to that of a direct-current shunt motor with a resistance in circuit with the armature. To hold constant speed with variable load, this resis- tance requires continual adjustment. Varying Primary Volt.4.ge The six method — that in which the. primary electro-motive force is varied while the secondary resistance is held constant — gives the same results as the fifth method, as the speed-torque curves are similar. To hold a constant low speed, the electro- motive force must be varied continually if the load is changing. 34 ELECTRICAL ENGINEERING PAPERS Like the fifth method, it is not efficient at low speeds, as the reduc- tion in speed is obtained by means of a corresponding loss of energy in the secondary circuits. ^^^ FIG. 20— POLYPHASE MOTOR WITH ONLY ONE SECONDARY CURRENT CLOSED. For crane work, hoisting, etc., where it is necessary to run at reduced speed for but a portion of the time, either of the methods five or six is satisfactory, but method five requires the use of a vari- able secondary resistance, and there must be a set of secondary leads carried out to a rheostat if the speed changes are to be gradual. This introduces complication, especially on a crane where several motors are to be controlled. In this case there must be trolley wires for both the primary and the secondary circuits of each motor. But by method six, the control is effected in the primary circuit and only primary trolley wires are needed, and these may be controlled from one pair of transformers, as explained before. The sixfh method is therefore the simplest and most practical one to use for hoisting, etc., and will be found to present many advan- tages for all classes of work, whether speed regulation is important or not. Method of Varying Primary Electro-Motive Force There are several methods of varying the electro-motive force applied for starting and varying the speed on the Type C motor. These may be classified under three headings : (1) — ^Varying the electro-motive force from the genera- tor. (2) — ^Varying the electro-motive force by transformers. (3) — Varying the motor connections. Varying Electro-Motive Force from the Generator A variable electro-motive force may be obtained from the generator in several ways. The generator may be run at low THE POLYPHASE MOTOR 35 speed, with the field charged. This gives lower electro-motive force and lower alternations at the same time. This is adapted only to places where all the motors are to be started at once. The generator may be run at normal speed and its field charge lowered- This gives the normal alternations with lower electro- motive force. This is practicable only where all the motors are to be started at once. A third method is to so arrange the generator windings that two or more electro-motive forces for each phase may be obtained. A lower electro-motive force may be used at start, and a higher for running. The different arrangements of the generator windings for this purpose are as follows : If the armature has but one winding closed on itself, like a direct-current machine, two or three phases may be taken off. For two phases four leads are used. Fig. 21 illustrates this. Between FIG. 21- RUNNING W!TH FULL VOLTAGE -CONNECTIONS FOR TWO-PHASE MOTORS STARTING -ON SIDE CIRCUITS. 1-3 and 2-4 is the maximum electro-motive force, and between 1-2, 2-3, 3-4 and 4-1 there is 0.7'the electro-motive force of 1-3. The electro-motive force 1-2 is at quarter phase to that of 4-1 and 2-3, and the electro-motive force 3-4 is at quarter phase to that of 2-3 and 4-1. Therefore, across any two adjacent side circuits we have quarter phase circuits of 0.7 the electro-motive force of the main circtiit. A motor may thus be started on any adjacent side cir- cuit and then switched to the main circuit. This method is well adapted for local plants where the generator electro-motive force is 200 or 400 volts. If there are many motors to be started, and 36 ELECTRICAL ENGINEERING PAPERS the starts are numerous, it is advisable to wire the starting switches so that the various motors are started on different side circuits. If the generator winding is of the "open coil" type, a similar arrangement may be obtained for two phases. The two windings may be connected to the middle point, thus giving side circuits of 0.7 electro-motive force. This is shown in Fig. 22. FIG. 22— WINDINGS OP "OPEN COIL," TWO-PHASE GENERATORS CONNECTED TOGETHER AT MIDDLE POINT TO ALLOW STARTING OF MOTORS FROM "SIDE CIRCUITS "- Three-phase connections do not allow any convenient com- binations with the generator winding. A fourth wire may be run, however, which will give about 0.58 electro-motive force for starting. fig. 23— connections ofjransformers on two-phase circuits to give .7 and .5 normal voltage for starting motors. Variable Electro-Motive Force from Transformers The method of varying the .electro-motive force by means of transformers admits of many differeiit combinations. Several of the simpler forms will be given. (1) The transformers may be so connected that two or more electro-motive forces may be obtained. For two-phase circuits, the secondaries may be connected together at the centre, as shown in Fig. 23. This gives two main circuits, and four side circuits of lower electro-motive force. If an extra wire be carried out from the point 5, then 1-5, 2-5, will form a two-phase combination- for 0.5 voltage, while 1-2, 2-3 form a THE POLYPHASE MOTOR 37 two-phase combination for 0.7 voltage, and 1-3 and 2-4 give full voltage. Another method is to connect the secondaries at one side of the centre, as shown in Fig. 24. Then 3-5 and 4-5 give one electro- motive force; 1-5 and 2-5 give a higher electro-motive force, and 1-3 and 2-4 give full electro-motive force. 2 1 FIG. 24— CONNECTIONS OF SECONDARIES OF TRANSFORMERS ON TWO-PHASE CIRCUIT AT A POINT ONE SIDE OF THE CENTER, TO OBTAIN LOWER ELECTRO-MOTIVE FORCES FOR STARTING MOTORS. These combinations are useful in certain cases, but are not as general in their application as the following method : (2) Auto-transformers with loops brought out for lower electro-motive forces. FIG. 25— DIAGRAMMATIC ARRANGEMENT OF AUTO-TRANSFORMERS AND CONTROLLERS FOR REGULATING SPEED OP TWO-PHASE MOTOR BY VAR- IANCES OF VOLTAGE. In this method, no special combinations of the lines, lowering transformers or generators are made, but, in connection with each motor, a small pair of auto, or one-coil transformers, is used for starting. If speed regulation is also desired, as for cranes, the 38 ELECTRICAL ENGINEERING PAPERS auto-transformers are made larger. From these auto-transformers several loops or connections are brought out. For regulating the speed these are connected to the contact plates or dials of a con- troller, as shown in Figs. 25 and 26. But for starting purposes only, when but one loop from each transformer is used, a pair of switches g { , - 3 J i f ^ 1 \ hXl, Jo oJ FIG. 26— DIAGRAMMATIC ARRANGEMENT OF AUTO-TRANSFORMERS AND CONTROLLER FOR REGULATING SPEED OF THREE-PHASE MOTOR BY VARYING THE VOLTAGE. are used in connection with the transformers. With the switches Open, the motor is disconnected. Throwing one direction starts the motor at reduced voltage and brings it up to almost full speed. The switches are then thrown over to full electro-motive force. Two small transforraers in a case with one four-jaw, throw- over switch, form what is called an "auto-starter." This is readily arranged for either two or three-phase circuits and motors. This makes a most flexible arrangement for starting, as the motor may be put at any location, and the auto-starter may be put in the most convenient position. It also loads all the line wires equally at start, and each motor and starter really form a unit separate from all the others. One pair of transformers may be connected to several sets of switches and thus be used for starting several motors. Where motors are close to reducing transformers, the second- aries of the transformers may have loops brought out, to which one or more switches are connected. The primaries of the trans- formers may have loops connected to proper switches, and the number of primary turns in the circuit may be varied instead of the secondary. This is applicable when the transformers supply only one motor, or when several motors are started at the same time. A regulator with secondary movable with respect to the primary may be used. Regulators of this type vary the electro- THE POLYPHASE MOTO-R 39 motive forces without any "make" or "break" devices, and con- sequently have no sparking tendency. But they are in general too complicated and costly to compete with the transformer with loops. Varying the Motor Connections This is not a method for changing the electro-motive force applied, but for varying the number of turns in series with a given electro-motive force, and the effect is the same as varying the ap- plied electro-EQotive force. This method is rather limited in its application owing to the complication involved. The simplest case for two-phase motors is a series-parallel combination of the windings of each phase. This is equivalent to using 0.5 electro- motive force at start. For three-phase motors, series-parallel may be used or the winding may be thrown from the star ■ system of connection at start to the delta system for running. This is equiv- alent to using about 0.6 electro-motive force for start. But, as the star connection is preferred for the running condition, this com- bination is not advisable. Choke Coils or Resistance in the Primary There is a fourth rtiethod of regulation which may be men- tioned, but which is not advisable in general practice. This is the use of choke coils or resistance in the primary-circuits of the motor, to reduce the electro-motive force. These really give varying electro-motive forces. With choke coils, the power factor at start is lowered, with correspondingly bad effect on the generator and system. With ohmic resistance in' the primary circuit, the reduc- tion of electro-motive force is accompanied by a cons amp tion of energy in the primary circuit which in no way represents torque. 40 ELECTRICAL ENGINEERING PAPERS WASHINGTON, BALTIMORE & ANNAPOLIS SINGLE- PHASE RAILWAY FOREWORD — This paper was presented before the American Institute of Electrical Engineers, September, 1902. It was the the very first information given out for publication regarding the single-phase alternating-current railway system which has been developed and installed so extensively since that time. Before the publication of this paper, it was generally assumed that the difficulties in the commutation of alternating current were so great that only motors of relatively small capacity could be built. Following its publication, many of the larger companies throughout the world began work on such motors and produced operating railw^ay equipments with more or less success. — (Ed.) The Washington, Baltimore and Annapolis Railway is a new high-speed electric line extending from the suburbs of Washing- ton to Baltimore, a distance of about 31 miles, with a branch from Annapolis Junction to Annapolis, a distance of about 15 miles. The overhead trolley will be used, and schedule speeds of over 40 miles per hour are to be attained. This road is to be the scene of the first commercial operation of an entirely new system of electric traction. The special feature of this system is the use of single-phase alternating current in generators, transmission lines, trolley car equipment and motors. It constitutes a wide departure from present types of railway apparatus, and while retaining the best characteristics of the present standard d. c. motor system, the use of alternating current makes it possible to avoid many of the bad features. The standard d. c. railway equipment possesses several characteristics which fit it especially for railway service. These characteristics have been of sufficient importance to over- balance many defects in the system. In fact, a far greater amount of effort and engineering skill has been required for over- coming or neutralizing the defects, than for developing the good features possessed by the system. By far the most important characteristic possessed by the d. c. system is foimd.in the type of motor used on the car. The d. c. railway motor is in all cases a series-wound machine. The series motor is normally a variable field machme and it is this feature which has adapted the motor especially to railway service. Shunt-wound motors have been tried and abandoned. All manner of combinations of shuntj SINGLE-PHASE RAILWAY 41 series and separate excitation have been devised and found want- ing, and in many casesthe real cause of failure was not recognized by those responsible for the various combinations. They all missed to a greater or less extent the variable-field feature of the straight series motor. It is true that a variable field can be obtained with shunt or separate excitation, but not without con- trolling or regulating devices, and the variation is not inherently automatic, as in the series motor Polyphase and single-phase induction motors do not possess the variable field feature at all, as they are essentially constant-field machines They are equivalent to direct current shunt or separately excited motors with constant field strength, which have been unable to compete successfully with the series motor The variable field of the series motor makes it automatically adjustable for load and speed conditions. It also enables the series motor to develop large torques without proportionately increased currents The automatically varying field is accompanied by corresponding variation3*in the counter e.m.f. of the armature, until the speed can adjust itself to the new field conditions. This feature is of great assistance in reducing current fluctuations, with a small number of steps in the regulating rheostat Any increase in current, as resistance is cut out, is accompanied by a momentary increase in the counter e.m f., thus limiting the current increase to a less value than in the case of constant field motor Next to the type of motor, the greatest advantage possessed by the D. c. system lies in the use of a single current or circuit, thus permitting the use of one trolley wire The advantages of the single trolley are so well-known that it is unnecessary to discuss them. For third rail construction, the use of single current is of even greater importance than in the case of overhead trolley It is seen, therefore, that it is not to the direct current that credit should be given for the great success of the present railway system, but to the series type of motor and the fact that up to the present time no suitable single-phase a c. motor has been presented. Some of the undesirable features of the d c. railway system should also be considered. The speed control is inefficient. A nominally constant voltage is supplied to the car, and speed con- trol is obtained by applying variable voltage at the motor ter- minals. This variation is produced by the use of resistance in series with the motors, witli a loss proportional to the voltage taken up by the resistance. By means of the series-parallel 42 ELECTRICAL ENGINEERING PAPERS arrangement, the equivalent of two voltages is obtainable at tbe motor terminals without the use of resistance. Therefore, with •series-parallel control,, there are two efficient speeds with any given torque, and with multiple control there is but one efficient speed with a given torque. All other speeds are obtained through rheostatic loss, and the greater the reduction from either of the two speeds, series or parallel, the lower will be the efficiency of the equipment. At start, the rlieostatic losses' are always relatively large, as practically all the voltage of the line is taken up in the rheostat. For heavy railroad service- where operation for long periods at other than full and half speeds may be necessary, the rheostatic loss will be a very serious matter. The controlling devices themselves are also a source of trouble. An extraordinary amount of time and skill has been expended' on the perfection of this apparatus. The difficulties increase with the power to be handled. The controller is a part of the equipment which is subjected to much more than ordinary mechanical wear and tear, and it can go wrong at any one of many points. The larger the equipment to be controlled, the, rriore places are to be found in the controller which can give trouble. The best that can be said of the railway controller is that it is a necessary evil. Another limitation of the d. c. system is the trolley voltage. Five hundred volts is common at the car and 650 volts is very unusual. By far the larger number of the railway equipments in service to-day are unsuited for operation at 600 volts, and 700 volts in normal operation would be unsafe for practically all. The maximum permissible trolley voltage is dependent upon inherent limitations in the design of motors and controllers. The disadvantages of low voltage appear in the extra cost of cop- per and in the difficulty of collecting current. In heavy railroad work the current to be handled becomes enormous at usual voltages. A 2400 h.p. electric locomotive, for example, will require between 3000 and 4000 amperes at normal rated power and probably 6000 to 8000 amperes at times. With the overhead trolley these currents are too heavy to be collected in the ordinary manner, and it is a serious problem with any form of trolley or third rail system which can be used. It is evident that for heavy service, comparable with that of large steam rail- ways, a much higher voltage than used in our present d. c. sys- tem is essential, and the use of higher voltage is destined to come, provided it is not attended by complications which more than SINGLE-PHASE RAILWAY 43 overbalance the benefits obtained. A further disadvantage of the D. c. system is the destructive action known as electrolysis. This may not be of great importance in interurban lines, chiefly because there is nothing to be injured by it. In city work its dangers are well-known, and very expensive constructions are now used to eliminate or minimize its effects. From the above statements it is evident that an a. c. railway system, to equal the d. c, should possess the two principal features of the d. c. system, viz: A single supply circuit and the variable field motor, and to be an. improvement upon the d. c. system, the A. g. should avoid some of the more important dis- advantages incident to the present d. c. railway apparatus. The system must, therefore, be single-phase. The importance of using single-phase for railway work is well known. The diffi- culties and complications of the trolley construction are such that- several a. c. systems have been planned on the basis of single-phase supplied to the car, with converting apparatus on the car to transform to direct current, in order that the standard type of railway motors may be used. Such plans are attempts to obtain the two most valuable features of the present d. c. system. The polyphase railway system, used on a few European roads, employs three Currents, and therefore does not meet the above requirement. The motor for the a. c. railway service should have the variable speed characteristics of the series d. c. motor. The polyphase motor is not suitable, as it is essentially a constant field machine, and does not possess any true variable speed characteristics. Therefore it lacks both of the good fea- tures of the D. c. railway system. A new type of motor must, therefore be furnished, as none of the alternating current motors in commercial use is adapted for the speed and torque require- ments of first-class railway service. Assuming that such a motor is obtainable for operation on a single-phase circuit, the next step to consider is whether the use of alternating instead of direct current on the car, will allow some of the disadvan- tageous features of the d. c. system to be avoided. The d. c. limits of voltage are at once removed, as transformers can be used for changing from any desired trolley voltage to any con- venient motor voltage. Electrolysis troubles practically disap- pear. As transformers can be used, variations in supply voltage are easily obtainable. As the motor is assumed to have the characteristics of -the direct-current series motor, speed control without rheostatic loss is practicable when voltage control is 44 ELECTRICAL ENGINEERING PAPERS obtained. This combination, therefore, allows the motor to operate at relatively good efficiency at any speed within the range of voltage obtained If the voltage be varied over a sufficiently wide range, the speed range may be car- ried from the maximum desired down to zero, and there- fore, down to starting conditions. With such an arrange- ment no rheostat need be used under any conditions, and the lower the speed at which the motor is operated, the less the power required from the line. The least power is required at start, as the motor is doing no work and there is no rheostatic loss. The losses at start are only these in the motor and traiisforming apparatus, which are less than when running at full speed with an equal torque. Such a system, therefore, permits maximum economy in power consumed by motor and control. This- economy in control is not possible with the polyphase railway motor, as this motor is the equivalent of the d. c. shunt motor, with which the rheostatic loss is even greater than with the series motor. The use of alternating current on the car allows voltage control to be obtained in several ways. In one method a transformer is wound with a large number of leads carried to a dial or con- troller driuii. The Stillwell regulator is a well-known example of this type of voltage control. This method of regulation is suitable for small equipments with moderate currents to be handled. The controller will be subject to some sparking, as in the case of d. c. apparatus, and therefore becomes less satisfactory as the car ecj uipment is increased in capacity. Another method of control available with alternating curreat is entirely non- sparking, there being no make-and-break contacts. This con- troller is the so-called " induction regulator," which is a trans- former with the .primary and secondary windings on separate cores. The voltage in the secondary winding is varied by shift- ing its angular position in relation to the primary. With this type of voltage controller, very large currents can be handled, and it is especially suitable for heavy equipments, such as loco- motives It is thus seen that there is que method of control, available with alternating current, which avoids the troubles inherent to the d. c. controller. The induction regulator is primarily a transformer, and all wear and tear is confined to the supports which carry the rotor. Therefore the objectionable controller of the standard d. c. system can be eliminated, pro- vided a suitable a. c. motor can be obtamed. This ideal type SINGLE-PHASE RAILWAY 45 of controller is not applicable to the polyphase railway motor, in which speed control can be obtained only through rheostatic loss. The polyphase control system is even more complicated than the d. c, as there must be a rheostat for each motor, and two or three circuits in each rheostat. It is thus apparent that by the use of single-phase alternating current with an a. c. motor having the characteristics of the d. c. series motor, the best features of the d. c. system can^ be obtained, and at the same time many of its disadvantages can be avoided. This portion of the problem therefore resolves itself into the construction of a single-phase motor having the characteristics of the D. c. series motor. There are several types of single phase A. c. motors which have the series characteristics. One type is similar in general construction to a d. c. motor, but with its magnetic circuit laminated throughout, and with such pro- portions that it can successfully commutate alternating current. Such a motor is a plain series motor, and can be operated on either alternating or direct current and will have the same torque characteristics in either case. Another type of motor is similar in general construction to the above, but the circuits are ar- ranged in a different manner. The field is connected directly across the supply circuit, with proper control appliances in series with it. The armature is short-circuited on itself across the brushes, and the brushes are set at an angle of approximately 45° from the ordinary neutral point. The first of these two types of motors is the one best adapted for operation in large units. This is the type of motor which is to be used on the Washington Baltimore and Annapolis Railway. Several motors have been built and tested with very satisfactory results, both on the test- ing stand and under a car. The results were so favorable that the system was proposed to the Cleveland Engineering Company, representing the Washington, Baltimore and Annapolis Railway, and after investigation by their engineers, the system was adopted. A description of the apparatus to be used on this road will illustrate the system to good advantage. Single-phase alternating current will be suppled to the car at a frequency of 16-| cycles per second, or 2,000 alternations per minute. The current from the overhead trolley wire is normally fed in by one trolley at approximately 1,000 volts. Within the limits of the District of Columbia two trolleys are employed, as by Act of Congress the use of rails as conductors is prohibited in this District, presumably on account of electrolysis. In this 46 ELECTRICAL ENGINEERING PAPERS case the trouble, of course, will not exist, but the contracting company has been unable to obtain permission for the grounded circuit. The alternating current to- the car is carried through a main switch or circuit breaker on the car, to an autp-transformer connected between the trolley and the return circuit. At approximately 300 volts from the ground terminal, a lead is brought out from the auto-transformer and passes through the regulator to one terminal of the motors. For starting and con- trolling the speed, an induction regulator is used with its second- ary winding in series with the motors. This secondary circuit of the regulator can be made either to add to, or substract from the transformer voltage, thus raising or lowering the voltage 6 -vwvwvv 1 Fig. 1. — a. Auto-Transformer, b. Induction Regulator c. Reversing Switch, of Motors, e. Armature of Motors. / Equalizing Transformer. d ?\a\i supplied to the motors. The regulator therefore does double duty. .The controller for d. c. motors merely lowers the voltage supplied to the motors but cannot raise it, but an a. c. regulator can be connected for an intermediate voltage, and can either raise or lower the motor voltage. In this way the regulator can be made relatively small, as it handles only the variable element of the voltage and the maximum voltage in the secondary wind- ing is but half of the total variation required. In the equipments irh question, the rang'e of voltage at the motor is to be varied from approximately 200 volts up to 400 volts or slightly higher. The transformer on the car will supply 315 volts, and the secondary circuit of the regulator will be SINGLE-PHASE RAILWAY 47 wound to generatevslightly more than 100 volts when turned to the position of its maximum voltage. This voltage of the regu- lator is about one-fourth of that of the motors at full voltage. The regulator can consequently be made relatively small, in comparison with the motor capacity of the equipment. It has been found unnecessary to use much lower tlian 200 volts in this installation, as this voltage allows a comparatively low running speed, and approximately 200 volts will be necessary to start with the required torque. The greater part of this voltage is required to overcome the e.m.f. of- self-induction in the motor windings, which is dependent upon the current through the motor and is independent of the speed of the armature. There will be four motors of 100 h.p. on each car. The full rated voltage of each motor is approximately 220 volts. The motors are arranged in two pairs, each consisting of two arma- tures in series, and two fields in series, and the two pairs are connected in parallel The motors are connected permanently in this manner. As voltage control is used, there is no necessity for series parallel operation, as with D. c. motors. To ensure equal voltage to the armatures in series, a balancing or equalizing action is obtained by the use of a small auto-transformer con- nected permanently across the two armatures in series with its middle point connected between them. The fields are arranged in two pairs, with two fields in series and two pairs in multiple. This parallels the fields independently of the armatures, which was formerly the practice with d. c motors. It was a defective arrangement with such motors, as equal currents m the field did not ensure equal field strengths in the motors, and the armatures connected in parallel would be operating in fields of unequal strength, with unequal armature currents as a direct result. With alternating currents in the fields, the case is different The voltage across the fields is dependent upon the field strengths, and the current supplied to the fields naturally divides itself for equal magnetic strengths. The chief advantage m paralleling the fields and armatures independently is, that one reversing switch may serve for the four motors and one balancing trans- former may be used across the two pairs of armatures. The usual D. c. arrangement of armatures m series with their own fields can be used, with a greater number of switches and con- nections. The general arrangement of the auto-transformer, regulator, jnatoTS, etc. , is shown in Fig. 1 48 ELECTRICAL ENGINEERING PAPERS The induction regulator or controller, resembles an mduction motor in general appearance and constructioh. The primary winding is placed on the rotor, and the secondary or low voltage winding on the stator. The rotor also has a second winding which is permanently short-circuited on itself. This function of this short-circuited winding is to neutralize the self-induction of the secondary winding as it passes from the magnetic influ- ence of the primary. The regulator is wound for two poles, and therefore is operated through 180° for producing the full range of variation of voltage for the motors. One end of the primary winding of the regulator is connected to the trolley, and the other to a point between the regulator and the motors. It thus receives a variable voltage as the controller is rotated. There are several advantages in this arrangement of the primary in this particular case. First, the regulator is worked at a higher induction at start, and at lower induction when running, the running position being used in these equipments for much longer periods than required for starting. Second, when the motors are operating at full voltage the current in the primary of the •regulator passes through the motors but not through the auto- transformer or the secondary of the regulator. This allows con- siderable reduction in the size of auto-transformer and regulator. The motors on the car are all of the straight series type. The armature and fields being connected in series, the entire current of the field passes througli tlie armature as in ordinary series D. c. motors. The motor has eight poles, and the speed is approximately 700 revolutions at 220 volts. The general con- struction is similar to that of a d. c. motor, but the field core is laminated throughout, this bemg necessary on account of the alternating magnetic field. There are eight field-coils wound with copper strap, and all connected permanently in parallel. The parallel arrangement of field-coils assists in the equalizing of the field strength in the different poles, due to the balancing action of alternating circuits in parallel. This arrangement is not really necessary, but it possesses some advantages and therefore has been used. With equal magnetic strength in the poles, the magnetic pull is equaHzed even with the armature out of center. The armature is similar in general construction to that of a D. c. motor. The fundamental difficulty in the opera- tion of a commutator type of motor, on single-phase alternating current lies in the sparking at the brushes. The working current passing through the motor should be practically no more difficult ^ SINGLE-PHASE RAILWA Y 49 to commutate than an equal direct current, and it is not this cur- rent which gives trouble. The real source of trouble is found in a local or secondary current set up in any coil, the two ends of which are momentarily short-circuited by a brush. This coil encloses the alternating magnetic field, and thus becomes a secondary circuit of which th^e field-coil forms the primary. In \ 1 sg t 1200 z 1 1100 100 1000 \ / \ \ / \ / \ PO* lXfai TOB_ / ' 90 900 80 800 70 700 60 600 50 .500 40 400 30 300 20 200 EF ■ICIENCY — \ \ T \; N. ^ V f / /' \ / ^ / / / / / 40 .50 80 tiO 100 no 120 1.30 140 HORSE POWER Fig. 2. — Westinghouse Alternating Current Railway Motor. No. 9L — Single-Phase. — 220 Volts. the motors of the Washington, Baltimore and Annapolis Rail- way, this commutation difficulty has been overcome by so con^ structing the motor that the secondary or short-circuit current in the armature coil is small, and the commutating conditions so 50 ELECTRICAL ENGINEERING PAPERS perfect that the combined working and secondary currents can be commutated without sparking. This condition being ob- tained, the motor operates like a d. c. machine and will give no more trouble at the commutator than ordinary d. c. railway motors.- Experience covering a considerable period in the opera- tion of motors of 100 h.p. capacity indicates that no trouble need be feared at the commutator. An extended series of tests were made at the Westinghouse shops at East Pittsburg, both in the testing room and under a car. Fig. 2 shows curves of the speed, torque, efficiency and power factor plotted from data from brake tests. It should be noted that >the efficiency is good, being very nearly equal to that of high-class d. c. motors. The power factor, as shown in these curves, is highest at light loads and decreases with the load. This is due to the fact that the power developed increases approximately in proportion to the current, while the wattless component of the input increases practically as the square of the current. The curve indicates that the average power factor should be very good. The calculations for the W. B. and A. Railway show that the average power factor of the motors will be approximately 96 per cent. Tlie average efficiency of these equipments will be much liigher during starting and acceleration than that of correspond- ing D. c. equipments, and rheostatic losses are avoided. When running at normal full speed, however, the efficiency will be slightly less than with d. c. This is due to the fact that the a. C. motor efficiency is slightly lower than the d. c, and in addition there are small losses in the transformer and the regulator. The A. c. equipments are somewhat heavier than the d. c, thus re- quiring some extra power, both in accelerating and at full speed. Therefore, for infrequent stops the d. c. car equipment is more efficient than the a. c, but for frequent stops the a. c. shows the better efficiency. Tests on the East Pittsburg track verified this conclusion. But the better efficiency of the d. c. equipment with infrequent stops is offset with the a. c. by decreased loss in tlie trolley wire, by reason of the higher voltage used, and the elimination of the rotary converter losses. The resultant effi- ciency for the system will therefore be equal to or better than that of the d. c. In the W. B. and A. Railway contract the guarantee given by the Westinghouse Electric and Mfg. Co. states that the efficiency of the system shall be equal to that of the d. c. system with rotary converter substations. SINGLE-PHA SE RAILWAY 51 There Is one loss in the a. c. system which is relatively much higher than in the d. c. This is the loss in the rail return. Tests have shown that at 2,000 alternations this is three to four times as grgat as with an equal direct current. This would be a Serious matter in cases where the d. c. rail loss is high. But the higher a. c. trolley voltage reduces the- current so much, that the A. c. rail loss is practically the same as with direct current at usual voltages. In many city railways the d. c. rail loss is made very low, not to lessen waste of power, but in order to reduce electrolysis. In such cases the a. c. rail loss could be higher than d. c, thus decreasing the cost of return conductors. More frequent transformer substations, with copper feeders connected to the rails at frequent intervals will enable the rail loss to be reduced to any extent desired. As a frequency of 2,000 alternations per minute is used, the lighting of the cars and the substations was at first considered to be a serious difficulty, due to the very disagreeable winking of ordinary incandescent lamps at this frequency. Two methods of overcoming the winking were tried, both of which were successful. One method was by the use of split phase. A two-phase induction motor was run on a single-phase 2,000 alternating circuit, and current was taken from the unconnected primary circuit of the motor. This current was, of course, at approximately 90° from the cur- rent of the supply circuit. A two-phase circuit was thus obtained on the car. Currents from the two phases were put through ordinary incandescent lamps, placed close -together. The resulting illumination a few feet distant from the lamps showed about the same winking as is noticed with 8,000 alts. With two filaments in one lamp the winking disappears entirely. A three- phase arrangement would work in the same way. A much simpler method was tried which worked equally well. This consisted in the use of very low-voltage lamps. I/OW volt- age at the lamp terminals allows the use of a thick filament with considerable heat inertia. Tests were made on lamps of this type at g. frequency of 2,000 alts., and the light appeared to be as steady as that from the ordinary highrfrequency incandescent lamp. The low voltage is not objectionable in this case, as a number of lamps can be run in a series, as in ordinary street railway practice, and any voltage desired can readily be obtained, as alternating current is used on the car. There will be an air compressor, driven by a series A. c. motor, on each car, for supplying air to the brakes and for operating 52 ELECTRICAL ENGINEERING PAPERS the driving mechanism of the controller; The details of this mechanism are not near enough to completion to permit a de- sctiption of it. The method used will be one which readily allows operation on the multiple-unit system. The generating station contains some interesting electrical features, but there is no great departure from usual A. C. prac- tice. There will be three 1,500 k.w. single-phase alternators. These are 24-pole machines operating at 83 revolutions and wound for 15,000 volts at the terminals. They are of the rotating field type, with laminated magnetic circuits a;nd field- coils of strap on edge. The field-coils are held on the pole-tips by coppier supports, which serve also as dampers to assist in the l)arallel running. The armatures are of the usual slotted type. The armature coils are placed in partially closed slots. There are four coils per pole. The proportions of these machines are such that good inherent regulation is obtained without saturation of the magnetic circuit. The rise in potential with nOn-inductive load thrown off will be approximately 4 per cent. An alterna- tive estimate was furnished for the generators proposing 20,000 volts instead of 15,000. The simplicity of the type of winding used, and the low frequency, are both favorable for the use of very high voltage on the generator. As 15,000 volts was con- sidered amply high for the service, the engineers for the railway considered it unadvisable to adopt a higher voltage. There are to be two exciters, each of 100 k.w. capacity at 250 revolutions. The exciters are wound for 125 volts normal. The armature of each exciter has, in addition to the commutator, two collector rings, so that single-phase alternating current can be delivered. It is the intention to use the exciters as alter- nators for -supplying current to the system for lighting when the large generators are shut down at night. The main station switchboard comprises three generator panels, one load panel, and three feeder panels. High-tension oil-break switches are to be provided, operated by means of controlling apparatus on the panels. The switches, bus-bars and all high-tension apparatus will be in brick compartments separate from the board. In each generator circuit there are two non-automatic oil-break switches in series ; and on each feeder circuit there are two over- load time-limit oil-break switches in series. The two oil-break switches in series on the same circuit can be closed separately and then opened to test the switches without closing the circuit. With the switches in the closed position they are both operated' SINGLE-PHA SE RAILWAY 53 at the same time by the controller, to ensure opening of the cir- cuit, and to put less strain on the switches, although either one is capable of opening the load. There will be nine transformer substations distributed along the railway line. Each station will contain two 250 k.w. oil-cooled lowering transformers, supplying approximately 1,000 volts to the trolley system. The transformers are used in each station so that in case of accident to one transformer the station will not be entirely crippled. It is the intention of the railway company to operate a d. c. road already equipped with the direct-current system. The present D. c. car equipments are to be retained, but the current will be supplied from a rotary converter substation fed from the main system of the W. B. and A. Railway. As this system is single- phase, it is necessary that single-phase rotaries be used in the substations. There are to be two k.w. 550-volt rotary con- verters. These are 4-pole, 500-revolution machines. The general construction of these machines is very similar to that of the Westinghouse polyphase rotary converters. The armature resembles that of a polyphase rotary except in the number of collector rings, and in certain details of the proportions made necessary by reason of the use of single-phase. The commutat- ing proportions are so perfect that any reactions due to the use of single-phase will result in no injurious effect. The field con- struction is similar to that of a polyphase rotary. The lamin- ated field-poles are provided with dampers of the " grid " or " cage " type, a form used at present in the Westinghouse poly- phase rotary converters. This damper serves to prevent hunt- ing, as in the polyphase machines, and also to damp out pulsa- tions due to single-phase currents in the armature. The damper acts to a certain extent as a second phase. Each rotary con- verter is started and brought to synchronous speed by a small series a. c. motor on the end of the shaft. The voltage at the motor terminals can be adjusted either by loops from the lower- ing transformer or by resistance in series with the motor, so that true synchronous speed can be given to the rotary converter, before throwing it on the a. c. line. From the preceding description of this system and the appar- atus used on it, some conclusions may be drawn as to the various fields where it can be applied to advantage. It is evident that a good field for it will be on interurban long-distance lines such as the W. B. and A. Railway. On such railways, high trolley voltage and the absence of converter substations are very important factors. 64 ELEi^lRICAL ENGINEERING PAPERS For heavy railroading also, this system possesses many ideal features.. It allows efficient operation of large equipments at practically any speed and any torque, and also avoids the con- troller, troubles- which are ever present with large direct current equipments. It. also permits the use of high trolley voltage, thus reducing the current to be collected In this class of serv- ice the .advantages of this a c. system are so great that is it possible that heavy railroading will prove to be the special field for it. For general city work, tins system may not find a field for some time to come, as the limitations m the present system are not so great that there, will be any great necessity for making a change. It is probable that at first this system will be appHed to new railways, or in changing over steam roads rather than in replac- ing existing city equipments. One difficulty with which the new system will have to contend, is due to the fact that the A. c. equipments cannot conveniently operate on existing city lines, as is the present practice where interurban lines run into the cities. It will be preferable for the a. c. system to have its own lines throughout, unless very considerable complication is permitted. When the a. c. system applied to interurban and steam railway systems finally becomes of predominant import- ance, it is probable that the existing d. c. railways will gradually be changed to a. c. as a matter of convenience in tying the vari- ous railway systems together. As was stated above, a. c. equipments cannot convenientljr be operated on direct current lines. It does not follow that the motor will not operate on direct current. On the contrary, the motor is a first-class direct current machine, and if supplied with suitable control apparatus and proper voltage it will operate very well on the d. c. lines. This would require that the motors be connected ^lormally in series, as the voltage per motor is low. A complete set of d. c. control apparatus would be needed when the a. c. equipment is to be run on direct current, and con- siderable switching apparatus would be necessary for disconnect- ing all the A. c. control system and connecting in the d. c. The complication of such a system may be sufficient to prevent its use, at least for some time to come. In some cities, very strict laws are in force in regard to the voltage variations in various parts of the track system. The permissible variations are so small in Some cases, that an enor- mous amount of copper is used for return conductors; and in SINGLE-PHASE RAILWAY 55 some cases special boosters are used in the return circuits to avoid large differences of potential between the various parts of the track system. The object in limiting the conditions in this manner is to avoid troubles from electrolysis. The a. c. system will, of course, remedy this. For city work, it is probable that voltages of 500 or 600 would be employed instead of 1,000 or higher. The transformers and controllers can be designed to be readily changed from full to half voltage, so that low voltage can be used on one part of the line and high voltage on another As the car equipments of such railways are usually of small capacity, it is probable that speed control will be obtained by means of a transformer with a large number of leads carried out to a control drum, rather than by means of the induction regulator, as the latter device is much more expensive in small units. This is chiefly a question of cost, and if the advantages of the induction regulator are found to over- weigh the objection of high first cost, then it will be used even on small equipments. In the W. B. and A. Railway, the generators are wound for single-phase. In the case of large power-stations with many feeders, the generators may be wound for three-pliase, with single-phase circuits carried out to the transformer substation; or three-phase transmissioa may be used, with the transformers connected in such a manner as will give a fairly well-balanced three-phase load. There are many arrangements and combinations of apparatus made possible by the use of alternating current in the car equip- ments, which have not been mentioned, as it is impracticable to give a full description of all that can be done. But enough has been presented to outline the apparatus and to indicate the possibilities of this new system which is soon to see the test of ■ commercial service. 56 ELECTRICAL ENGINEERING PAPERS SYNCHRONOUS MOTORS FOR REGULATION OF POWER FACTOR AND LINE PRESSURE FOREWORD — At the time when this paper was written, the use of synchronous motors as condensers had, as yet, been given little consideration. In 1890, the author of this paper discovered, during certain experiments in the Westing- house testing room, that a synchronous motor could affect the power factor of the supply system, by variations in its field strength. Sometime later, he proposed the use of such a machine for regulating the pressure of a supply system and for changing the relation between e.m.f . and current in alternating- current systems, and a broad patent was obtained eventually. However, even as late as 1904, when the paper was presented, the value of this method of operation was but little appreciated. This paper should be read from the viewpoint of the time when it was written. Automatic field regulators were not then in general use, and hand regulation was the ordinary practice. Consequently, alternators with in- herently good regulation, that is, which would give three to four times full load current on sustained short circuit, were preferred, in order that rnuch hand regulation would not be needed. A number of facts and relations brought out in this paper have been published niany times by others in recent years. This paper was first presented at a meeting of the American Institute of Electrical Engineers in June, 1904. — (Ed.) It is well known that the synchronous motor, running with- out load oft an alternating -current circuit, for instance, can have its armature current varied by varying its field strength, A certain adjustment of field strength will give a minimum armature current. Either stronger or weaker fields will give increased current. These increased currents are to a great ex- tent wattless. If the field is weaker than the normal (or field for minimum armature current), the increased armature cur- rent is leading with respect to the e.m.f. waves in the motor and lagging with respect to the line e.m.f. The current in the motor is therefore corrective in its nature. For stronger thad the normal field, the current is to a great extent lagging and tends to lessen the flux in the motor and the current is leading with respect to the line e.m.f. A. synchronous motor therefore has an inherent tendency to correct conditions set up by im- proper adjustment of its field strength. The correcting current in the motor is drawn from the supply system and this current also has a correcting effect on the supply system, tending to produce equalization between generated pressures in the motor and the supply pressure. This characteristic of the synchronous motor can readily be utilized for two purposes; namely, for varying the amount of leading or lagging current in a system for producing changes in the power-factor of the system (in- cluding transmission line, transformers, and generators), or a synchronous motor can be utilized for pressure regulation in a system. POWER-FA CTOR REG ULA TION 57 As the synchronous motor can be made to impress a leading current upon the system, and as the amount of this leading current will depend upon the field adjustment of the synchronous motor, it is evident that this property can be used for neutral- izing the effects of lagging current due to other apparatus on the system. The resultant leading or lagging current can be varied and the power-factor controlled over a fairly wide range depending upon the location of the synchronous motor or motors, and upon the current capacity of the motor, etc. As the wattless current in the motor is primarily a corrective current, it is evident that for most effective purposes for ad- justing power-factor on the system the corrective action of this current on the motor should not be too great. When used for such purpose the synchronous motor should therefore be one which would give a comparatively large current if short- circuited as a generator. Also the motor should preferably be one in which the magnetic circuit is not highly saturated, for in the saturated machine the limits of adjustment in the field strength are rather narrow. As has been noted above, if the field strength of the motor be varied, a leading or lagging current can be m.ade to flow in its armature circuit, this current being one which tends to adjust the pressure of the armature and that of the supply system. It is evident that if the armature pressure is held con- stant and the supply pressure varied, a leading or lagging current would also flow. If for instance the line pressure were dropped below that of the motor, then a lagging current would flow in the motor tending to weaken its field, and a leading current would flow in the line, tending to raise the pressure on the line; If the line pressure should be higher than that of the synchronous motor, then the current in the motor would be leading, tending to raise its pressure; while it would be lag- ging with respect to the line, tending to lower it^ pressure. The resultant effect would be to equalize the pressures of the line and motor, and there would thus be a teodency to regulate the line pressure to a more nearly constant value. It is evident that. the less the synchronous motor is affected by the correc- tive current and the more sensitive the line is .to such corrective action, the greater the tendency will be toward constant pres- sure on the- line. It is therefore evident that the synchronous motor which gives the largest current on short circuit as a gen- erator would be the one which gives the greatest corrective action as regards ];rcssurc regulation of the system. 58 ELECTRICAL ENGINEERING PAPERS For such regulation, the synchronous motor which gives a com- paratively large leading or lagging current with small change to the pressure of the system is the most suitable one. Or, the motor which gives the greatest change in the leading or lagging current is the one which gives best regulation. It is the change in the amount of wattless current which produces the regulation. This current could vary from zero to 100 leading, for example, or could change from 50 leading to 50 lagging, or could change from 100 lagging to zero lagging. Any of these conditions could produce the desired regulating tendency, but all would' not be equally good as regards the synchronous motor capacity. If in addition to the regulating tendency it is desired to correct for lower power-factor due to other apparatus on the circuit, it would probably be advisable to run a comparatively large leading current on the line due to the synchronous motor, and the regulating tendency would be in the variations in the amount of leading current, and not from leading to lagging, or vice versa. A larger synchronous motor for the same regu- lating range would be required than if the motor were used for pressure regulation alone. It is evident that the current capacity of a -motor regulating from 50 leading to 50 lagging need be much less than for current regulating from 100 leading to zero. It is evident therefore that if there is to be compensa- tion for power-factor as well as. regulation of pressure, that additional normal current capacity is required. In case such synchronous motors are required for regiilation purely, it may be suggested that such machines be operated at very high speeds compared with ordinary practice. At first glance it would appear that such a synchronous motor could be operated at the highest speed that mechanical conditions would allow, but there are other conditions than mechanical ones which enter into this problem. For instance, it is now possible to build machines of relatively large capacity for two poles for 60-cycle circuits, and for very large capacities — say 1500 kilowatts — having four poles. Therefore mechanical con- ditions permit the. high speeds, and the electrical conditions should be looked into carefully to see whether they are suitable for such service. As such synchronous motors should give rela- tively large currents on short circuit the effect of high speeds and a small number of poles on short-circuit current should be considered. In order to give full-load current on short circuit, the field POWER-FACTOR REGULATION 59 ampere-turns of such a machine should be practically equal to the. armature ampere-turns, taking the distribution of windings, etc., into account. By armature turns in this case is not meant the ampere wires on the armature, but the magnetizing effect due to these wires. Therefore to give, for instance, five or six times full-load current on short circuit, the field ampere-turns should be relatively high compared with the armature. This means that the field ampere-turns per pole should be very high, or the armature ampere-turns per pole very low Ex- perience shows that for very high speed machines, such as used for turbo-generators, there is considerable difficulty in finding room for a large number of field ampere-turns, and therefore in such machines it is necessary to reduce the armature ampere- turns very considerably for good inherent regulating charac- teristics. This in turn means rather massive construction, as the magnetic circuit in both the armature and field must have comparatively large section and the inductions must be rather high. This in turn means high iron losses in a relatively small amount of material compared with an ordinary low-speed ma- chine, and abnormal designs are required for ventilation, etc., and for mechanical strength. An increase in the number of poles usually allows increased number of field ampere-turns without a proportionate increase in the number of armature ampere-turns. This condition is trtie until a large number of poles is obtained when the leakage between poles may become so high that the effective induction per pole is decreased so that there is no further gain by increas- ing the number of poles, unless the machine is made of abnormal dimensions as regards diameter, etc. Experience has indi- cated that in the case of very high-speed and very low-speed alternators, it is miore difficult to obtain a large current on short circuit than with machines with an intermediate number of poles. For example, it is rather difficult to make a 600 kilovolt- ampere, 3600-rev. per min., 2-pole machine which will give three times full-load current on short circuit. A 4-pole, 1800 rev. per min. machine can more easily be made to give three times full-load current on short circuit and with comparatively small additional weight of material. The material in the ro- tating part of the four-pole machine, while of greater weight, may be of considerably lower cost per pound. The stationary part of the four-pole machine may have a sofriewhat larger in- ternal diameter, but the radial depth of sheet-steel will be less 60 ELECTRICAL ENGINEERING PAPERS than in a two-pole machine. The total weight of material in the armature of a four-pole machine may be practically no greater than in a two-pole machine. Therefore a two-pole machine of this capacity should cost more than a four-pole machine, if designed to give the same current on short circuit. A six-pole machine would show possibly a. slight gain over the one with four poles, but not nearly as much as the four-pole machine would over the one with two poles. The real gain of the six-pole over the four-pole construction would be in ob- taining a machine which would give more than three times full-load current on short circuit. It would possibly be as easy to obtain four, times full load current on short circuit with a six-pole machine as to obtain three times full load cur- rent on fourrpole machine. An eight-pole machirie would be in the same way somewhat better than the six-pole machine. Therefore if a 600 kilovolt-ampere m.achine giving six times full-load current on short circuit is desired, it would be advan- tageous to make the machine with possibly eight to twelve poles. The question of which would be the cheaper would de- pend upon a number of features in design. If very large short-circuit currents are desired, then, as in- dicated above, the number of poles for a given capacity should be increased, or the normal rating of the high-speed machine should be decreased. If, for example, the 600 kilovolt-ampere, 3600 rev. per min. machine, mentioned above, should be rated at 200 kilo volt -amperes, then it could give nine times full-load current on short circuit ; but such a method of rating is merely dodging the question. In general, the following approximate Imuts for speeds and short circuit currents for 40-cycle apparatus can be given. These limits are necessarily arbitrary, and are intended to rep- resent machines which could probably be made without using too abnormal dimensions; 600 kilovolt-amperes, 3600 rev. per min., two to three times full-load current on short circuit. 1000 kilovclt-amperes, 1800 rev. per min., three to four times full-load current on short circuit. 1500 kilovolt-amperes, 1200 rev. per min., four to five times full-load current on short circuit. 2500 kilovolt-amperes, 900 rev. per min., four to five times full-load current on short circuit. For 25 cycles it is more difficult to give limiting conditions, POWER-FACTOR REGULATION 61 as the choice of speeds is very narrow. If, for example, a 1500 kilovolt-ampere, 2-pole, 1500 rev. per min. machine can be made to give three times full-load current on short circuit, then as machines of smaller rating cannot run at higher speed, the limiting condition of such machines must be the amount of current which they will give on short circuit. In the same way a 4-pole machine running at 750 rev. per min. may be made for 5000 kilovolt-amperes for three times full-load current as the limiting rating, and there is no choice of speeds for ratings between 1500 kilovolt-amperes and 5000 kilovolt-amperes. It should be noted that the above speeds are very high com- pared with ordinary alternator practite and are up to high- speed turbo-generator practice, but machines with the above short-circuit ratings and speeds are probably more costly to build than machines of corresponding ratings at somewhat lower speeds. It will probably be found therefore that for the above maximum current on short circuit the cheapest synchronous motors for the given ratings will have somewhat lower speeds than those indicated above. It is certain that the lower-speed machines will be easier to design and will be slightly quieter in operation. Probably best all-round condi- tions will be found at about half the above speeds. The above limiting conditions are given as only approxi- mate and are based upon machines having ventiliation as is usually found on rotating field generators for high speed. Arti- ficial cooling, such as obtained with an air-blast or blowers could modify the above figures somewhat; but in general it has been found that high-speed alternators can be worked up to the Hmit imposed by saturation before the limit imposed by temperature is attained. Therefore if higher saturation is not permissible, then there may be relatively small gain by using artificial cooling. One of the principal applications of such regulating syn- chronous motors would be for controlling or regulating the pressure at the end of a long transmission line for maintaining constant pressure at the end of the line, independent of fluc- tuations of load or change of power-factor. In this case, in- creased output of the transmission line may more than con- pensate for the cost of the regulating synchronous motor. In such a case the synchronous motor not only acts as a regulator on the system but costs nothing in the end. In general, the more current that such a synchronous motor will give on short- 62 ELECTRICAL ENGINEERING PAPERS circuit, the better suited it will be for its purpose at the end of a long transmission line. Where a number of such synchronous motors are installed in the same station, the field adjustment must be rather care- fully made, to avoid cross-currents between machines; and the saturation characteristics of the various machines should be very similar. The better such machines are for regulating purposes, the poorer they are for equalizing each other by means of cross-currents. As to the use of dampers with such synchronous motors, it is difficult to say just what is required. A synchronous motor on a line with considerable ohmic drop is liable to hunt to some extent, especially if the prime mover driving the gen- erator has periodic variations in speed. If the synchronous motor gives very large current on short circuit, then its syn- chronizing power is high ; this will tend to steady the operation of the motor and decrease the hunting. The writer believes that such motors in practice will be found to operate better and have better regulating power for constant pressure if pro- vided with rather heavy copper dampers effectively placed on the field poles. With such heavy dampers reaction of the armature on the field is retarded, and therefore the armature niay give a larger momentary current than would flow if there were no damping effect; in other words, the motor is more sluggish than one without dampers. Therefore the addition of heavy dampers on such a machine may produce the same regulating effect which would be obtained by a machine without dampers which gives a larger current on short circuit. Also a machine with heavy dampers will usually be the one with the least hunting tendency and therefore will have the least effect on the transmission line due to hunting currents. In the above, the synchronous motor has been considered only as a regulator and not as a motor. It may be worth considering what would be the effect if the synchronous motor can do. useful work at the same time that it regulates the system. In this case, with a given rated output, one com- ponent of the input will be wattless, and the other part will be energy. The ratio of these two components could be varied as desired. For example, considering the input as 100, the wattless component could be 60 when the energy component is 80; or the synchronous motor could carrv a load of 80% of its rated capacity, this load including its own losses, and could POWER-FACTOR REGULATION 63 have a regulating component of 60% of its rated capacity. If the motor is used as a regulating machine only, then its wattless component can be practically 100. It appears there- 'fore that the machine could be used more economically as both motor and regulator than as a regulator alone, but in such case it would probably be advisable to run the motor at somewhat lower speed than if operated entirely as a regulator. This reduction in speed may practically offset the gain in apparent capacity by using the machine for a double purpose. Also there is comparatively limited use for large synchronous motors for power purposes, as better results are usually obtained by subdividing the units and locating each unit nearest to its load. If a load could be provided which would permit very high-speed driving, then it would probably be of advantage to utilize the synchronous motor for driving. As the synchronous converter is one form of synchronous motor, the question of utilizing such machines for regulators should be mentioned. Upon looking into the question of dis- tribution of losses in the converter, it will be noted that the losses in the armature winding are not uniform. Investigations show that at 100% power-factor, the lowest heating in copper is obtained, and that any departure from this power-factor shows considerably increased loss in the copper, such loss being very high in certain portions of the winding. Next to the taps which lead* to the collector there are strips of winding which at times are worked at a very high loss. Experience shows that it is not advantageous to operate converters at a low power-factor, and that if so operated continuously, or for any considerable periods, the winding should be made much heavier than for higher power-factors. Also in the usual de- sign of converters the field is not made as strong compared with the armature as in alternator practice, and therefore the regulating tendency of the converter compared with a generator or ordinary synchronous motor, is low. Synchronous con- verters can and do act as regulators of pressure for sudden changes of the supply pressure, but such correcting or regu- lating action should not be continual; that is, the pressure supplied to a converter from a hne should nominally be that required by the converter for best operation as a synchronous converter. Unless designed for the purpose, a synchronous converter should not be used to correct low power-factors due to other apparatus on the circuit. 64 ELECTRICAL ENGINEERING PAPERS In the above considerations only general reference has been made to the cost of synchronous motors for regulating pres- sure and power-factors. It is difficult to give even approxi- mate figures for relative cqsts of such apparatus. As inti- mated before, there is some mean speed or number of poles which will be the most suitable for giving a certain maximum current on short circuit. For speeds slightly above or below such mean speed, the cost of the synchronous motor should vary almost in proportion to the speed, provided the maximum short-circuit current can be diminished somewhat at the same time. If the speed is further increased or further decreased, the cost will tend to approach a constant figure. As the ex- treme conditions are approached, tTie cost will begin to rise. The above assumptions are on the basis of continuous opera- tion at a given current capacity, this being the same in all cases. The above assumption is on the basis of decrease in the max- imum short-circuit current, as the machine departs from the mean, or best speed. If the same maximum current is re- quired, then the lowest cost should be at the mean or best speed, while at either side the cost should rise. It is evident that it would be difficult to give any figures on relative costs of such apparatus. The machine for the best or mean condition, should cost practically the same as an alter- nating-current generator of the same speed, output, and short- circuit characteristics. As this speed would probably be some- what higher than usual generator speeds, the cost of such machine would therefore be somewhat lower. This cost would be to a considerable extent, a function of the current on short circuit for a given rated capacity of machine. As mentioned before, in giving a table of limiting speeds and short circuits, it is probable that one-half this limiting speed would be near the best condition. Such machines would probably cost from 60% to 80% as much as similar machines for usual commercial high-speed conditions, neglecting turbo-generator practice. The frequency has considerable effect on this, as, for example, there is small choice of speed as regards high-speed 25-cycle machines. Taking very general figures only, it is probable that in the case of a given capacity of machine for say three or four times full-load current on short circuit the cost cannot be expected to be lower than one-half that of machines of similar rating at ordinary commercial speeds, turbo-generator practice being ex- cluded. The costs in general should approximate more nearly POWER- FACTOR REGULATION 65 those of turbo-generators; but again, an exact comparison cannot be made because in usual practice the turbo-generators do not give three to four times full-load current on short circuit. There are a number of other conditions in this general problem, such as advantage or disadvantage of placing synchronous motors in the main pciwer-house, or distributing them in a number of sub-stations. Also there is the question of the effect of the cost on the generating plant when used with such regu- lating synchronous motors. If higher power-factors are main- tained on the transmission system and generator, a cheaper form of generator can probably be used. The high power- factor permits a larger output from the transmission system and thus represents a gain. If the synchronous motor can be operated at its best speed and also do work, then there is a further gain. If the synchronous motor should be located "at the center of power distribution, and the power is distributed through induction motors, then there is a possibility of re- ducing the cost of such motors by lowering the power-factor, this being compensated for by the synchronous motor deliver- ing leading currents. As the cost per horse power of small motors will be much greater than the cost per horse power of a large regulating motor, there is a possibility of gain from this source. If the induction motors are distributed over wide territory, this gain would be lessened and might disappear. It should be mentioned that the power-factor of a system as influenced by difference in wave form has not been con- sidered in the preceding discussion. It is obviously impossible to neutralize by a synchronous motor the effect of currents in a system due to difference in wave form. Such currents will in general be of higher frequency than the fundamental wave of the system, and the synchronous motor obviously could not correct for them, unless it impressed upon the system opposite waves of the same frequency. This would mean a synchronous motor with a different wave form from that of the system. The power-factor of a system will also be affected by any hunting of the apparatus on the system. It is evident that the synchronous motor could not correct or neutralize s-uch effects, except through exerting a damping effect on the system and other apparatus on the system. A synchronous motor ■with heavy dampers can reduce the hunting in a system, but such hunting can also be damped by induction motors with low-resistance secondaries, especially if of 'the cage type. This 66 ELECTRICAL ENGINEERING PAPERS correcting effect should therefore be credited to the damper rather than to synchronous-motor action. There are a number of other questions which arise in connection with this regu- lating feature of the synchronous motor, but the subject is too broad to permit even mention of them. The substance of the preceding statements can be summarized as follows: 1. A synchronous motor' can be. used to establish leading or lagging currents in its supply system by suitable field adjust- ment, and can thus affect or control power-factor or phase relations of the current in the alternating current system. 2. A synchronous motor will set up leading or lagging cur- rents in its supply system if its field strength is held constant, and the pressure of the supply system is varied above or below that generated by the synchronous motor. Such leading or lagging currents in the supply system will tend to vary the pressure of the system. A synchronous motor can thus act as a regulator of the pressure of its supply system. 3. This regulating action is greatest with synchronous motors which have the closest true inherent regulation (as indicated by high field magnetomotive force compared with the armature magnetomotive force) in distinction from machines which have close apparent regulation obtained by saturation of the mag- netic circuit. •4. If the synchronous motor is used both for regulating the power-factor for neutralizing the effect of other apparatus on the circtiit, and for regulating or steadying the pressure of the supply system, its normal capacity for regulating will be diminished. 5. The most suitable speeds for best electrical conditions will in general be considerably below highest possible speeds as limited by mechanical conditions. 6. Heavy dampers will increase the effectivfeness of the reg- ulating tendency. 7. If the synchronous motor can be used for power purposes as well as for regulation, its apparent capacity is increased. This is due to the fact that the regulation is obtained by means of a wattless component and the power from the energy com- ponent, and the algebraic sum of these two is greater than their resultant which fixes the current capacity of the machine. 8. Synchronous converters in general are not suited for reg- ulating the pressure or controlling the power-factor of an alter- POWER-FACTOR REGULATION 67 9. The costs of synchronous motors for regulating purposes will in general be lower than for alternating-current motors or generators of customary speeds, and will approach more nearly to turbo-generator practice. 68 ELECTRICAL ENGINEERING PAPERS DATA AND TESTS ON 10 000 CYCLE PER SECOND ALTERNATOR FOREWORD — In 1902, the author undertook the construction of a 10 000 cycle per second alternator. This problem was a very new and radical one at that time and it was considered worth while to put the record of results in permanent form. Therefore, this paper was prepared on the subject and presented before the American Institute of Electrical Engineers in May, 1904. This is interest- ing merely as a record of a relatively early construction. — (Ed.) In the early part of 1902, M. Leblanc, the eminent French engineer, was in this country, and spent considerable time at the. Westinghouse Electric & Manufacturing Company's worKs at East Pittsburg. M. Leblanc was very much interested in cer- tain special telephone work, and in connection with such work he desired for experimentation a current of very high frequency. He took up with the writer the question of building a successful alternator for generating current at frequencies between 5000 and 10 000 cycles per second. He was informed that the ma- chine would necessarily be of very special construction, but that it was not an impossible machine. Later he took up the matter with Mr. Westinghouse, who, upon receiving satisfactory assurance that such a machine was possible, advised that the generator be built. A preliminary description of the general design was given M. Leblanc before he returned to Paris. He was somewhat surprised at certain of the features proposed, especially at the fact that an iron-cored armature was consid- ered feasible for a frequency of 10 000 cycles per second. The machine was designed and built on practically the lines of the preliminary description furnished M. Leblanc. The fre- quency being so abnormal, the writer believes that many features in the machine, with the results obtained, will be of scientific interest, and therefore the data of the machine, and the tests obtained are presented herewith. The starting point in this machine was the sheet-steel to be used in the armature. No direct data were at hand showing losses in sheet-steel at such high frequencies, nor was there at 10 000 CYCLE ALTERNATOR 69 hand any suitable apparatus for determining such losses. As preliminary data, tests at frequencies up to about 140 cycles per second were used, and results plotted in the form of curves; these results were plotted for different thicknesses of sheet-steel. Also, tests were obtained showing the relative losses due to eddy currents and hysteresis, and these were plotted, taking into account the thickness of the sheets. These data were not consistent throughout; but the general shape of the curves was indicated, and in this way the probable loss at the frequency of 10 000 cycles per second was estimated for the thinnest sheet-steel which could be obtained. The steel finally obtained for this machine was in the form of a ribbon about 2 in. wide, and about 0.003 in. thick, which was very much thinner than any steel used in commercial dynamos or transformers, which varies from 0.125 to 0.O2S0 mch. Therefore the machine had to be designed with the intention of using this narrow ribbon of steel for the armature segments. A second consideration of great importance in the construc- tion of such a machine is the number of poles permissible for good mechanical construction. For instance, at 3000 revolu- tions ^which was adopted as normal speed — the number of poles required is 400 for 10 000 cycles per second. The fre- quency, expressed in terms of alternations peir minute, multi- plied by. the pole-pitch in inches, gives the peripheral speed in inches. At 1 200 000 alternations per minute (or 10 000 cycles pjer second) and a pole pitch of 0.25 in., for example, the peri- pheral speed of the field will be 25 000 feet per minute. It was therefore evident that either a pole construction should b« 70 ELECTRICAL ENGINEERING PAPERS adopted which would stand this high peripheral speed, or the pole-pitch should be less than 0.25 in. It was finally decided that an inductor type of alternator would be the most convenient construction for this high frequency; with the inductor type alternate poles could be omitted, thus allowing 200 pole projec- tions, instead- of 400. The field winding could also be made stationary instead of rotating, which is important for such a high speed. This construction required a somewhat larger ma- chine for a given output than if the usual rotating type of machine were adopted; but in a machine of this type where everything was special the weight of material was of compara- tively little importance, and no attempts were made to cut the weight or cost of the machine down to the lowest possible limits. The following covers a general description of the electrical, and magnetic features of the machine: Armature. — The armature was built up in two laminated, rings dovetailed into a cast-iron yoke, as indicated in Fig. 1. The laminations were made in the form of segments dovetailed to the cast-iron yoke (Fig. 2). Special- care was taken that the laminations made good contact with the cast-iron yoke, as" the magnetic circuit is completed through the yoke. The armature sheet-steel consisted of plates of 0.003 in. thickness. The sheet-steel was not annealed after being re- ceived from the manufacturer; it was so thin that to attempt annealing was considered inadvisable. To avoid eddy currents between plates each segment was coated with a thin paint of good insulating quality. This painting was a feature requiring; considerable care and investigation, as it was necessary to obtaia a paint or varnish which was very thin, and which would adhere properly to the unannealed laminations. These laminations had a bright polished appearance quite different from that of ordinary steel. Thej^ were so thin that the ordinary paint or varnish used on sheet -steel made a relatively thick coating, possibly almost as thick as the plates themselves. A very thin varnish was finally obtained which gave a much t?;inner coating than the plate itself, so that a relatively small part of the arma- ture space was taken -up by the insulation between plates. 10 000 CYCLE ALTERNATOR 71 Each armature ring or crown has 400 slots. Each slot is circular and 0.0625 inch diameter (Fig. 3). There is 0.03125 inch opening at the top of the slot into the air-gap, and the thickness pf the overhanging tip at Ithe thinnest point is 0.03 125 inch. ya 1/82 Fig.3 Fig. 5 ^<^ /r=^ :«--- Pig.l r>; l?iB radijis 1 -H- Fig.6 The amiature winding consists of No 22 wire, B & S gauge, and there is one wire per slot The entire winding is con- nected in series (Fig. 4). The measured resistance of the wind- ing is 1.84 ohms at 25° cent. After the sheet-steel was built up in the frame, it was ground OJt carefully. The laminations were then removed, all burred edges taken off and the laminations again built up in the frame. The object of this was to remove all chances of eddy currents 72 ELECTRICAL ENGINEERING PAPERS between the plates due to any filing or gnndmg. The finished bore of the armature is 25.0.625 inch. Field or Inductor. — This was made of a forged-steel disc 25 in. diameter turned into the proper shape, and the poles were formed on the outside by slotting the periphery of the ring. The general construction is indicated in Figs. 1 and 5. The poles were 0.125 m. wide and about 0.75 in. long radially and were round at the pole-face. Fig. 6 shows the general dimen- sions of a pole. The field winding consisted of No. 21 wire, B. & S. gauge. There were 600 turns total arranged in 30 layers of 20 turns per layer. The field coil after being wound was attached to a light brass supporting ring. The general arrangement of the field or inductor, armature yokf, and bearings, is as indicated in Fig. L The measured resistance of the field winding is 53.8 ohms at 25° cent. Tests. — The machine was designed primarily for only a small output, but was operated on temporary test up to 2 kw. A. series of curves were taken at 500, 1000, 1500, 2000, 2500, and 3000 revolutioiis, giving frequencies from 1667 to 10 000 per second. At each of the above speeds, saturation curves, iron losses, and short-circuit tests were made. Friction and wind- age were also measured at each speed. On account of the high frequency, the machine was worked at a very low induction; consequently there is an extremely wide range in pressure, the .normal operating pressure being taken at approximately 150 volts. On curve sheet No. 1, the saturation curves for the various speed's are given. These curves check fairly well, the pressure being practically proportional to the speed with a given field charge. This is to be expected at the lower speeds, but it was considered possible that at 3000 revolutions the air-gap might be slightly lessened, due to the expansion of the rotor under centrifugal action; and it was aiso thought that eddy-current loss due to the high frequency might affect the distribution of magnetism at the armature face; but the armature iron losses were comparatively small, and there appeared to be no such effect. Also there appears to be no effect due to expansion at high speed. The air-gap specified for this machine is 0.03125 in. on each side or 0.0625 in. total gap A very small varia- tion in the diameter of the inductor or the bore of the armature would make a relatively large per cent in the effective air-gap 10 000 CYCLE ALTERNATOR 73 Therefore no reliable calculations can be made on the saturation, curves of this rnachine based upon the specified air-gap. Curve sheet No. 2 shows the iron losses at various speeds from 500 to 3000 rev per min. — 1667 to 10 000 cycles per second. These losses are plotted i»i terms of watts for a given exciting 4UU 375 350 / 1 .... HIGH FREQUENCY ALTERKATOI ICOOO Cycles per Second Saturation Curves ' / / 3sj5 / 300 / ..^'^^ f ^5 2S0 - / /. 4/ Ci irve St leetNc 1.1 / / <^ / < 200 ^ i) V // '' 1 130 1« 100 i/ / A '/^ A / / rfA y / /. / A ,^ / 7 / / ^€-^ ^ ^ // 7 / ^"^ V 75 30 25 // / y y ^"^ ^ o y^ ^ vsii w^ -^ 0.2 0.4 0.6 O.S 1.0 1.2 1.4 rield Amperes 1.0 1.8 ■1:1 current. These curves show a rather unexpected "condition as regards the losses. According to the original data showing the relative losses due to eddy currents and hysteresis, the eddy- current loss eK^en with these thin plates should have been much liigher than the hysteresis loss, but these iron -loss curves show 74 ELECTRICAL ENGINEERING PAPERS losses with a given field charge almost proportional to the fre- quency, which is the ratio that the hysteresis loss alone should show. As the eddy-current loss varies as the sc^uare of the frequency, the writer expected this to be a large element in the total iron loss, especially at the higher inductions. The six curves shown on this test-sheet are fairly consistent 1 1400 1300 1200 1100 1000 900 800 700 000 HIGH-FREQUENCY ALTERNATOR 10000 Cycles per Second Iron-Loss Tests / / / .51 / 1 / Ci i-\e SI eetNc .2 4 i h V /f 4y Y 400 aoo / V y f .,^\ y^ // 6 ^y .f> f / % ^.A > 100 A ^ /^ (SlO ^ ^^ ^as^ 2 i 6 .8 1 1 1 5^eldA % L mpei;e s 6 1 8 i 2 2 with each other, but it should be remembered that in making measurements of such abnorrtlal apparatus little discrepancies in the curve? eould easily creep in. For instance, in the satu- ration cun'e a series of experiments were first made to find whether usual types of voltmeters were satisfactory, and a num- 10 000 CYCLE ALTERNATOR 75 ber of different methods for checking these readings were used. In determining the iron losses in curve sheet No. 2, the machine was driven by a small motor and the losses measured with dif- ferent field charges. Under most conditions of test the iron loss was a small element of the total loss, and therefore slight varia- tions in the friction loss would apparently show large variations HIGH-FREQUENCY ALTERNATOR 10000 Cycles per Second Sbort-Circuit Tests w / / / / s Ci rve SI eeiNc z / / 1 /c ¥ ■< ^ 4 /4 V. i 1 / m V 8- i/ V JW7V 5- //si V 1/ / // V if if /, f 3- — i- r^ f / / // / 1 1- 'y> X 0.1 0.2 0.3 0.1 FleM Amperes 0.5 in the iron losses. Also the fly-wheel capacity of the rotating part of the alternator was comparatively high. Therefore, if there are any variations in the circuits supplying the driving motor, there would tend to be considerable fluctuations in the power supplied. Considering all the conditions of test, the curves appear to be remarkably consistent. 76 ELECTRICAL ENGINEERING PAPERS Curve sheet No. 3 shows the short-circuit curves at speeds, of 1000, 2000, and 3000 rev. per min., or frequencies of 3333,. 6667, and 10 000 cycles per second, respectively. It should be noted that at a given frequency the short-circuit current is pro- portional to the field current over the entire range measured i)7S0 8900 3250 3000 2750 2500 2350 HIGH FREQUENCY ALTERN>TOB lOOCO Cycles per Second Windage and Bearing Friction 1000 1500 rev. per nun. 2000 2300 3000 but that the short-circuit current is not the same for the same- field current at the vanous frequencies. According to these curves the current on short circuit increases somewhat with the given field charge as the frequency is increased. Curve sheet No. 4 shows the measured windage and friction losses plotted at speeds from 500 to 3000 rev. per min. This 10 000 CYCLE ALTERNATOR 77 curve indicates clearly that the windage is the principal friction loss at the higher speeds. The writer has added two curves, one showing the estimated bearing friction loss, and the other the estimated windage, based upon the assumption that the bearing friction varies directly as the revolutions and the wind- age loss with the third power of the revolutions. The small circles lying close to the measured loss curve show the sum of these estimated losses, and the agreement with the measured loss is fairly close over the entire range. 3 S HtGH-FKEQUENCr ALTERNATOa Regulation Test atlOOOO Cjclee per Second 0.8 lao ^'o.2 Jonstan t e.m.f, 0.7 145 0.6 140 ^->. "^ ::^ So.2 newc ^rgiiSlS^ S iio.l Cuubtaiit Field ,CliM.ri;ti 0.5 135 \ h Ci irveS .eetN .5^ % 0.3 125 0.2 120 \ \ ^ 0.1 116 Amperes Load Curve sheet No. 5 shows regulation tests made at 150 volts. The- power-factor of the load on this test was not determined, as it was extremely difficult to make accurate measurements. The load consisted of incandescent lamps and the wiring from the machine to the laimps was non-inductive for the usual fre- quencies; but at the abnormal frequency of 10 000 cycles per second it' is more difficult to obtain a true non-inductive load with ordinary apparatus. The tested regulation indicates that the load was practically non-inductive. In first undertaking tests on this machine there was consid- 78 ELECTRICAL ENGINEERING PAPERS crable difficulty in measuring the pressures. It was found that at a frequency of 10 000 cycles per second the Weston voltmeter did not work satisfactorily. Practically the same deflection was obtained on the high and Idw scales of a 60-120 volt Weston, alternating-current direct-current voltmeter with the same pressure. Very good results were obtained by the use of a form of static voltmeter devised by Mr. Miles Walker. This voltmeter is of the ■same' form as the static wattmeter described by Mr. Walker before the American Institute of Electrical En- gineers, May, 1902.* Tests .were also made with the Cardew hot-wire voltmeter with the high frequencies, and the results checked very satisfactorily with the static voltmeter. For measuring the current a current dynamometer was used which had wood upright supports and a celluloid dial. The only metal parts outside of the copper coils were brass screws It was found that the current dynamometer is not affected by frequency, unless there are adjacent metal parts in which eddy currents can be generated which react upon the moving element. The dynamometer used had but a few turns in order to reduce the pressure drop across it. This dynamometer was checked very carefully at different frequencies and apparently gave similar results for any frequency between 25 and 10 000 cycles Several temperature tests were made on this machine. The heaviest load on any test was 13.3 amperes at 150 volts, or 2-kw. output. This test was of two hours' duration, and at the end the armature iron showed a rise of 16° cent.; the armature copper 21° cent, by resistance, and the field copper 17.3° cent. Air temperature 19° cent. The machine showed a relatively small increase in temperature at this load over the temperature rise with one-third this load. This was probably due to the fact that the windage loss was so much higher than the other losses of the machine that the temperature was but little affected by the small additional loss with increase in load. Attempts were made to utilize the current from this machine for various experiments, but difficult}' was at once found in transforming it. At this high frequency no suitable iron-cored transformer was available. Transformers with open magnetic circuits were tried and operated better than those with iron cores but were still rather unsatisfactory. It was decided that nothing could be done in this line without building special transformers. 5*Transactions of the A. I. E. E., Vol. xix, p. 1035.] 10 000 CYCLE ALTERNATOR 79 Amorig the few experiments made was that of forming an arc with current at this high frequency. This arc appeared to be like an ordinary arc so far as the hght was concerned, but had a very high-pitched note corresponding to the high frequency. This npte was very distressing to the ears. This machine is iii reality of the nature of a piece of labora- tory apparatus; and at present it has no commercial value. It was designed primarily for scientific investigation, and appears, to be a very good machine for that purpose. 80 ELECTRICAL ENGINEERING PAPERS THE SINGLE-PHASE COMMUTATOR TYPE RAILWAY MOTOR FOREWORD — This paper was presented before the Philadelphia Section of the American Institute of Electrical Engineers in February, 1908. It describes, as simply as possible, the general construction and characteristics of com- pensated series single-phase motors. — (Ed.) The broad statement may be made that it is no more difficult to commutate an alternating current than an equal direct current. Such a statement would appear to be entirely contrary to the usual experience, but a little study of the matter will show where the apparent discrepancy lies. In commutator type alternating- current motors, as usually built, a relatively large number of commutator bars pass ofE under the brush during one alternation of the supply current. While the current supplied is Varying from zero to maximum value and back to zero, possibly 50 bars have been passed under the brush, and therefore 50 coils in the armature have been reversed or commutated. Some of these reversals occur at the top of the current wave which has a value of about 40% higher than the mean or effective value which is read by the ammeter The motor is therefore at times commutating 40% higher current than that indicated by the instruments. It is thus evident that in comparing the com- mutation of 100 amperes direct-current with 100 amperes alternating-current we should actually compare the direct- current with 141 amperes alternating. In other words, for com- mutating equal currents alternating-current or direct-current, the alternating-current ammeter should register only 71% as much current as the direct-current. Another way of expressing it is that we have to commutate the top or maximum of the alternating-current wave, while our instruments only record the mean value. If the above represented the only difference between the alternating current and direct current the problem to be solved in commutation of alternating current would not be serious. SINGLE-PHASE RAILWAY MOTOR 81 However, the current to be commutated by an alternating- current motor is not merely the working current supplied to the motor and measured by the ammeter, but there is, in addi- tion, a current which is generated in the motor itself, both at standstill and during rotation, which has to be reversed or com-, mutated along with the working current. It is this latter cur- rent, usually called the local or short-circuit current, which has been the source of greatest trouble in commutating alternating current; for this short-circuit current may have a value any- where from three to ten times the working current, depending on the design of the machine. Therefore in comparing the com- mutation of an alternating current, as indicated by an ammeter. with an equal direct current, we should, in reality, consider that the alternating-current motor is commutating a maximum current from five to ten times the value of the indicated current. Furthermore, it would not do to reduce the ammeter current to one-fifth or one-tenth value in order to compare commutation with direct current, because by so doing we would simply be reducing the small applied component of the total current commutated by the brushes, the local or short-circuit current still retaining a rather high value. In order to compare with direct-current commutation, it would be necessary for the total maximum of the combined supply and the short-circuit current to be reduced to the same value as direct current. 82 ELECTRICAL ENGINEERING PAPERS It is the local current in the armature turn short-circuited by the brush which is the source of practically all the trouble in commutating alternating currents. Fig. 1 illustrates a portion of. the field and armature structure of a commutator type alternating-current motor. It will be noted that the armature conductor, which is in the neutral position between poles, sur- rounds the magnetic flux from the field pole, just as the field turns themselves surround it. The field flux being alternating, this armature turn will have set up in it an electromotive force of the same value as one of the field turns. Short-circuiting the two ends of this armature turn should h9,ve the same effect as short-circuiting one of the field turns, which is the same thing as short-circuiting a turn on a transformer. Such a short- circuited turn, if of sufficiently low resistance, should have as many ampere-turns set up in it as there are field ampere-turns. In single-phase motors of good design the field ampere-turns per pole are about twelve to fifteen times the normal ampere- turns in any one armature coil. Therefore, if the armature coil in the position shown in this Fig. 1 should have its ends closed on themselves the current in this coil would rise to a value of twelve to fifteen times normal. In reality, it would not rise quite this much, because this armature turn is placed on a separate core from the field or magnetizing turns with an air- gap between, so that the magnetic leakage between the primary (or field winding) and this armature (or secondary winding) would tend to protect this coil somewhat, just as leakage between the primary and secondary windings of a transformer tends to reduce the secondary electromotive force and current. Also, this armature coil is embedded in slots, thus adding somewhat to its self-induction, and tending further to reduce the short- circuit current. In consequence, with its ends closed together the current in this armature coil would probably not rise more than ten to twelve times above normal value under any con- dition. It is evident, therefore, that if the brush shown in Fig. 1 as bridging across two commutator bars to which the ends of this coil are connected is of copper or other low-resistance material, then there could be an enormous local current set up in the coil when thus short-circuited by the brush. This lopal current of about ten times the normal working current would have to be commutated as the brush moves from bar to bar, and therefore the operation of the machine would be similar to that of a direct-current motor if overloaded about ten times iii^ current. In other words, there would be vicious soarkine-. SINGLE-PHASE RAILWA Y MOTOR 83 Even if the low-resistance brush were replaced by one of ordinary carbon, the short-circuiting current would still be rela- tively high, due to the fact that it is not possible to make the brush contact of very high resistance by reducing the size or number of the brushes, because these -same brushes must carry the working current supplied to the motor, and there must be brush capacity sufficient to handle this current. This brush capacity will, in practice, be of such amount that the resistance in bridging from one bar to the next is still rather low, although much higher than if a copper brush were used. Experience shows that with not more than four or five volts generated in this short-circuited coil by the field flux, the resistance of the carbons at the contact with the commutator would be such that a short-circuit current of three to four times the normal working current in the coil can still flow. Therefore, if the motor were equipped with carbon brushes and had but four or five volts generated in the short-circuit coil, the motor would have to commutate the main or working current and also a short-circuit current of possibly three times the amount. This short-circuit current would also have a maximum or top of its current wave. Assuming 100 amperes as the current supplied to the motor, the machine therefore actually commutates a supply current of 141 amperes and an additional short-circuit current of possibly three times this value, or from 400 to 500 amperes; therefore, the motor actually comtautates the equivalent of about 600 amperes direct current when the alternating-current ammeter is reading 100. It is evident from this that any one who tries to commutate alternating current with an ordinary type of commutating machine would at once draw the conclusion that alternating current in itself is very difficult to commutate, naturally overlooking the fact that it is the excessive current handled by the brush that is back of the trouble, and not the current indicated by the ammeter. From what has been stated, it is evident that the excessive local current is back of the difficulty in commutating alternating current. All efforts of designers of alternating-current com- mutator motors have been in the direction of reducing or elimina- ting this local current. The present success of the motor, in the various forms brought out, is largely due to the fact that this current has been successfully reduced to so low a value that it does not materially add to the difficulties of commutating the main current. No successful method has yet been practically 84 ELECTRICAL ENGINEERING PAPERS developed for entirely overcoming the effects of this sTiort-circtlit current under all conditions from standstill to highest speed. Some of the corrective methods developed almost eliminate this current at a certain speed or speeds, but have little or no cor- rective effect under other conditions; other methods do not effect a complete correction at any speed, but have a reFatively good effect at all speeds and under all conditions. The former methods would appear to be applicable to rfiotors which run at, or near, a certain speed for a large part of the time; the latter method would be more applicable to those cases where the motor is liable to be operated for considerable periods with practically any speed from standstill to the highest. While several methods have been brought forward for correcting local current when the motor has obtained speed, yet up to the present time but one successful method has been developed for materially re- ducing this current at standstill of very low speeds. It may be suggested that the short-circuit voltage per coil be reduced to so low a value, say four or five volts, that the local current is not excessive and does not produce undue sparking. This would certainly reduce the sparking difficulty, but is open to the very great objection that the capacity of the motor is directly affected by a reduction in the shoi-t-circuit voltage. This voltage per turn in the armature coil is a direct function of the value of the alternating field-flux and its frequency. Assuming a given frequency, then the short-circuit voltage is a direct function of the induction per field pole, and the lower the short-circuit volt- age the lower must be the field flux. But the output of the machine, or the torque with a given speed, is proportional to the product of the field flux per pole by the armature ampere- turns. In a given size of armature the maximum permissible number of ampere-turns is pretty well fixed by mechanical and heating considerations, and therefore with a given armature the torque of the motor is a direct function of the field flux. Using the maximum permissible armature ampere-turns, the output of a given motor would be very low if the field flux were so low that the short-circuit voltage would not be more than three or four volts. Increasing the field induction, and there- fore increasing the short-circuit voltage, increases the output. Experience shows that on large motors, such as required for railway work, the induction per pole must necessarily be so high that the electromotive force in the short-circuit coil must be about double the figure just given; therefore, with such heavy SINGLE-PHASE RAILWAY MOTOR 85 flux the short-circuited current will necessarily be excessive unless some corrective means is used for reducing it. I will consider the standstill or low-speed conditions first. For this condition only one practical arrangement has so far been suggested for reducing the local current to a reasonably low value compared with the working current. This method involves the use of preventive leads, or, as they are sometimes called, resistance leads. These consist of resistances connected between the commutator bars and the armature conductors. Fig. 2 illustrates the arrangement. The armature is wound like a direct-current machine, except that the end of one arma- ture coil is connected directly to the beginning of the next o o o o o o Lead. Fig. 2 without being placed in the commutator Between these con- nections separate leads are carried to the commutator bars, and in these leads sufficient resistance is placed to cut down the short-circuit current. The arrangement is very similar in effect to the preventive coils used in connection with step-by-step voltage regulators which have been in use for many years. In passing from one step to the next on such regulators, it is common practice to introduce a preventive coil or resistance in such a way that the two contact bars are bridged only through this preventive device. In. an armature winding arranged in this way, the working current is introduced through the brushes and the leads to the armature winding proper. After entering the winding, the 86 ELECTRICAL ENGINEERING PAPERS current does not pass through the resistance leads because the connections between coils are made beyond these leads. In consequence, only a very sm-all number of these leads are in circuit at any one time, when the armature is in motion all the leads carry current in turn so that the average loss in any one lead is very small. As the brush generally bridges across two or more commutator bars, there is usually more than one lead m circuit, but generally not more than three. When the brush is bridging across two bars, there is not only the working cur- rent passing into the two leads connected to these two bars, but there is the local current, before' described, which passes in through one lead, through an armature turn, then back through the next lead to the brush. There are losses in these two leads due to these two currents. By increasing the resistance, the loss due to the working current is increased, but at the same time the short-circuit current is decreased. As the loss due to this latter is equal to the square of the current multiplied by the resistance, it- is evident that increasing this resistance will cut down the loss due to the local current in direct proportion as the resistance is increased. When the working current is much smaller in value than the short-circuit current, an increase in the resistance of the leads does not increase the loss due to the working current as much as it decreases the loss due to the short-circuit current. Both theory and practice show that when the resistance in the leads is so proportioned that the short-circuit current in the coil is equal to the normal working current, the total losses are a minimum. Calculation, as well as experience, indicates that a variation of 20% to 30% at either side of this theoretically best resistance gives but a very slight increase in loss, so there is considerable flexibility in the adjust- ment of this resistance. The resistance of the brush contacts and of the coil itself must be included with the resistance of the leads in determining the best value. In practice it is found that with ordinary medium-resistance brushes, the resistance in the leads themselves should be about four or five times as great as the resistance in the brush contact and the coil; that is, we usually calculate the total necessary resistance required and then place about 70% or 80% of it in the leads themselves.- When leads of the proper proportion are added to the motor, it IS found that practically twice as high field flux can be used' as before with the same sparking and burning tendency as when the lower flux is used without such leads. But even with six SINGLE-PHASE RAILWA Y MOTOR 87 to eight volts per commutator bar as a limit, we are greatly handicapped in the design of the motors, especially when the frequency is taken into account. This limited voltage between bars also indicates at once why single-phase railway motors are wound for such relatively low armature voltages. Direct- current railway motors commonly use from 12 to 20 volts per commutator bar, or from 2 to 2.5 times the usual practice on alternating-current motors. With this low voltage between bars in alternating-current machines, with the largest practic- able number of bars, the armature voltages become 200 to 250, or about 40% of the usual direct voltages. The choice of low voltage should, therefore, not be considered as simply a whim of the designers; it is a necessity which they would gladly avoid if possible. Assuming preventive leads of the best proportions, let us again compare the current to be commutated in an alternating- current motor with that of the direct-current. Considering the ammeter reading as 100, the working alternating current has a maximum value of 140 and in addition there is a short- circuit current of same value. Even under this best condition, the alternating-current motor must commutate a current several times as large as in. the corresponding direct-current motor. The design of such a motor, therefore, is a rather difficult prob- lem, even under the best conditions. While resistance leads theoretically appear to give the most satisfactory method for 'obtaining good starting and slow- speed running conditions, yet other methods have been pro- posed. The only one of any practical importance is that in which the short-circuit voltage is reduced at start and at slow speed by sufficiently reducing the field induction. As this reduced field induction would give a proportionately reduced torque, it is necessary at the same time to increase the armature ampere-turns a corresponding amount above normal. This is only a part solution of the problem, however, for the decrease in short-circuit current by this means is partly offset by the increase in the working current, so that the total current to be commutated is not reduced in proportion to the field flux. Where the period of starting and slow running is very short, this method is fairly successful in practice. However, with this arrangement it is rather dangerous to hold the motor at stand- still for any appreciable length of time, for in such a case the large short-circuit current is confined to a single coil and the 88 ELECTRICAL ENGINEERING PAPERS effect is liable to be disastrous if continued for more than a very short period. With this method of starting, the total current handled by the brushes will usually be at least two to three times as great as when preventive leads are used. The preceding statements refer mainly to. starting or slow- speed conditions. When it comes to full-speed conditions, however, there are various ways of taking care of the commuta- tion. One of these methods is based on the use of preventive leads, as described; the other methods depend upon the use of commutating poles or commutating fields in one form or another. It is evident, from what has been said, that at start the pre- ventive leads which reduce the short-circuit current to low values will also be effective in a similar manner when running at normal speed. Such a motor with proper proportion of leads will, in general, commutate very well at full speed when the starting conditions have been suitably taken care of. Nothing further need be. said of this method except that the tests show that the short-circuit current has considerably less value at high speed than at start. The other methods of commutation at speed, involving com- mutating poles and commutating fields, necessarily depend upon the armature rotation for setting up a suitable electromotive force in the short-circuit coil to oppose the flow of the short- circuit current. As the electromotive force in the short-cir- cuited coil is a direct function of the field flux, and is inde- pendent of speed, while the correcting electromotive force is a function of the armature speed, it is evident that either the commutating pole can produce the proper correction only at one particular speed, or the strength of this commutating pole must be varied as some function of the speed. Usually the strength of these poles is made adjustable with a limited number of adjustments and approximate compensation only is obtained on the average. In the Siemens-Schuckert motor the com- mutating poles are of small size and placed between the main poles. These are for the purpose of obtaining commutation when running. In addition the armature is provided with pre- ventive leads for improving the operation at start and at slow speed. In the Alexanderson motor, according to published description, no separate commutating poles are provided, but the edges of the main poles are used as commutating poles, the armature coil having its throw shortened until its two sides come under the edges of the main poles. Tn this motor the field SINGLE-PHASE RAILWA Y MOTOR 89 'is weakened and the armature ampere-turns are increased while starting. The commutating-pole scheme in this motor is, in some ways, not as economical as in the Siemens-Schuckert arrangement, as the motor requires a somewhat higher mag- netization with a consequent reduction in power-factor. The Winter-Eichberg motor is quite different in arrangement from any of those which I have mentioned I will not attempt to describe this motor in full,- but will say that it has two sets of brushes in the armature, one of which is short-circuited on itself, and carries the equivalent of the working current in the types I have described, while the other carries the magnetizing or exciting current which is supplied to the armature winding instead of the field. The arrangement is such as to give prac- tically the same effect as a commutating pole or commutating field. When starting, the field flux is decreased and the arma- ture ampere-turns increased. All of the above motors are nominally of low armature voltage and all of them appear to commutate reasonably well at speed. Two of them use the full-speed induction at start, while the •other two use reduced induction and increased armature ampere- turns at start. There has been considerable discussion during the last year or two regarding the most suitable frequency for single-phase commutator type motors. It may therefore be of interest to consider what effect reduction in frequency would have on the commutation, output, and other characteristics of the inotor. The short-circuit voltage, as I have stated before, is a function of the amount of field flux and of the frequency. For a given short-circuit voltage the induction per pole can be increased directly as the frequency is decreased. If a certain maximum induction per pole is permissible at 25 cycles, then with 12.5 cycles, for example, the induction per pole may be double, with the same short-circuit voltage. This would at once permit double output if the saturation of the magnetic circuit would permit the doubling of the induction. But on 25-cycle motors, as usually built, we work the magnetic flux up to a point just on the verge of saturation, so to speak, as indicated in Fig. 3. It is evident that double induction, under such conditions, would not be practicable unless the 25-cycle motor had been worked at an uneconomically low point. However, an increase of 30% to 40% in the induction would appear to be obtainable, but a large iricrease in excitation is required. With but 30% 90 ELECTRICAL ENGINEERING PAPERS to 40% higher induction, and with the frequency halved, the short-circuit voltage would be but 65% to 70% of that with 25 cycles or, in other words, the voltage per turn in the field coil is but 65% to 70%. As the higher induction raises the armature counter electromotive force the field electromotive force can be increased in proportion for the same power-factor. Or can be 30% to 40% higher than with 25 cycles. As the total .field voltage, therefore, can be 30% to 40% higher, and the voltage per field turn is but 65% to 70%, it is evident that the number of field turns can be doubled without changing the - - "~>^ V^ j ■i ^^ ' "o -ji^ 1 a /I 1 L / i « / 1 * O. / 1 1 c / I o / 1 ■*^ / I 1 o / ^ 3 / \ ' / 1 Field AropereZTurn*. Fig. 3 ratio of the field inductive volts to the armature electromotive force. In other words, the field turns can be doubled if the frequency is halved. With the double field turns the field excitation can therefore be doubled, which is the requirement for the increased induction shown in Fig. 3. It is thus evident that halving the frequency will permit higher pole inductions, and therefore higher torque and output, with lower short-circuit voltage and better commutating conditions throughout. Also, this higher field induction is not necessarily accompanied by an increased iron loss, for the lower frequency of the alternating SINGLE-PHASE RAILWAY MOTOR 91 flux compensates for this. On the above basis it may bfe asked why a reduction to 15 cycles is proposed instead of to 12.5, or even to 10 cycles. There are several reasons for the choice of 15 cycles. 1. The motor can be worked up to so high a saturation at 15 cycles that there is relatively small gain with a reduction to 12.5 cycles, which would be about the lowest frequency to con- sider when the transformers and other, apparatus is taken into account. 2. As the torque of the single-phase motor is pulsating in- stead of being constant, as in a direct-current machine, there is liability of vibration as the frequency of the pulsation is de- creased. This effect becomes more pronounced the larger the torque of the motor, and is, therefore, of most importance in the case of a. large locomotive. Experience shows that this ten- dency to vibrate can be damped out effectively in very large motors with a frequency of 15 cycles, but becomes more difficult to suppress as the frequency is further reduced. This is, in reality, one of the fundamental reasons for keeping up to 15 cycles instead of reducing to 12.5 or lower. 3. The lower the frequency the heavier the transforming apparatus on the car or locomotive. It is probable that with 12J cycles instead of 15 cycles, the increase in weight and cost of the transforming apparatus would about counter-balance the decrease in the same items in the motors themselves, al- though the efficiency and power factor of the equipment would be slightly better with the lower frequency. 4. As synchronous converters will be used to some extent in connection with the generating plants for single-phase systems in order to feed existing direct current railways, the frequency of 15 cycles will be slightly more favorable than 12.5 as regards cost of the converters and the step-down transformers. The same will be true if motor-generators are used for transforming to direct current, also for induction motors. Against the choice of 15 cycles may be cited the fact that there are other frequencies which represent a better ratio to 25 cycles when frequency-changers are to be taken into account. A low-frequency railway generating plant may require to tie up with some existing 25-cycle or 60-cycle plant; this can be done by interposing frequency-changers. Or it may be desired to obtain a lower frequency with a single-phase current from some existing higher frequency, polyphase plant, By inter- 92 ELECTRICAL ENGINEERING PAPERS posing the frequency-changer the single-phase railway load will not exert any unbalancing effect on the polyphase supply circuit, and at the same time the railway circuit can be regulated up or down independently of the three-phase generator circuit. In case the three-phase plant is operated at 25 cycles, then a two-to-one ratio of frequencies; that is, 12.5 cycles on the rail- Way circuit, would give the best conditions as regards choice of poles and speeds in the frequency-changer sets. A five-to- three relation is given by 15 cycles, which is not nearly as good as the two-to-one ratio. A frequency of 16§ cycles would give a three-to-two ratio, which represents considerable improve- ment over the five-to-three ratio. Therefore, this slightly higher frequency may prove of advantage in some cases. The choice of this frequency, however, does not mean a new line of apparatus; for a well designed line of 15-oycle motors tran- formers, etc, should operate very well on a 16§-cycle circuit without any change whatever When transforming from 60 cycles, however, the 15 cycle gives a four-to-one ratio which is very good, and neither 12.5 nor 16f cycles is very satisfactory. Therefore this 15-cycle frequency represents the best condition in transforming from 60 cycles, and fairly good conditions for transforming from 25 cycles; and by operation of 15-cycle apparatus at 16f cycles a very good transformation ratio is obtained from 25 cycles. It may be of interest to recall that the old Washington, Baltimore and Annapolis Railway, which was the first road contracting for single-phase commutator motors, was laid out for 16f cycles. There was considerable criticisms at that time of the use of this frequency, but the statement which I have just made shows one very good reason for this frequency A second rea- son is that 16| cycles per second is 2000 alternations per minute, which permits a steam turbine driving a two-pole generator to use a speed of 1000 rev. per min., which is a very good one for large turbo-generators. I have gone into the question of induction and frequency as affecting the commutation and torque. I will now take up the question of power-factor in the single-phase commutator motor In a direct-current motor we have two electromotive forces which add up equal to the applied electromotive force; namely, the counter electromotive force due to rotation of the armature winding in. the magnetic field, and the electromotive force absorbed in the resistance of the windings and rheostat. SINGLE-PHASE RAILWA Y MOTOR 93 In the alternating-current motor there are these two electro- motive forces, and there is also another one not found in the direct-current machine; namely, the electromotive force of self- induction of the armature and field windings due to the alter- nating magnetic flux in the motor. This inductive electro- motive force exerts a far greater influence than the ohmic electromotive force for it has much higher values. The inductive electromotive force lies principally in the main field or exciting winding of the alternating-current motor. There is a certain voltage per turn generated in the field coils, depending upon the amount of the field flux and its frequency, as stated before. This electromotive force per field turn is practically of the same value as the short-circuit electromotive force generated in the armature coil, as already referred to. I have stated that a short-circuit voltage of three or four volts per armature turn gave prohibitive designs and that it was necessary practically to double this. This means that the field coils also have six to eight volts per turn generated in them. The total number of field turns must, therefore, be very small in order to keep down the field electromotive force, for this represents simply a choke-coil in series with the armature. If the armature counter electromotive force should be 200 volts, for instance, which is rather high in practice with 25-cycle motors, then a field self-induction of half this value would allow about 14 turns total in the field winding. Compare this with direct-current motors with 150 to 200 field turns for 550 volts, or 60 to 80 turns for 220 volts. The alternating-current 25-cycle motor, therefore, can have only about 20% to 25% as many field turns as the ordinary direct-current motor. This fact makes it particularly hard to design large motors where there must be many poles. In the single-phase motor the induction per pole being Hmited by the pei-missible short-circuit voltage, it is necessary to use a large number of poles for heavy torques; but the total number of field turns must remain practically constant on account of the self induction, while in reality the number of turns should be increased as the number of poles is increased. With a given number of poles we may have just sufficient field turns to magnetize the motor up to the required point; but if a large number of poles should be required, then we at once lack field turns and mu^t either reduce the field induc- tion, and thus reduce the output, or must add more field turns and thus get a higher self-induction or choking action in the 94 ELECTRICAL ENGINEERING PAPERS field, with a consequent reduction in powei-- factor. Here is where a lower frequency comes in to advantage, fdr, as I showed before, with the same relative inductive effect, the field turns can be increased directly as the frequency is decreased. The use of 15 cycles thus permits 67% more field turns than 25 cycles and raises our permissible magnetizing limits enormously. This problem is encountered particularly in gearless locomotive motors of large capacity. For increased capacity the driving wheels are made larger, thus permitting a larger diameter of motor, the length, axlewise, being fixed. But with increased diameter of drivers, the number of revolutions is decreased for a given number of miles per hour. With 25-cycle motors we soon encounter the above mentioned limiting condition in field turns; beyond this point the characteristics of the motor must be sacrificed, and even doing this we soon reach prohibitive limits. By dropping the frequency to 15 cycles, for instance, we change the whole situation. The induction per pole can be increased and the number of poles, if desired, can also be in- creased. The practical result is that, in the case of a high-speed passenger locomotive with gearless motors, a 700-h.p. 15-cycle motor can be got in on the same diameter of drivers as required for a 500-h.p. 25-cycle motor. Also a 500-h.p. 15-cycle motor goes in on the same drivers a»s a 360-h.p., 25-cycle motor. At the same time these 15-cycle motors have better all round characteristics than the 25-cycle machines as regards efficiency, power-factor, starting, over-load commutation, etc. Returning to the design of the motor, there is one other 'electromotive force of self induction which may be considered; namely, that generated in the armature winding and in the opposing winding in the pole face, usually called the neutrahzing or compensating winding. Fig. 4 shows a section of the field and armature corresponding to the usual direct-current motor, or an alternating-current motor without compensating winding. In the direct-current motor the armature ampere-turns lying under the pole face tend to set up a local field around themselves, producing what is known as cross-induction. This produpes no harmful effect except in crowding the field induction to one edge of the pole, thus shifting the magnetic field slightly and possibly affecting the commutation in a small degree. But if the armature is carrying alternating current this cross flux will generate an electromotive force in the armature winding, and this will be SINGLE-PHASE RAILWA Y MOTOR 95 added to the field self-induction, thus increasing the self-induc- tion or choking action of the machine. As the armature turns on such motors are much greater, in proportion, than the field turns, it is evident that the arnpere-turns under the pole face can exert a relatively great cross-magnetizing effect. This high cross-magnetization generates a high armature self-induction which may be almost as much as the field self-induction. Further, this great cross-induction would tend to shift the magnetic field quite appreciably, thus affecting the commutation to some extent. To overcome this serious objection, the neutralizing winding is added. This is a winding embedded in the pole face and so / I / V / I I I ^' o/o/o/o/o/o/o Fig. 4 arranged that it opposes the armature cross-magnetizing action. The arrangement is shown in Fig. 5. As it opposes and thus neutralizes the cross-induction set up by the armature winding, it eliminates the self-induction due to the cross-magnetization. It also prevents shifting of the magnetic field and thus eliminates its injurious effect on commutation. As the cross-flux is practically cut out the armature winding becomes relatively non-inductive. There is, however, a small self-induction in the armature and neutralizing windings, due to the small flux which can be set up in the space between the two windings, they being on separate cores with an air-gap between. I have stated that the field turns of the alternating-current motor can be only 20% to 25% as "many as in ordinary direct 96 ELECTRICAL ENGINEERING PAPERS current practice. It may be questioned how the field can be magnetized with so few field turns. This has been one of the most difficult problems in the motor. Obviously, one solution would be the use of a very small air-gap, but in railway practice there are objections to making the air-gap unduly small. Furth- ermore, if the armature has large open slots, as shown in Fig. 6, experience shows that a reduction in the clearance between the armature and field iron does not represent a corresponding de- crease in the effective length, of the air-gap, due to the fact that the fringing of the magnetic flux from the tooth tip of the pole face changes as the air-gap is varied. The most effective con- struction yet used consists in making the armature slots of the partially closed type as in the secondary of an induction motor. This is shown in Fig. 7. / o' o/o,/o/o,/^o';o X-:" \ i^i^i \ o \o vo\0\oVo\o\o'o/oyo/ O'OlQi o O/ O/ v^", \ yy Fig. 5 With this construction practically the whole armature surface under the pole becomes effective, and the true length of air-gap is practically the same as the distance from iron to iron. With the increased effective surface, due to this construction, the length of air-gap need npt be unduly decreased, which is of con- siderable importance in railway work. A further assistance in reducing the required field turns is the field construction used in the single-phase motor. The magnetic circuit consists of laminations of high permeability and usually without joints across the magnetic path. The iron is also worked either below the bend in the saturation curve or, at most, only slightly up on the bend, except in the case of very low frequency motors where more field turns are permissible. SINGLE-PHASE RAILWA Y MOTOR 97 Taking the whole magnetic circuit into account, on 25-cycle motors about 80% of the whole field excitation is expended in the air-gap, while in direct-current motors, even with a much larger air-gap, as much as 40% to 50% of the magnetization may be expended in the iron and in the joints. This armature construction with the partly closed slots has been found very effective in large, slow-speed, single-phase motors in which a relatively large number of poles is required. This construction is used on the New Haven 250-h:p., 25-cycle A 227" VU ll.'W yWWIJJ \U\llyJ Fig. 6 motors; also on the 500 h.p., 15-cycle motor on the Pennsylvania locomotive exhibited at Altantic City at the Street Railway convention, last October. Geared motors for interurban service can be constructed with ordinary open slots with bands, and many have been built that way. The semi-closed slot, however, allows more economical field excitation. It may be asked what the objection is to low power-factors on single-phase railway motors, aside from the increased watt- less load on the generating station and transmission circuits. T"! „ .V „^ r^Uiar'+inr, fn f>if> InTO- nnTO-pr-fartor in Riirh motors. 98 ELECTRICAL ENGINEERING PAPERS a very serious one. This lies in the greatly reduced margin for overload torque in case the supply voltage is lowered. In railway work it is generally the requirement of abnormal loads or torques which causes a reduction in the line voltage; that is, the overload pulls down the trolley voltage just when a good voltage condition is most necessary. This is true of direct cur- rent as well as alternating current. In the direct current motor, however, such reduction in voltage simply means reduced speed but in the alternating current motor the effect may be more serious. To illustrate, assume a motor with a power-factor of 90% at full load. The energy component of the input being 90%, Pig. 7 the inductive component is about 44% or, putting it in terms of electromotive force the inductive volts of the motor are 44% of the terminal voltage, Neglecting the resistance of the motor, a supplied electromotive force of 44% of the rated voltage would just drive full-load current through it and develop full- load torque. With full voltage applied the motor could develop from five to six times full-lpad torque. Under abnormal con- ditions a drop of 30% in the line voltage would still give suffi- cient voltage at the motor terminals to develop two and one half to three times full-load torque. Let us next take a motor of 80% power-factor at full load. The inductive voltage would then become 60% of the terminal voltage, and therefore 60% ■S-f +Vlo i-Q+orl -»!+. Tfi I "T.n rti^ f,+ u« ««^i;«j a r-_ii i_- SINGLE-PHASE RAILWAY MOTOR 99 through the motor. This neglects the resistance of the motor, which, if included, means that* slightly more than 60% of the voltage is required. With full voltage applied, this motor would develop about three or four times the rated torque. With 30% drop in the line voltage the motor could develop from one and one half to two times rated torque, which is hardly enough for an emergency condition Taking, next, a motor with 70% power-factor at full load it would require 70% of .the rated voltage to send full-load current through the motor; with 30% drop in line voltage the motor could just develop full-load torque, and even with 15% drop it would develop only about one and one half times torque. As 15% drop is liable to occur on any ordinary system", this latter motor would be a very unsafe one. It is evident from the above that it would be bad practice in railway work to install motors with very low full-load power- factors. In general, the higher the power-factor the more satisfactory will be the service, other things being equal. I have endeavored to explain some of the problems which have been encountered in the design of single-phase commutator railway motors of sizes suitable for all classes of railway service. Here is a type of machine which, has been known for a great many years, but which, until the last few years, has been con- sidered utterly bad. In a comparatively short time it has been changed from what was considered an unworkable machine to a highly satisfactory one and this has been accomplished, not by any radically new discoveries, but by the common-sense application of well known principles to overcome the apparently inherent defects of the type. As an indication that the motor is making progress in the railway field, I will mention that the first commercial single-phase railway motors have not been in use more than four or five years, and yet at the present time there have been sold by the various manufacturers in this country and Europe, a total capacity of approximately 200,000 to 250,000 h.p., a very considerable part of which has been put in operation. Considering that the motor was a newcomer in a well estabhshed field, the above record is astonishing. How- ever, it may be safely predicted that what has been done in the last five years will hardly make a showing compared with what will be done during the next five years, for the real field for such motors, namely, heavy railway work, has hardly been touched. 100 ELECTRICAL ENGINEERING PAPERS COMPARISON OF SERIES AND REPULSION TYPE A. C. COMMUTATOR MOTORS FOREWORD — This is part of a discussion by the author, of papers by Dr. Steinmetz and Mr. Slichter before the American Institute of Electrical Engineers, January 2d, 1904. The major part of the author's discussion covered the comparison between the aeries and repulsion type motors, in which he showed that the repulsion type was simply a series type motor with a transformer added. The several references to Mr, Slichter's paper given in the discussion have but little bearing on the technical matter contained but could not be eliminated without considerable remodeling of the paper. — (Ed.) Discussion of Steinmetz and Slighter Papers IN the paper presented before the American Institute of Elec- trical Engineers, in September, 1902, the speaker called atten- tion to the fact that there were but two types of single-phase al- ternating-current motors having suita,ble characteristics for rail- way service; viz., that called the "Series Type," and the "Repul- sion Type." Both of these motors have armatures like direct- current motors with commutators and brush holders. Attention was called to the fact that both motors have suitable characteris- tics for railway service, as both automatically give variable-speed characteristics with changes in load. That paper primarily de- scribed a single-phase railway system, and the motor formed but an element in the general system. It was a very general opinion at that time that the success of the commutator type of motor for large sizes was doubtful, and the sparking feature was considered a fundamental source of trouble. It was generally conceded that if a motor with series characteristics could be made to operate suc- cessfully, it would be a great step in advance in the railway field. Since that time single-phase railway systems have been more ftilly developed. Practically, no departures from the general system then indicated have been furnished, and the types of motors developed have been along the lines of the two motors indicated in that paper. Up to the present time the only suitable motors suggested for this work have been of the commutator type, and have been those having series characteristics. The speaker has suggested that all these motors can be considered broadly under the one class of series motors, as they all have the series characteristics SERIES AND REPULSION MOTORS. 101 of the direct-current series motor. The speaker further sug- gested that they be sub-divided into the "Straight-Series" type and the "Transformer-Series" type. The transformer-series could also be arranged in two classes; viz., one in which the armatiu-e or field is supplied by an external transformer, and one in which the transformer is placed in the motor itself; this latter is the reptilsion type of motor. Figs. 1, 2, and 3 illustrate the three classes. Fig. 1 being the straight-series, Fig. 2 the transformer-series and Fig. 3 the repul- sion motor. Fig. 2 would be considered as a true series motor, although the armature and field are not directly in series, yet most of the characteristics described as belonging in the repulsion { TRANSFORMER f^'V FIG. 1. FIG. 2. FIG. 3. motor apply directly to the transformer motor shown in the figure. Comparing the relations of these motors, viz., the straight-series and the repulsion motor, we will first take up the straight-series. In this motor, if properly designed, two pressures can be con- sidered; vi^., that across the field circuit, and that across the armature circuit. The armature pressure can be made practi- cally non-inductive so that the input of the armature will repre- sent practically true energy. The pressure across the field is practically at right angles to the armature pressure, and repre- sents very closely the wattless component supplied by the motor. The resultant of these two pressures will then be the line pressure. The power-factor of the motor when running is represented prac- 102 ELECTRICAL ENGINEERING PAPERS tically by the pressure across the armature winding, increased slightly by the losses in the field-core and winding. Therefore, for high power-factors it is important that the pressure across the armature circuit be made as high as possible, relatively to the applied pressure, and that across the field as low as possible. There are three ways in which to increase the pressure across the armature; viz., by increase in speed, by increase in the number of wires in series on the armature, and by increase in, flux through the armature. By increase in speed and increase of the wires in series, the armature pressure will be increased without affecting the field pressure, and therefore the ratio of the armature pressure to the line pressure is increased. Increasing the flux in the armature also increases the flux in the magnetizing-coil in the field, and the pressures of both are increased. Therefore this increase does not improve the power-factor of the machine. Instead of increasing the armature pressure, the pressure across the field winding may be decreased; this can be done in two ways; viz., by reducing the turns in the field coil, or by reducing flux through the coil. Reducing the flux through the field re- duces the flux in the armature winding also, and therefore repre- sents no gain; reduction in field-turns, therefore is the feasible means of reducing the field pressure. Reduction in field-turns can be accomplished. in two ways; viz., by decreasing the effec- tive length of air-gap in the motor, and by increasing the cross- section of gap. By making the gap very small the pressure across the field could be made very small compared with the line pressure, and extremely high power-factors could be ob- tained, whether the motor is of the straight-series or the repul- sion type. Also by increasing the section of the air-gap the turns of the field can be decreased with a given total fiux through the coil, and the power-factor can thus be very considerably increased. The first method, viz., decrease in gap, is limited by practical conditions which have 'been determined from long ex- perience with direct-current work. • It should be borne in mind when published descriptions of such motors are given, that the results, as regards power-factor, generally depend upon data which is not given in the description; such as the magnetic dimensions of the armature and field, the length of gap, etc. Therefore, a machine may be described as showing an extremely high power-factor, which may in practice not be a commercial machine, from the standpoint of American railway experience. SERIES AND REPULSION MOTORS 103 Increasing the section of air-gap without decreasing the length of gap also improves the power-factor, but makes a larger and heavier machine, as a rule. Both these modifications reduce the ampere-turns in the field. The direction of the improvement in the armature was shown to be in increased armature ampere-turns with a giyen speed. It therefore follows that almost any result desired can be ob- tained as regards power-factor by increasing the armature am- pere-turns and decreasing the field, or exciting ampere-turns. Reference will be made to this point -in considering the repulsion motor. It should be noted that in all these motors there should be but little saturation in the magnetic circuit and but few ampere- turns expended in saturation of the iron under normal conditions. This consequent low saturation in such motors leads to certain characteristics in the torque curves which have been cited this evening as an indication of superiority of alternating-current motors over direct-current motors; namely, a torque increasing approximately as the square of the current. In fact, this superi- ority of torque should be charged to the low flux-density of the motor rather than to the alternating current. If direct-current motors were worked normally at as low density as the alternating- current motor, then the direct-current motor would show better torque characteristics, and would be comparable with the alter- nating-current motor. This claim for a better torque in the alternating-current motor compared with the direct-current motor seems to be making a virtue of a necessity. It is evident from what has been said that the power-factor of the straight-series motor can be made anything desired, it being a question of proportion between armature and field, length of air-gap, amount of material used, etc. In practice a compromise would naturally be made among the various characteristics, and a slight reduction in power-factor is probably of less importance than a corresponding reduction in size and weight. Also large clearance is probably of more importance than an extremely high power-factor at normal load. In practice it will be found that the armatures of such motors have a large number of ampere- turns compared with the fields, in order to obtain comparatively high power-factors with large air-gaps. The number of poles need not be made such that the product of the poles by the normal speed represents the frequency of the supply circuit; good series 104 ELECTRICAL ENGINEERING PAPERS motors can be made, and have been made, in which the number of poles were very much larger or much smaller than represented by this relation. Taking up next the transformer type of motors — Fig. 2 ; the field is in series with the primary of the transformer, the second- ary of which is connected to the terminals of the motor. I would call this a true series motor, although it is not a straight-series motor. In this motor the pressure across the armature can be made practically non-inductive and the pressure across the primary of the transformer will be practically non-inductive. The voltage across the field winding will have practically 90° phase relation to that across the primary of the transformer, and the magnetic field, set up by the field winding, will have a 90° relation in time to the magnetic field in the transformer, as in the repulsion motor. In this motor the voltage across the trans- former ^^ill be highest at light loads and will decrease with load until zero speed is reached. At start there is lowest flux in the transformer and highest flux in the field winding. Such a motor will have speed-torque characteristics very similar to those of a straight-series motor, except as affected by the actions taking place in the transformer itself. If the- transformer possesses no reactance, then at start the current in the armature should be the same as if connected as straight-series motor, and the condi- tions of torque at start should be the same. If the transformer has reactance, then at start the current in the armature will not be quite equal to the current which the armature will receive if coupled as a straight-series motor, assuming the transformer to have a 1 to 1 ratio. Neithet will the armature current be ex- actly in phase with the field current; therefore the starting torque of a motor connected in this way will be slightly less than the torque of the same motor if connected in straight-series. This is on the assumption that the transformer is one propor- tioned for small reactance; but if the primary and secondary windings of the transformer should be on separate cores with air-gap between, then the reactances of the windings are con- siderably greater than in the above case. Therefore, we should expect a motor with such a transformer to give still lower torque than the straight-series with the same current supplied from the line. In a repulsion motor the transformer is combined with the motor itself and the primary and secondary windings are upon SERIES AND REPULSION MOTORS 105 different cores with an air-gap between. The starting condi- tions of such a motor as indicated above should be poorer than the straight-series motor, or for the same starting torque some- what greater apparent energy should be required. It stands to reason that applying the current directly to the armature wind- ing should give greater ampere-turns and better phase relations than generating this current in a secondary circuit, and hot under ideal transfojrmer conditions. The tests which have been made, as well as the restdts shown in the curves of the papers given tonight indicate this. It is to be noted that the torque curve is not the same shape near the zero speed point as the torque curve of the series motor. ^ FIG. 4. FIG. 5. Series motors and repulsion motors may be indicated in the simple form shown in Figs. 4 and 5. In the diagrams of the repvdsion motor (Fig. 5) , two field-poles FF, are shown, and two transformer-poles, TT. To obtain high power-factors on such a motor the ampere-tums in T must be very much greater than in F, which means that the ampere-turns in the secondary or arma- tiu-e are much greater than in the exciting field, as in the series motor. The high power-factor obtained with these motors is therefore due principally to the small ampere-turns in the field and the Small pressure across the field. For instance, with brushes set at an angle of 16°, from the primary or resultant field, the ratio of armature to exciting field- turns would be almost 5 to 1, a ration which will also permit of extremely high power-factors in well-designed straight-series 106 ELECTRICAL ENGINEERING PAPERS motors over wide ranges of speed. To this feature should be credited the good power-factors claimed for the repulsion motor. In either the series of repulsion type of motors, high power- factors, especially at low speeds, are directly dependent upon this fact of high ratio of armature to field, and with a high ratio, high power-factors should be obtained without crediting the result to leading currents in the armature. In the diagram of the repul- sion motor, the- line current indicated flows through both the field winding and the transformer winding. The primary cur- rent sets up a magnetic field in the exciting windings in phase ■ with the line current. If it also set up a field in the transformer in phase -with the line current, then the electromotive force gen- erated in the armature winding due to rotation would have a 90° relation to the electromotive force set up lay the transformer, and a correcting or magnetizing, current would flow. This flow is in such direction that it corrects the relation between the two pressures in the armature by shifting the transformer magnetism one-quarter phase later than the exciting field magnetism. This armature corrective current may thus be considered as mag- netizing the transformer, making the primary input to the trans- former practically non-inductive; but this magnetizing or cor- recting current may be considered as flowing in a circuit at right angles to the field magnetic circuit, and having practically no effect on the field circuit. Therefore as a rough approximation, the exciting field may be considered to represent the wattless component of the input, and the transformer field the energy component, as in the series motor. As to the statement that the magnetizing current in the armature reduces the wattless com- ponent of the exciting field, the speaker does not accept it broadly. If this component is reduced, then another component of practi- cally equal value is introduced somewhere else, for the power- factors obtained with such motors can be accounted for by the high ampere-turns in the armature winding, compared with the field or exciting ampere-turns. If the armature current improves the power-factor by diminishing the magnetizing or exciting field, then the curves in Pigs. 1 and 4 of Mr. Slichter's paper should show it. The speaker has gone over both sets of curves calculat- ing the wattless components from the power-factors. From this and other data in these curves, he finds that beginning near synchronous speed the wattless component in the motor goes up slightly faster than would be represented by the field excitation. SERIES AND REPULSION MOTORS 107 assuming it to be entirely wattless. Therefore, according to these curves, the power-factors at lower speeds are not quite as good as would be obtained by a field entirely inductive and the armature entirely non-incjuctive, in a straight series motor. These calculations are rather approximate, as the curves do not check at all well with each other. For instance, the output of the motor as represented by the input multiplied by the power- factor and by the efficiency, does not check with the output as represented by the product of speed by torque, in either set of curves, the discrepancies beinjg as high as 10 percent. In Fig. 4, for instance, either the torque or the speed is too high for the lower speeds. Checking back on this curve, using either the speed and torque or the power-factor and efficiency for deter- mining the output, the speaker finds that the wattless component in the motor at 190 revolutions is approximately 20 percent higher than it would be if the field excitation alone were wattless,, assuming at 440 revolutions the wattless component is represented purely by field excitation; that is, from 440 to 190 revolutions the wattless component is increased 20 percent over that which would be represented by field excitation alone. This indicates that not only should the field excitation be considered as practi- cally wattless, but that in addition there is a wattless component due to reactances in the armature windings. The armature current can be split into two components, one of which is partly magnetizing and represents no torque. The other component is in phase with the field magnetism and there- fore represents torque. The magnetizing or wattless element may be comparatively small, as the number of turns in the arma- ture is relatively large, but the armature thus carries at times a slightly larger current than the straight-series motor. A further inspection of the diagram (Fig. 5) indicates how the power-factor of the motor can be made very high at synchronous speed. At all speeds the pressure generated in the armature due to rotation in the field of F. is practically equal to the pressure generated by the transformer T, thus making zero presstire across the terminals. But also at synchronous speed the pressure generated by the exciting field acting as a transformer, between the points a b, will be practically equal to the pressure generated in the winding by rotation of the winding in the transformer field. Therefore across a b the pressure is practically zero with these conditions, but the frequency remains the same as that'in 108 ELECTRICAL ENGINEERING PAPERS the field. If now the magnetizing current be supplied across the points a b, then the required ampere-turns for magnetizing the motor can be supplied at practically zero pressure, and the turns of the external magnetizing field can be omitted. Therefore, under this condition the wattless component is practically zero and the power-factor becomes practically 100 per ceiit. This is the method of excitation used on certain European single-phase motors in which high power-factors are claimed for full-load running. But this method of excitation does not improve con- ditions at start, as the same excitation will be required at stand- still, whether the excitation be supplied to the armature or to the field. Therefore this method of excitation does not help the motor at that condition of load which is the severest on the generating and transmission systein. It has the advantage of omitting the field exciting winding, but has the great disadvan- tage of requiring a double set of brushes on the commutator, with but half the distance between the brushes found in the straight-series or the ordinary repulsion motor. I do not believe that such raethods of compensation are of sufficient advantage to . overcome the complications attendant upon them. At zero speed, both the straight-series and the repulsion motors have low power-factors and with equal losses in the motors, the repulsion shotild have slightly lower power-factor than the series. This question of power-factor at start is largely a question of internal losses in the motor at rest, and the repul- sion motor in individual cases may show higher than the series motor, because it may be designed with higher internal losses. The real meastire of effectiveness is not the power-factor at start, but the apparent input or kilovolt-amperes at start required for a given starting torque. With equally good designs of motors, the speaker's experience is that the kilovolt-amperes will be found to be considerably less with the straight-series than with the repul- sion motor, due to the fact that the current is fed directly into the armature and not by transformer action, and therefore the conditions of phase-relation- and amount of current in the arma- ture windings are more favorable. Therefore it follows that in order to have the same kilovolt-ampere input for the same' start- ing torque, the repulsion motor should have a smaller length of air-gap than the corresponding straight-series motor, or should have a greater section of air-gap, which means greater weight of motor. This is one of the conditions which'has led the speaker SERIES AND REPULSION MOTORS 109 to the advocacy of the series motor rather than the repulsion motor, as he has considered this condition of starting of more importance than running; although he is satisfied that many of the running conditions of a well-designed series motor will be found. in practice to be superior to those of an equally well-de- signed repulsion motor. Referring again to Fig. 5, it will be noted that two fields are set up in such a motor, and that at synchronous speed these two fields are equal. In the straight-series motor there is but one field set up,' the other being omitted. It is evident that the straight-series motor with the current supplied directly to the brushes can have a smaller section in certain parts of the mag- netic circuit than is required for the repulsion motor, and that therefore the weight of material would be less, and the external FIG. 6. PIG. 7. dimensions can be less. In Fig. 7 the heavy fine represents outlines of series and the dotted line those of repulsion motor; therefore, it follows that for equally good designs and same fre- quency, the straight-series motor should be more compact, and should weigh less than the repulsion motor. It is reasonable to expect this, as the repulsion motor contains a transformer in addition to the other parts found in the straight-series motor. Futhermore, the transformer found in such a motor is one with an air-gap, and with the windings on two separate elements, and therefore cannot be so well proportioned as a separate trans- former cotild be. Also, there is a transformer for each motor, and in a 4-motor railway equipment, for instance, there wotild be four transformers of smaller size against one larger transformer used with the series motor, this larger transformer having a no ELECTRICAL ENGINEERING PAPERS closed magnetic circuit, and of a highly efficient design com- pared with the transformers in the motors themselves. A further point should be taken up in the comparison of these motors; viz., the current in the coil short-circuited by the brushes. This coil is a secondary to the field and the current in it is neces- sarily greatest at the period of strongest field. Therefore, this current will be greatest at the time of starting. If the repulsion motor and the straight series motor have the same field strength at start, then the short-circuited current should be the same in each. ' But as the current is fed into the armature in the repulsion motor through transformer action, it will as a rule' be found that STRAIGHT SERIES MOTOR REPULSION MOTOR the starting field strength of such a motor is slightly greater and the starting armature strength slightly less for a given torque than is found in the straight-series motor having same ratio of armatiure to field windings. Therefore the short-circuit current at start will be somewhat larger for the repulsion motor than for the corresponding straight-series motor. This short-circuit cur- rent may be somewhat less near full speed than in the straight- series motor, but it is not the full-speed condition which is the serious one. The short-circuit current at start is one of the most serious conditions which confronts us in alternating-cur- rent motors, and is also of great importance where there is any considerable operation on low speeds. The speaker advocates SERIES AND REPULSION MOTORS 111 a type which he considers gives the easiest condition in this regard. This short-circuiting cannot be entirely avoided in any of the motors brought out without adopting abnormal and question- able constructions, although devices like narrow brushes, sand- wich windings, etc., have been proposed. In certain foreign motors the brushes used are so narrow that they cover practically the width of one commutator-bar. As such motors are gener- ally built with a very large number of bars, the brushes used are extremely narrow, being approximately 0.2 inch thick at the tip. This will undoubtedly lessen the short-circuiting, but simply transfers trouble to another point; a brush 0.2 inch thick is not practicable for commercial railway service; at high speeds, with only a moderately rough commutator, such brushes will be liable to chip and break; further, the brush on a street-car motor should bridge at least two bars to give good, smooth, brush operation; in practice, a 0.5 inch brush on motors of 100 h.p. should be used. The sandwich winding, which consists of two or more wind- ings side by side, will prevent short-circuiting at the brushes, but is only another way of transferring trouble to another point; it has been found in practice that it is difficult to run a sandwich winding without trouble at the commutator with direct current, without a tendency to blackening and pitting the commutator, and with alternating current this tendency to pitting and burn- ing of the bars would be equally great. As a rule, there is little difference between the operation of repulsion and straight-series motors as regards sparking, except that the repulsion motors generally have greater current in the short-circuited coil near zero speed, and therefore show greater tendency to heat and spark. At or near synchronous speed, there appears to be very little difference in the commutation, although the speaker has never given the repulsion motor the same test of long-continued service as he has in the case of the series motor. These series motors have never shown any tend- ency to give trouble on the commutator, and on an exhibition car equipped with four 100-h.p. motors, the commutators have never been sandpapered since the equipment has been put into service. This exhibition car is used principally for showing the accelerating properties of the motors; therefore, the speaker does not hesitate to say that the commutation of the straight- series motor will prove to be equal to that of the direct-current 112 ELECTRICAL ENGINEERING PAPERS motor. Wide brushes are used with it, such as have been used in street-railway motors. It is weE known that with large direct-current motors, espe- cially when operated at very high speeds, there is a tendency to flash across the commutator, or to the frame of the motor, if the field circmt be opened for a period long enough to allow magnet- ism to drop to zero, and then the field be closed again. In this case there is a rush of ctirrent before the field has had time to build up, and this rush of current, together with field distortion, may cause serious flashing. In the alternating-current motor, whether of the straight-series or the repulsion type, this ten- dency should be entirely absent. In the straight-series motor the magnetism falls to zero once in each alternation, and therefore if^ this tendency existed, flashing woiild occur continuously. Furthermore, a properly designed straight-series motor can be short-circuited across the brushes without injury to the motor, and can continue to operate in this way; therefore, if the ma- chine can be short-circuited in this way, there is evidently no tendency to maintain an arc. Returning to the subject of power-factors it should be noted that high-power-factors are very frequently found in motors of low or only moderately good efficiency. This low efficiency to a slight extent explains the high power-factor in some motors, both polyphase and single-phase. Low efficiency means higher true energy expended, and with a given wattless component it means higher power-factor. It is the old problem of increasing the power-factor by wasting energy in a circuit instead of reducing the wattless component. The power-factor of any altemating- ctirrent motor can be very considerably increased by putting resistance in series with it. Instead of this resistance the internal losses of the motor may be made higher, which will accomplish the same results. The motor will therefore appear to have a higher power-factor than it really deserves, if efficiency of the motor is taken into accoimt. If, for instance, the efficiency' at 300 rev. shown in Mr. SHchter's Fig. 4 would be made as high as on direct-current motors, then the power-factors with the same magnetizing and other conditions, would have been approxi- mately four percent lower. This lower power-factor woiild not have made any harder condition on the supply circuit, but actually would have made a somewhat easier condition, as the supply sys- tem would have furnished about eight percent less Idlovolt- SERIES AND REPULSION MOTORS 113 amperes. For lower speeds this difference in power-factor will be greater, and less for higher speeds. A high power-factor at start, obtained by the use of resistance in series with the motor by high internal losses which do not represent torque, is there- fore a detrimental condition rather than a good one, as it means increased kilovolt-ampere expenditure for a given torque. This is merely given as an illustration showing that power-factor in itself is not a true indication of conditions, but must be accom- panied by other data; this is not a criticism of these motors, but is a general condition, found to a greater or less extent in all alternating-current motors. 114 ELECTRICAL ENGINEERING PAPERS COMPARATIVE CAPACITIES OF ALTERNATORS FOR SINGLE AND POLYPHASE CURRENTS FOREWORD — This article was prepared many years ago for the infonnation of the younger technical men of the Westinghouse Company. It was considered of sufficient value by the Company to publish in pamphlet form, of which there were several editions from time to time. It should be considered as purely educational. Only the types of windings which were in use up to the time the paper was prepared, are included. — (Ed.) THERE are a number of popular misconceptions regarding the relative polyphase and single-phase capacities which can be obtained from a given winding. For instance, there appears to be a half-formed opinion that a given winding connected for two- phase will give a slightly less output than when connected for three- phase; but, on the other hand, it seems to be generally assumed that the various three-phase windings all give the same rating. Also, it is a widespread idea that when any polyphase machine carries a single-phase load the permissible rating, with the same temperature, is approximately 71 percent of the polyphase rating. While there are a few cases where this may be true, yet, in general, it is far from being the fact, as will be explained below. This fallacy regarding single-phase ratings arose partly from early practice with polyphase machines, which were ofttimes designed with a view to carrying single-phase load almost exclu- sively, the polyphase load being a possible future development. In consequence, the type of armature winding chosen was, in many cases, that which gave a high output on the siagle-phase, with some sacrifice in the polyphase rating, and the single-phase rating in many cases was a relatively large percent of the polyphase rating simply because the polyphase rating was less than could have been ob- tained with a different type of winding. This point will be illus- trated later. The 71 percent (or 70.71 percent) ratio of single-phase to poly- phase ratings in a given aramature arose partly from the fact that at these relative loads the total armature losseis were practically equal. On old designs of machines, in many cases it could be as- sumed safely that with equal armature losses the temperature of the armature parts would be practically equal. This assumption CAPACITIES OF ALTERNATORS 115 does not hold, in general, on modern design's of machines in which each individual part is proportioned for a specified result. The distribution of the armature losses is just as important as the total losses. If the temperature drop between the inside of the arma- ture coU and the armature core is small compared with the tem- perature drop from the core to the air, then the temperature of the armature, or its rating, will depend largely upon its total losses, equal ventilation being assximed in all such comparisons. If, how- ever.the temperature drop from the coil to the iron, or from the in- side of the coU to the outside, is relatively high, then the temper- ature limit may be fixed by the loss in an individual coil rather than by the total loss. This is particularly the case in high voltage machines where there is a considerable amount of insulation over the individual armature coUs. Also, in many of the later designs of machines (especially turbo-generators) each armature coil is practically separated from all other coils, so that one coil can have but little direct influence on the temperature of its neighboring coils. In such an armatiire it is possible to completely roast out an indi- vidual coU or group of coils without, seriously heating any other coils or groups of coils. It is obvious that in such a machine the loss in the individual coils is what fixes the rating of the machine, and not the armature loss as a whole. It is evident, therefore, that when a polyphase machine, with such a winding, is loaded single- phase, the maximum current which can be carried in any single coil must be the same for either polyphase or single-phase rating. As this type of winding is used in the majority of large capacity machines of the present day, the following comparison will show 116 ELECTRICAL ENGINEERING PAPERS the relative rating of- such machines on polyphase and single- phase loading. Three-phase ratings will be considered first, because the great majority of modem machines are wovrnd for three-phase. Three-Phase Windings All the various types of commercial three-phase windings with their current and voltage relations can be derived in a very simple manner from the consideration of a ring armature with its windings arranged in six symmetrical groups, each covering 60 degrees of the ring, which may all be closed together to form the ordinary closed winding, or which may be separated into either three or six groups and connected to form various delta and star types of windings. F/g.4 Let Fig. 1 represent such a ring armature closed on itself and with six taps brought out, these being designated as A, a, B, b, C, c. By connecting together the points Aa, aB, Bb, etc., as shown in Fig. 2, a six-sided figure is obtained, which represents the various voltage and phase relations which can be obtained with all commercial three-phase (and six-phase) windings. It wUl be noted that Aa and bC are of equal length and are parallel in di- rection. . The length represents e. m. f. and the direction re- presents phase relation. Therefore, these two groups or legs are of equal e. m. f.'s and of the same phase. The same holds true of aB and Cc, and of Bb and cA. Beginning at Aa, these groups have also been numbered consecutively from 1 to 6, so that in the following diagrams a given leg or group can be identified by number. CAPACITIES OF ALTERNATORS 117 Fig. 3 is the same as Fig. 2 with three leads carried out from A, B and C to form the three terminals of a three-phase winding. The dotted lines from A to B, B to C, and C to A represent the voltage and phase relations obtained from this combination. This is known as a closed coil- type of winding and is the standard arrangement of winding on a three-phase rotary converter. The comparative e- m. f . values for this and other combinations will be given later. By opening the closed arrangement of Fig. 2 at the points A, B and C, as shown in Fig. 4, then an open coil arrangement is obtained and the threfe parts resulting can be recombined in several ways, keeping the same voltage and phase relations' of the individual parts. F/'^.S However, only one of these combinations, — that shown in Fig. 5 — has been used to any extent. This is one form of star winding which is sometimes used to give certain voltage combinations, as will be explained later. By splitting Fig. 2 at six points instead of three, as shown in Fig. 6, various other open coil combinations of windings can be obtained while keeping each group or leg in its proper phase and voltage relations. fc,: One of these combinations is shown in Fig. 7, in which the groups which are similar in e. m. f. and phase are connected in parallel and the three resulting combinations are connected to form a delta winding. In Fig. 8 the two groups of similar phase are shown in series instead of parallel and connected to form a delta. Obviously 118 ELECTRICAL ENGINEERING PAPERS Fig. 7 and 8 are equivalent, except that the terminal e. m. f . of one is double that of the other. By reconnecting the three components of Fig. 7 in the manner Fi^.8 shown in Fig. 9, a parallel star winding is obtained. Two arrang- ments are shown, one with all the legs connected together at the middle point, and the other with the two stars not connected at the middle. Fig. 10 is equivalent to Fig. 9 except that the two e. m. f.'s of . XX F/0. 9 equal phase are in series instead of in parallel, thus giving just twice the voltage of Pig. 9. Six-Phase Windings The foregoing covers all of the usual combinations for three- phase windings, open and closed coil types. The same general scheme may be used to illustrate the usual six-phase combinations of windings which are frequently used in connection with rotary converters. CAPACITIES OF ALTERNATORS 119 In Fig. 3, a three-phase winding is shown with terminals at A, B and C. If three other terminals be formed by a, b and c, then a second three-phase winding is obtained. The dotted lines in Fig. 11 illustrate the voltage and phase relation of these two windings. This is the so-called double delta arrangement some- times used with six-phase rotaries, the dotted lines representing the voltage and phase relations of the transformers which supply the rotary converters. 'yXiJiixxt."..txixsJiJ> a « a.t>.«.9 a a » a /' pmynnrrinrryTra »'rinrd"inr\ F/^. II It is evident that the voltage represented by ac is equal in value and phase to that represented by BC. Therefore, one transformer with two secondaries of equal value could be tapped across these two circuits. Similar arrangements can be applied to the other two-phase relations injthis diagram. In Fig. 2 it is evident that, if six terminals are used, a voltage can be obtained across f^ 6. |^Similar voltages can be obtained across aC and Be. These threejvoltages are equal in value but have the 60-degree relation to each other. It is evident therefore that three transformers connected to a three-phase circuit can have their secondaries connected tojthisjwinding across the indicated 120 ELECTRICAL ENGINEERING PAPERS points. This arrangement is indicated in Fig. 12, and is the so-called diametral connection of six-phase rotaries. The middle points of the transformer winding from these three circuits can be connected together, if desired. Relative E. M. F. Obtained from the Foregoing Combinations From an inspection of the above diagrams, and the application of but very little mathematics, all the e. m. f. relations of these various combinations can be readily obtained. In the following comparisons the magnetic field is asstmied to be of such distribu- tion that the e. m. f. waves will be of sine shape, as this greatly simplifies the various relations. Let E represent the effective e. m. f . of any one of the six legs or groups in Fig. 2. Then, combining the various groups geo- metrically, taking into account the angular relation between the legs in the diagram, the various e. m. f.'s can be readily derived. The results are as follows: In Pig. 3, the e. m. f. across AB, BC, etc., = V~3~ x £ = 1.732 E. In Fig. 5, the e. m.f. Ad is the same as AB in Fig. 3 and is therefore equal to V's" x E, but the e. m. f . AB in Fig. 5 is equal to -s/~F X Ad. Therefore, the e. m. f . of AB = V~a» x VT' x E = 3£. In Fig. 7, AB is evidently equal to one group or side of Fig. 2 and therefore the e. m. f. of AB = E. In the same way the e. m. f. of Fig. 8 = 2E. In Fig. 9 the e. m. f . Ad is evidently equal to E and the e. m. f . AB = VT X e. m. f . of Ad. Therefore the e. m. f . of AB = VT x E = 1.732 E, or same as Fig. 3. In the same way the e. m. f. of AB in Fig. 10 = 2 times V~S~ xE = 3.464 E. For the six-phase combinations the following e. m. f.'s are obtained : In Fig. 11 each of the deltas is the same as in Fig. 3 and therefore the e. m. f.'s are the same and are equal to 1.732 E. In Fig. 12, Ab is geometrically equal to twice aB and the e. m. f. Ab is therefore equal to 2E. Three-Phase Capacities It might be assumed from casual inspection that all of the different - combinations of three-phase and six-phase would give CAPACITIES OF ALTERNATORS 121 the same capacities when carrying the same limiting current per armature coil, or per leg. This however, is not correct, as will be shown by the following: Let A equal the limiting current which can be carried by one coil or by one group of windings. This is not necessarily the ctirrent per terminal, but it is the current permissible in an indi- vidual coil without exceeding a certain prescribed temperature. Then the following ratings are obtainable with the above combina- tions of windings. In Fig. 3 the rating = 3A x VT x £ = 5.196 AE. In Fig. 5 the current per coil and per terminal = A. The e. m. f. becomes A x VlT x 3£ = 5.196 AE. Therefore the three- phase ratings of the windings in Figs. 3 and 5 are equal. In Fig. 7 the current in each leg of the delta is 2A, as there are two groups in parallel, each carrying current A . As the e. m. f . across terminals is E, the rating becomes 3 x2AxE = 6 AE. In Fig. 8 the current per side or leg of the delta is equal to A and the e. m. f. is 2E. The capacity therefore becomes the same as for Fig. 7 or = 6 AE. In Fig. 9 the current per terminal is 2A as there are two groups in parallel for each terminal. The e. m. f . across the term- inals is \/~3' X E. The capacity is therefore 2A VlT x -s/~3~ -E = 6AE. The rating of Fig. 10 is also 6 AE, the same as Fig. 9. In 'Figs. 11 and 12 the ratings can be determined by direct inspection from the following method of considering the problem : In a closed coil, polyphase machine, for example, such as shown in Fig. 2, one circuit can be taken off 'from A and a, a second circuit from a and B, etc., and the total number of circuits which can be taken off corresponds to the number of armature taps. Each circuit can be considered as having its own rating. There- fore, the effective voltage of each of such circuits times the current per circuit, times the number of circuits, equals the rating. In Figs. 11 and 12 six circuits can be taken off, each with voltage E and carrying current A. The rating therefore becomes 6 AE. The same method could be applied to any other number of phases from closed coil windings. It is evident from the foregoing that the same rating can not be obtained from the armature winding with all methods of con- nection. In those three-phase arrangements in which two groups of similar phase relations are thrown in series or parallel, the high- 122 ELECTRICAL ENGINEERING PAPERS est output is obtained. In those cases where two e. m. f.'s out of phase with each other are combined to form one leg of the three- phase circuit, it is evident that the restiltant e. m. f. is at once reduced by such combinations and that the capacity of the ma- chine is therefore reduced, simply because the most effective use of the windings is not obtained'. The three-phase closed coil winding is therefore not as effective as the true delta or star type of winding. For this reason the closed coil winding is used in only those cases where some condition other than the current capacity itself is of greater importance. Otherwise, delta and star windings are always used, the star being preferred as it gives a higher voltage with a given number of conductors, or a smaller number of conduc- tors for a given voltage, and is therefore somewhat more effective in the amount of copper which can be gotten into a given space. Single-Phase Rating — Any three-phase machine with one of the above windings can be used to carry single-phase load by using two of the three terminals. The single-phase e. m. f.'s obtained will therefore be the same, in each case, as the three-phase. The current capacity per coil, or group, on single-phase can be no greater than on three-phase. On this basis, therefore, the following single-phase ratings are obtained with the above combinations: Fig. 3, calling A and B the single-phase terminals, then with the limiting current A per coil, the windings 1 and 2 in the diagram will carry current A, and 3, 4, 5 and 6 will carry 3^ A. The total current at the terminals will therefore be \}/2A and the e. m. f. per terminal will be ^~z' E. The single-phase rating then becomes 1.5 A X VlT E = 2.598 AE. The corresponding three-phase rating is ?)A X \/~z'E = 5. 196 AE. The single-phase rating is therefore just 50 percent of the three-phase for this combination. In Fig. 5, the current per leg is A, while the e. m. f. is 3£. The single-phase rating therefore becomes 3 AE. The correspond- ing polyphase rating is A x VX x 3E = 5.196 AE. The single- phase rating is therefore 57.7 percent of the polyphase rating. In Fig. 7 the total current in two legs is 2 A, while in the other four legs of the delta the total current is A. The total current at the terminals therefore becomes 2>A. The e. m. f. is E and therefore the single-phase rating becomes 3 AE. The corres- ponding three-phase rating is 6 AE. The single-phase rating is therefore 50 percent of the polyphase for a true delta winding. The same holds true for Fig. 8. CAPACITIES OF ALTERNATORS 123 In Fig. 9 the ctirrent per group is A and with two groups in parallel the current per terminal is 2A. The e. m. f. across the terminals is V^ -E. The single-phase rating therefore becomes 2A X V"3~ -E or 3.464 AE. The three-phase rating for the same ■combination is 6 AE. The single-phase rating therefore becomes 57.7 percent of the three-phase when a true star winding is used. Fig. 10 gives the same results as Fig. 9. It may be noted that in the three-phase star arrangement two legs are carrying all of the current, while the third leg is idle and could be omitted. This means that the active winding covers two-thirds of the armature surface, while an idle space of one-third the surface lies in the middle of the winding. In the delta winding it may be noted that one leg, covering -one-third the surface, is directly in phase with the single-phase, e. m. f. and is therefore in its most effective position. The other two legs carry current also, but are relatively ineffective as the •e. m. f.'s generated in these two legs are displaced 60 degrees in phase from the single-phase e. m. f . delivered. The delta arrange- ment therefore has two-thirds of its winding acting in a very ineffective manner. One-third of the winding is very effective. In the star arrangement, two-thirds of the winding is almost in phase with the terminal e. m. f. (being 86.6 percent effective), while one leg is entirely idle. The star arrangement is about 15 percent more effective than the delta arrangement. The single-phase rating which can be obtained from the two six-phase combinations shown in Figs. 1 1 and 12 should also be con- sidered. In either of these diagrams, if two opposite terminals, such as AB, be taken as the single-phase terminals, then the e. m. f . will be 2E. As each half of the winding can carry the current A, the total which can be handled is 2A. The single-phase rating therefore becomes AAE. The corresponding polyphase rating is 6AE. The single-phase rating is therefore 66.7 percent of the polyphase, or is higher- than in any of the other three-phase com- "binations shown. It should be noted, however, that in order to •obtain three-phase from this combination, ■ transformers are neces- sary in order to transform from six-phase at the winding to three- phase on the line. Therefore, while this combination gives the highest single-phase and polj^hase ratings, yet if three-phase is used on the transmission circuit, transformers must be interposed. Therefore, the highest obtainable rating of single-phase and three- phase from the same winding impHes the use of transformers. 124 ELECTRICAL ENGINEERING PAPERS The high single-phase rating obtained in this case is due to the fact that the arrangement is equivalent to the star arrangement with the idle leg added, as illustrated in Fig. 13. The addition of this extra leg increases the terminal e. m. f . in the ratio of 100 : 86.6, while the current per terminal remains the same. This arrange- ment, when used for both single-phase and three-phase, implies the use of a closed coil type of winding which, as shown before, cannot give the maximum three-phase rating unless six terminals are used. It should be noted that the three legs shown in Fig. 13 have the same phase relations as a delta winding when used on single- phase ; that is, one of the three legs is in phase with the terminal voltage, while the other two legs have a 60-degree relation, How- ever, these two legs, with the 60-degree relation, carry the full cur- rent A ; while in the delta arrangement they carry one-half current. F/^./J Therefore, although the voltage relations are the same, the current relations are quite different; which accounts for the increased capacity with the groups connected as in Fig. 13 or Fig. 12. Fig. 13, like Fig. 12, is equivalent to covering the entire arma- tiu-e surface with copper which is equally active in carrying current when the machine is operated single-phase. However, compared with the three-phase star arrangement where two legs only are active, it may be seen that the voltage and the output have been increased in the ratio of 100: 86.6, or about IS percent, by the addition of 33 1-3 percent in copper, and 33 1-3 percent in total armature copper loss. It is evident, therefore, that the addition of a third leg when operating single-phase does not give results in proportion to the material used. CAPACITIES OF ALTERNATORS 125 Comparison of Single-Phase and Three-Phase Ratings on THE Basis of Equal Total Armature Copper Loss All the foregoing comparisons have been on the basis of equal losses in a given coil or group; but it has been shown that with some of the windings, when operated on single-phase, the currents are not divided equally. In consequence, in such cases the total copper loss in the windings must be less than where the current is divided equally. In the following comparisons the total copper losses for three-phase and single-phase are given, and the possible increase in single-phase rating for the same total copper loss is indicated. Let r = the resistance of one group. Let A = the limiting current per group, which has been used in the above comparisons. Then in Fig. 3, for three-phase, 6Ah = the armature copper loss. For single-phase I — I x4r -|- 2^4 V = 3AV = total armature copper loss. The three-phase loss is therefore twice the single-phase on the basis of equal limiting' current. For equal total loss the single- phase current could therefore be increased as the V 2, as the loss varies as the square of the current. As the former single-phase output was 50 percent of the three-phase, then for equal losses the single-phase output becomes 50 x •\/~2~ = 70.7 percent of the corresponding polyphase rating. In Fig. 5, the three-phase loss = 6 Ah. The single-phase loss with the same limiting current = 4AV, as there are but four legs in circuit instead of six, each leg carrying the same current as when operating three-phase. The three-phase loss is thus 6/4 single- phase, and for equal losses the single-phase current can be in- creased in the ratio of Ve/i- The former single-phase rating was 57.7 percent. This therefore can be increased to 57.7 x V6/4 = 70.7 percent of the corresponding three-phase rating. In Figs. 7 and 8, the three-phase loss = 6Ah. The single-phase loss = 3Ah, as determined by direct inspection of currents and resistances. For equal losses, therefore, the single-phase current can be increased as the \/~2~- The output then becomes 50 xV 2 = 70.7 percent of the corresponding three-phase rating. In Figs. 9 and 10, the three-phase loss = 6AV. Single-phase loss = 4AV. The single-phase output = 57.7 percent and for 126 ELECTRICAL ENGINEERING PAPERS equal loss this can be increased in the ratio of •v/6/4- The output . then becomes 70.7 percent of the corresponding three-phase output. In Fig. 12, the six-phase loss = 6AV. The single-phase loss = 6^4 V, as all the groups carry equal currents and all are in circuit. Therefore the single-phase current cannot be ftorther increased and the single-phase output remains at 66.7 percent of the six-phase output (or three-phase beyond the transformers.) From this it would appear that most of the above wind- ings would give, forequal armature copper loss, 70.7 percent of the three-phase rating. However, it should be taken into account that the three-phase ratings are not all equal on the basis of equal copper loss. In Figs. 3 and 5, for instance, the three-phase ratings are equal to 5.196 AE. The three-phase ratings with the arrangement shown in Figs. 7, 8, 9 and 10, are equal to dAE. Therefore Figs. 3 and 5 have only 86.6 percent of the three-phase ratings of 7, 8, 9 and 10. The single-phase ratings of Figs. 3 and' 5 therefore are 70.7 percent of 86.6 percent, or 61.2 percent of the best three-phase rating which can be obtained. Therefore, on the basis of 6AE being the best three-phase output, then with equal copper loss, the arrangements in Figs. 3 and 5 give 61.2 x 6AE = 3.792AE as the single-phase rating with equal copper loss, while Figs. 7, 8, 9 and 10 give 4.243AE as the single-phase ratings with equal copper loss, and Fig. 12 gives iAE as the single-phase rating with the same copper loss. Therefore, the arrangements in Figs. 7, 8, 9 and 10 are better than any of the others for single-phase rating, if total copper loss is the limit rather than the loss in an individual coil or group. However, if total copper loss is the limit, then there is still a difference between the true delta and star windings. With the delta winding the current A is increased 41 percent, which means that one of the groups will have double the copper loss which it has on three-phase, while with the star winding the current A will be increased slightly over 22 percent, which means that two groups of the winding will have their copper losses increased 50 percent. The star arrangement, even with the same total copper loss, works the individual coils on single-phase easier than in the delta arrange- ment. The following table summarizes the above relationships. CAPACITIES OP ALTERNATORS 127 a 1 .■< s| H tJ H •< s < Ratio Capacity Best 3-Ph. CO CO ^ t^ g ^ b- R o s 8 g o In II <<; ^11 co^ m < 1^- < <3 In-* < i^.t <; Armature Copper Loss WITH Amps. A Resist, r. Per Group S .i CO 1 * S < 1 CO CO CO ^ 5 S < to M a M < b 3 ls°lf CO CO i O !5 i 6? s g 6? i 1-1 C0< Mco < 11 "^co^co .S II w II mW WW S II 8 CO E 1^ 0=2 11 .-H 128 ELECTRICAL ENGINEERING PAPERS Two-Phase Windings The two-phase windings may be analyzed in a manner similar to the preceding. Starting with a closed-ring arrangement, just as in the three-phase, the various relations may be readily determined. Assuming a ring, as in Fig. 14, with four taps brought out at 90 degrees apart and assuming that this winding is the same in every way as that in Fig. 1, then the following e. m. f.'s and capacities are obtained. F/g./4 Fig. 15 represents a closed coil two-phase winding correspond- ing to the three-phase winding in Fig. 3. Calling E the e. m. f. of the groups of legs, then the e. m. f.'s AC and BD = V 2 x £. Opening Fig. 15 at two opposite points as in Fig. 16, the two parts may be rearranged to give Fig. 17. This is an interconnected open coil two-phase winding; that is, the central points are con- nected together so that there are fixed e. m. f . relations between all four terminals. The e. m. f . Ad is equal to E, and the e. m. f . across AB,BC, etc. = •\/~2~ X Ei, while the e. m. f. across AC and BD = V^ X El. Splitting the winding of Fig. 16 at four points, then the ar- rangements shown in Figs. 18 and 19 are obtained. These two windings are equivalent, except that in Fig. 18 the two legs which are in phase are connected in parallel, while in Fig. 19 they are in series. If the middle points in Fig. 19 are connected together the arrangement becomes equivalent to Fig. 17. In Fig. 18, e. m. f.'s AC and BD are equal to Ei, while there is no fixed e. m. f. relation between AB, BC, etc. In Fig. 19 the e. m. f.'s AC and BD axe equal to 2Ei and there is no fixed relation between AB, BC, etc., unless the middle points CAPACITIES OF ALTERNATORS 129 are interconnected, in which case the e. m. f.'s become the same as in Fig. 17. In Figs. 20 and 21 the usual two-phase, three-wire arrange- ment is shown. In Fig. 20, AB = Ei and AC = VT x Ej. In Fig. 21 AB = 2Ei and AC = 2 x VT^i. Fi^.17 Capacities of Two-Phase Windings : Let A equal the cur- rent per coil, this current being the same as for the three-phase winding. Then — In Fig. IS the capacity equals 17 18 19 20 21 A£i A£i A£i 4 AEx It is obvious therefore that the two-phase capacities are equal for all the various windings which have been' commonly used. Comparison of Two-Phase Capacities with Three-Phase As the same winding has been assumed for both two-phase and three-phase, it is of interest to compare their ratings. Comparing E and E\ in Fig. 22, it may be seen that Ex = ■\/~2 x E. Therefore the two-phase capacities given above, when put in terms of three- phase e. m. f.'s become, in all cases, 4A x \/~2~ x £ = 5.656 AE. The closed coil three-phase capacity = 5.196 AE. The closed coil six-phase capacity = 6 AE. The open coil (star or delta) three- phase capacity = 6 AE. Therefore, the three-phase closed coil arrangement gives the least output, while the two-phase, (which is. 130 ELECTRICAL ENGINEERING PAPERS in reality, four-phase with a closed coil winding) gives somewhat better results and the six-phase closed coil gives still better results. Single-Phase Rating from Two-Phase Winding Two of the terminals of the two-phase windings may be used for single-phase. Assuming the same current A per coil as in F/^./S Fig. 19 two-phase or three-phase, then the single-phase capacity becomes In Fig. IS = 2A X VT £i = 2.828 A£i In Fig. 17 = Ax 2£i= 2A£i In Figs. 18 and 19 = 2 AEi In Figs. 20 and 21 = 2.828 A£i Fig. 21 Fig. 22 Comparing the best single-phase obtained from the two- phase with the best single-phase from the three-phase windings, E\ being equal to V 2 .E, the following is obtained : CAPACITIES OF ALTERNATORS 131 Then 2.828 EiA = 4 AE, or same as obtained from the six-phase closed coil winding. Comparing the three-phase closed coil winding with the two- phase closed coil winding for both polyphase and single-phase ratings, the following is obtained on the basis of same loss per coil : The 3-phase closed coil winding gives 3-phase rating of 5 . 196 AE. " 3 " ' " single" " ' " 2.596 " 2 " " " " " 2 " " " 5.656 A£ " 2 " '■ " " " single" " " 4 AE. It is therefore apparent that with the closed coil winding the two-phase arrangement (or four-phase in reality) gives higher out- puts, for both polyphase and single-phase, than the three-phase closed coil arrangement will give, It may be of interest to note that in the earlier Westinghouse polyphase machines, when the single-phase rating of a polyphase generator was frequently of more importance than the polyphase rating, the closed coil two-phase winding shown above was gener- ally used. One reason for the selection of this type of winding was the high single-phase rating which could be obtained without un- due sacrifice in the polyphase rating. Special Connections for Single-Phase All of the preceding comparisons have had to do with sym- metrical arrangement of windings. However, by putting on one or more additional connections, which are used for single-phase operation purely, the windings can sometimes be made to give larger single-phase ratings than where the straight polyphase con- nections are used for single-phase operation. Two such arrange- ments will be shown below: — It is shown in Fig. 12 that by taking off single-phase at Ah, a high single-phase rating can be obtained. For supplying three- phase circuits, however, it was stated that transformers would have to be interposed to transform from six-phase to three-phase. However, by using A and h as the single-phase terminals and using A, B and C as the three-phase terminals, thus having fottr terminals total on the winding, as shown in Fig. 23, the machine can supply three-phase directly to the circuit and can also deliver single-phase with the best utihzation of winding. In this case the three-phase rating equals 5.196 AE and the single-phase rating equals 4 AE. The single-phase thus becomes approximately 77 percent of the polyphase. This high relative rating, however, ELECTRICAL ENGINEERING PAPERS iue to the fact that the three-phase rating is only 86.6 percent the maximum three-phase which could be obtained. In a similar way, with the delta winding shown in Fig. 8, an proved single-phase rating can be obtained by putting an ditional terminal at the middle of one of the legs, as shown in Fig. . The single-phase is then taken off at A and b, while tI, B and C i the three-phase terminals. In this case two of the delta legs are nost in phase with the single-phase, while the third leg is prac- ally idle as far as voltage is concerned, although it carries the 1 current. If the e. m. f . of AB is 2E then the e. m. f . of AB is YE. The total single-phase ctirrent is 2.A. Therefore, the -^/ '^-^ y i ^ i \ '*^N. y' i "^ M \ Ni^ >. • i ■V^ X • "-. .-^ B F/g, 23 igle-phase rating becomes 3.464 AR. The single-phase rating in is case is therefore 57.7 percent of the three-phase, instead of 50 Tcent where the single-phase was taken off at the terminals AB. The above two arrangements are therefore more effective than e usual single-phase from the same types of windings. However, will be shown later, the true delta and the closed coil three- lase windings are seldom used on alternating-current generators id therefore the above special arrangements are of no partictdar immercial advantage. dmparison of alternating and direct-current ratings From Same Armature Winding If direct current be taken from the same winding as described, .e limiting current per coil shoiild be the same as the effective r square root mean square) current when delivering alternating CAPACITIES OF ALTERNATORS 133 current. This is the value A used in the preceding comparisons. The direct-current e. m. f. is taken off from two opposite points of the armature. This e. m. f. therefore corresponds to the two op- posite terminals of either the two-phase closed coil or six-phase closed coil .winding shown in the preceding diagrams. The direct- current e. ra. f . will be equal to the maximum or peak value of the alternating-current e. m. f . taken off from these two points. This will be y/~2 times the effective value used in the preceding comparison. . For the six-phase diametral arrangement, it was shown that the effective alternating-current e. m. f. = 2E. Therefore the peak value of direct-current e. m. f . will be equal to V 2 x IE. As the limiting current is A, and as there are two direct-current branches, the total direct current will be 2E. The direct-current output therefore becomes 4 x V 2 AE = 5.656 AE. The following interesting comparisons can therefore be made: Direct-current capacity =5.656 A£ 1-Phase closed coil capacity =4 A£ =70.7% of D. C. 3-Phase " " " =5.192 AE =91.8% " " 2 " (4-phase) " " =5.656 AE =100% " " 6 " " " " =6 AE =106.1%" " 3 " open " " =6 AE =106.1%" " Prom the above it appears that the two-phase closed coil (and two-phase open coil) capacity is equal to the direct-current capacity from the same armature winding. The three-phase closed coil is less than the direct current, while the six-phase is greater than the direct current. The three-phase true star or delta winding and the six-phase closed coil winding are all slightly more effective than when the same winding is used for direct-current. The question may be raised whether still higher ratings could not be obtained from a given winding by taking off more phases. An examination will show that higher ratings can be obtained with the number of phase increased, with the closed coil winding; but it can be shown that the possible increase over the six-phase arrangement is very small. An easy way of comparing the ratings of closed coil windings, with different numbers of phases, is to compare the niimber of circuits, which can be taken off between adjacent taps or terminals all arottnd the winding, as referred to in first paragraph of page 123. This is equivalent to comparing^^the^' perimeters of the poly- 134 ELECTRICAL ENGINEERING PAPERS gonal figures shown in the diagrams for the various closed coil combinations and is illustrated in Figs. 25, 26, 27 and 28. In Fig. 25, calling one side E, than the perimeter = 6E. In Fig. 26, the perimeter = 5.656 E. In Fig. 27 the perimeter = 3 V 3 -E = 5.196 E. In Fig. 28, which represents single- phase, the two sides of the polygon coincide, making a straight line. Therefore, double the length of this line should represent the perimeter, which = AE. A comparison of these values shows that they are exactly in proportion to the alternating-current capacities given above. . F/g. 25 Fig. 26 It is evident that the greater the number of phases obtained from the closed coil winding, the more nearly the perimeter of the polygon approaches the circumference of the circle. With an infinite munber of phases a true circle would be obtained and in this case the perimeter becomes li^E = 6.283 E. Therefore, the maximum 6 28S possible polyphase rating is -—— = 1.047, or 4.7 percent greater than the six-phase closed coil rating or the true star and delta rat- ing. Also, the greatest possible polyphase rating is greater than the direct-current rating in the proportion of 6.283: 5.656, or approx- imately 1 1 per cent. Field Heating In the aboA^e comparisons of the relative ratings of the three- pliase, two-phase and single-phase windings, only the armature copper losses have been taken into account : but if the problem is to be considered in its completeness, other armature conditions and tlie field conditions must also be taken into account. CAPACITIES OF ALTERNATORS 135 A comparison of the thxee-phase and two-phase ratings shows that they are usually so close together that the field conditions woiild probably not exert a controlling influence on the relative capacities. In general, it may be taken that those combinations: of polyphase windings which give lower ratings at the same time give lower armature reactions. In comparing single-phase with polyphase ratings, however, the field conditions, both as regards the field winding and field core, must be taken into account. The armature reaction of the single- phase winding is pulsating and tends to produce magnetic disturb- Fig.2F ZE Fig. 28 ances in the field poles or core which may result in very considerable iron losses, both eddy and hysteretic. In general, these disturb- ances are relatively much greater on larger capacity machines, so that provision, must be made on such machines for suppressing or avoiding the ill effects of the armature reaction. This can be accomplished to some extent, by completely laminating the fielci poles. Another method which has been used on very large ma- chines is the employment of heavy cage damper in the pole faces, similar to that of the secondary of an induction motor. This damper must have current capacity such that when developing ampere turns sufficient to completely neutralize the armature pulsations, the heating effect in the damper winding, due to the current in it, is relatively low. Field copper heating, in most cases, is not a controlling con- dition, owing to the fact that the single-phase rating, defined by the armature heating, as indicated above, is so much lower than the polyphase rating that the field copper is usually worked some- what easier than on the polyphase loading. This is partictilarly 136 ELECTRICAL ENGINEERING PAPERS true when the rating is fixed by the heating of individual armature coHs. However, if the single-phase rating is determined by the total armature loss and not by the loss in individual coils, then the penmissible armature capacity on single-phase may be such that in some instances the field copper is worked harder than on polyphase. In such cases, if the field copper is the limiting condition, then the single-phase rating cannot be as high as the armature would permit. It may be assumed, however, that in large machines the armature conditions, as fixed by the loss in individual coils, determine the safe single-phase rating; and under this assumption the field conditions, except in regard to the use. of dampers or the elimination of the effects of armatvire reaction, need not be con- sidered. Application of Various Types of Alternating-Current Windings The three-phase true star type of winding is the one which, in general, lends itself to best advantage to the various types of alternating-current machinery. It may be a question then as to why any other types of windings are used. However, it was intimated before that where other than the true star winding is used, there is usually some condition other than the output which is of first importance. In the following will be given some of the principal applications of the different types of windings: — Closed Coil Types The closed coil type of winding is always used with rotary dbnverters. The controlling featiu-e in this case is that the rotary converter carries a commutator, which naturally requires a closed coil type of winding. Rotary converters are, in practice, wound for three-phase as in Fig. 3, four-phase (usually called two-phase) as in Fig. 15 and six-phase as in Figs. 11 and 12; and the number of collector rings is 3, 4 and 6 respectively. The three-phase winding is generally used in small capacity rotaries. While the three-phase winding allows legs output than the four-phase or six-phase, on small rotaries the capacity is usually not limited by the armature copper loss, while the use of three rings somewhat simplifies the machines. Four-phase rotaries are used to a very considerable extent in connection with two-phase circuits. However, where the supply CAPACITIES OF ALTERNATORS 137 circuit is three-phase it is rare that the transformation is from three- phase on the supply circuit to the two-phase on the rotary, as there are certain disadvantages in such transformation which more than offset the sUght advantage of the four-phase rotary over the three- phase. Moreover, where a higher number of phases is of advantage in a rotary converter, it is practicable to transform from the three- phase supply circuit to six-phase for the rotary. Two arrange- ments of such six-phase transformation are in use, as illustrated in Figs. 11 and 12. One of these is the so-called " Double Delta" arrangement, in which each of the step-down transformer circuits is equipped with two secondaries, as indicated in Fig. 11. These are connected to form two separate deltas, one being inverted with respect to the other. The other arrangement is the so-called " Diametral" arrange- ment, as shown in Fig. 12. This has advantages over the double delta in that only one secondary circuit is required for each phase and the middle points of these secondary circuits may be connected together for a neutral or middle wire between the direct-current leads from the rotary converter. In a rotary converter the armature copper loss is generally so small, compared with that of the straight direct-current or straight alternating-current machine with the same winding, that all con- siderations of the comparative heating of three-phase, four-phase and six-phase windings, as on alternating-current generators, has practically no bearing on the rotary converter rating. In a rotary converter, an increase in the number of phases over six represents a considerable reduction in the armature copper loss, — much more so than in the closed coil alternating-current generator. This is due, in the rotary converter, to the fact that one arrnature winding carries both the direct and the alternating currents, which are to a certain extent, flowing oppositely. Closed coil windings are also occasionally used on the second- aries of induction motors in order to give a better choice in the number of slots than would be allowed otherwise. Such windings when used on induction motors are usually of the two-circuit or series type, for the purpose of increasing the voltage as much as possible and at the same time keeping the mmiber of conductors as small as possible, while retaining the closed coil arrangemeiit. A two-circmt closed coil winding will close upon itself symmetrical- ly if the number of turns or coils is one more or less than a multiple 138 ELECTRICAL ENGINEERING PAPERS of the number of pairs of poles. This sometimes allows the use in the secondary of an induction motor, of a number of coils or slots which has no close numerical relation to the number of primary slots. For instance, if the primary of a four-pole induction motor has 48 slots with an open coil, star or delta winding, then with 39 coils and slots in the secondary, a symmetrical' closed coil three- phase winding could be obtained, while if an open coil secondary were used, the number of slots should preferably be 36 or 42, which might not be as desirable as 39 in some cases. This simply illus- trates an occasional use of the closed coil winding. Closed coil windings were at one time used very extensively on low voltage, rotating armature, two-phase generators. Such genera- tors were A^ery satisfactoty for delivering a relatively large percent- age of their rating as single-phase. Furthermore, with one conduc- tor per slot and with bolted-on end connectors, the potential bet- ween adjacent end connectors was at all points relatively low. The symmetrical arrangement of such windings also rendered them very suitable for use with supporting bands or end bells over the end winding. However, with the advent of the rotating field ma- chines, and particularly with the use of higher voltages, the open coil star winding has entirely superseded the closed coil type of generator winding. Three-Phase Star Windings Two types of star windings have been shown, namely,' those in Figs. 5 and 10. That of Fig. 5 gives less output than that of Fig. 10 in the ratio of 86.6; 100. There would appear therefore to be no use for the Fig. 5 arrangement; but, in certain cases, in using a given winding it may be desired to reduce the voltage from 12 percent to 15 percent while retaining normal conditions otherwise. In such a case the lower voltage could be obtained, if a new winding were used, by simply chording the winding one-third the pitch. On the other hand, if an existing winding is to be used, the same result cotdd be obtained by coupling as in Fig. 5. In induction motors the arrangement shown in Fig. 5 may be used occasionally where the windings are arranged for coupling for two different speeds. In some cases this type of winding may give better average field distribution for the two numbers of poles than the one shown in Fig. 10. In this case therefore it is the dis- tribution of the magnetic field, and not the capacity of the winding, which is the important feature. CAPACITIES OF ALTERNATORS ' ISff- The arrangement shown in Fig. 10 is the true star winding which is used almost universally on three-phase machines. For a given voltage it requires fewer conductors than any other type of winding. This is of very material advantage in allowing, with a given number of slots, a smaller number of conductors per slot, which, as a rule, allows a better utilization of the star space: — ■ That is, raore copper can be gotten into a given slot. Furthermore, in relatively high voltage machines where the conductors may be- very large in nmnber and small in size, the star 'winding with its smaller number of conductors, each of much larger size, gives more substantial coils than any other arrangement. Another advantage of the three-phase winding is its fairly good utilization of copper when operated on single-phase. When operated on purely single- phase load, oile leg of the star could, of course, be omitted, but if it is retained it becomes a reserve winding which may be used in case of an accident to one of the active legs of the winding. By opening any defective coils in an active leg and connecting in the reserve leg in place of the defective one, the machine can still develop its specified rating on single-phase. Another advantage of the star type of winding is the readiness with which the central or neutral point can be grounded, which is a very considerable advantage in some high voltage systems. Delta Type Windings The true delta type winding, as illustrated in Figs. 7 and 8, is not used to any great extent in either alternating-current generators or induction motors. For a given voltage it requires 73 percent more conductors, each of 58 percent of the capacity of those of the true star type of winding. As the terrninals of all three legs are connected in a closed circuit it is necessary that the e. m. f .'s generated in the three legs should balance each other at all instants or there is liable to be circulating current around the windings. This means that the winding must be applied only where the conditions are favorable, or the conditions in the design must be made -to suit .the type of winding. This winding is occasionally used on low voltage turbo generators of fairly large capacity, due to the fact that the delta type winding requires more conductors than the star type. For example, in a large capacity two-pole turbo-generator, wound for relatively low voltage, the number of conductors for the star winding may be so small that a satisfactory number of slots is not obtained, even with only one conductor per 140 ELECTRICAL ENGINEERING PAPERS slot and even using the double-star winding, shown in Fig. 9. In such case a double delta winding will allow 73 percent more con- ductors and slots than the double star will give. Also, each conductor will be much smaller than in the star arrangement, which may be of considerable advantage in the case of low voltages and very heavy current per conductor. Delta windings are occasionally used on machines which are arranged for connection for two different voltages, such as 6600 volts and 1 1 ,000 volts. If an armature is wound for star connection at 11,000 volts, then it can be coupled in delta for 6600 volts with practically the same inductions, losses, field currents, etc. The delta type of winding is also used occasionally in the primaries of induction motors for special purposes, such as multi-speed combinations where the winding is changed from one number of poles to another. In general, however, the star type winding is used on induction motors. The delta winding is not well adapted for single-phase opera- tion on account of its low capacity. Also, it does not admit of grounding of the neutral or central point of the system. Taking everything into account, the true delta winding is only used where some special condition is imposed upon the winding which puts the star, arrangement at a disadvantage. Single-Phase Alternators All the foregoing comparisons have been made on the basis of the same armature winding being used for three-phase, two-phase or single-phase. The relations shown do not hold true in general for machines which are woiind initially for single-phase service, such as for single-phase railway or electro-chemical or electro-fusion work. In such cases the amount of armature copper used and its distribution are such that the armature coils, either individually or as a whole, do not determine the true limits of output; but the armature as a rule can carry anything that the field winding will stand, so that the field temperature becomes the true limit in such machines. Also, very massive, well distributed cage, dampers are used with such machines when they are of large capacity and these, in turn, have a certain effect on the characteristics, such as the regulation, and thus have an indirect influence on the permissible capacity. It is well known that if the inherent regulation of an al- ternator is made poorer, the capacity can usually be increased with the same limiting field temperature. In large single-phase gener- CAPACITIES OF ALTERNATORS 141 ators, especially for railway service, the capacity is increased by sacrifice in the inherent regulation of the machine. However, the massive dampers greatly improve the regulation for quick changes in load; while the poorer inherent regulation only affects the regulation over considerable intervals of time, and automatic regulators, acting on the alternator excitation, readily take care of the slow fluctuations. In consequence, single-phase generators of large capacities may be built for ratings which bear no definite relation to any of those given above. The armature windings of single-phase generators, when ar- ranged for single-phase purely, are frequently distributed over only part of the surface. Usually they cover considerably more than half the surface, and in extreme cases they cover 80 percent or more. Of course, when spaced like a true three-phase winding they cover two-thirds the siurface. This arrangement admits of an extra leg being added to the winding, which is normally idle, if the winding is connected in star, this leg being a reserve in case of accident, as mentioned before. However, when such a leg is not added, the winding generally covers more than two- thirds the surface, rather than less, but rarely covers the whole surface. 142 ELECTRICAL ENGINEERING PAPERS DAMPERS ON LARGE SINGLE-PHASE GENERATORS FOREWORD — This formed part of the discussion of a paper presented before the Institute of Electrical Engineers, December 1908, by Mr. Murray, describ- ing the operation of the New Haven single-phase railway. The effect of the addition of the massive dampers on the rotors of the New Haven generators was so pronounced, and the results were so beneficial, as a whole, that it was considered advisable to publish it as new and interesting material, in the form of a discussion of Mr. Murray's paper. Immediately after the publication of this, heavy dampers were adopted very generally by manufacturers of large single-phase generators, throughout the world, who had encountered more or less trouble of the same nature as found in the New Haven generators. Prac- tically all large single-phase generators since then have been built with such dampers as part of their construction. — (Ed.) WHEN the New Haven single-phase generators were put on load test, the first, and most pronoimced, difficulty was in heating, not in the winding, but in the field or rotor structure, due to the pulsating reaction, of the armature winding when carrying a 'heavy load in single-phase current. This reaction was known previously to building these machines, but on machines of smaller capacity it had not developed destructive tendencies. It was proved later that this was simply because it had not been tried out under the conditions which would develop its most harmful effects. This pulsating armature reaction may be analyzed in the following manner; Consider the armature winding as a magnetizing coil fixed in space and carrying an alternating current. This coil may be considered as setting up ari alternating field fixed in space. For analysis, this alternating-current field, fixed in space, may be considered as made up of two constant fields of half value, rota- ting in opposite directions at the synchronous speed of the machine. One of these fields therefore rotates at the same speed and in the same direction as the rotor. The other field is traveling round the rotor core in the direction opposite to its rotation. This field may therefore be considered as equivalent to one fixed in space with the rotor running in it at double speed. This armature, carrying single-phase current, could be replaced by an external direct-cur- rent field, and if the rotor were run at double speed in this field, an equivalent result, as regards heating, shoxild be obtained. This core thus becomes an armature core subject to a heavy in- duction at a high frequency. DAMPERS ON SINGLE-PHASE GENERATORS 143 When the first rotor was built, the structure was laminated as completely as mechanical conditions would permit. However, in the case of high-speed turbine-generators of very large capacity, it is almost impossible completely to laminate everything, due to the fact that the mechanical requirements call for rigidity in some of the structural features. Upon testing the first machine it was found that there was local heating, with heavy load, sufficient to create hot spots in the core ; and in a comparatively short time in turn these hot spots damaged the insulation on the coils from the outside, thus causing grounds on the winding. As soon as this was noticed, an effort was made to eliminate these hot spots ; but it was found, after several attempts, that as soon as one was eliminated others would show up in some different place as soon as a higher load condition was reached. It was evident, after considerable work had been done, that the correct remedy was not being applied to this trouble. It was then decided to take a bold step by at- tempting to eliminate all pulsating reactions from the armature by putting a short-circuited winding on the rotor, of such value that a very large current coiild flow in it with but very little loss. It was the idea to damp out the field in very much the same way that the armature of a polyphase alternator will demagnetize, or kill its magnetic field, if the armatiu-'e terminals, are all short-circuited together. It is known that under this condition the armature current will rise to such a value that the field flux is practically eliminated. In order to maintain this condition indefinitely without overheating, it is only necessary to put enough copper on the armature so that the I^R losses in it under this condition are within the temperature capacity of the windings. Working on this theory, a complete cage winding was placed on one of the rotors of the New Haven generators. This rotor had not been designed originally for this purpose, and it was therefore difficult to adopt the most suitable proportions in this winding, but what was put on, immediately showed in practice that a practicable remedy had been applied for this trouble. Meanwhile the 'new rotors designed for the application of heavy cage windings were under construction, and upon the installation of these, the field or rotor trouble all disappeared. It is interesting to note that the fourth machine installed, which has a 4260 Idlovolt-ampere single- phase rating, has a solid steel core, in the surface of which the copper cage winding is embedded. As this winding completely eliminated the pulsating armatvire reaction, there was no further 144 ELECTRICAL ENGINEERING PAPERS occasion for laminating the field as a protection from magnetic pulsations. I might add that a number of the earlier tests, leading up to the design of the first New Haven rotors, were misleading, in the fact that turbine-generators were used for obtaining the prelimin- ary data for single-phase operation and, in all cases, the machines had solid steel cores. These cores acted as dampers to a certain extent, and this in itself eliminated part of the pulsation. It thus developed afterwards, that in the very act of lamination to avoid the trouble, we had gotten into it deeper. Practically all this work on the generators was done before the full electric service was estabHshed, and while only one or two generators were required to be operated at one time. With one generator running, there was apparently but little or no disturb- ance due to short-circuits on the system. As the service was in- creased and two generators put in operation, the effect of short- circuits became more pronotinced. When, in June, 1908, the entire electric service was established, and three generators were con- nected to the system, it soon became evident that there was some serious condition existing in the system, as indicated by the ex- tremely violent shocks to everything in case of a short-circuit. This was particularly noticeable in the switching system, and, as Mr. Murray intimates, in the case of a short-circuit, all the switches in the system felt it their duty to jump in and open the circuit. This indicated an abnormal current condition. It was calculated that these machines would give possibly six or seven times full-load current on the first rush, in the case of a dead short- circuit, this excess current dying down to possibly two or three times normal full-load current. All indications were, however, that this current was being greatly exceeded, and therefore a series of oscillograph tests were made to determine the current rush when the lines were purposely short-circuited under various conditions. These tests indicated that vm.der certain conditions each machine could give, at the moment of short-circuit, almost 5000 amperes on one phase, the normal full-load current being 340. With three machines in parallel, this would therefore mean that approximately 15,000 amperes could be delivered momentarily. This enormous cttrrent rush was sufficient to explain many of the difficvilties, but this was not all the explanation. The oscillo- graph tests also showed that this short-circuit current would be maintained at almost its maximvma value for a very considerable DA MPERS ON SINGLE-PHA SE GENERA TORS 145 period, due to the cage winding on the rotors of the generators. Apparently this current at the first rush, was not appreciably- greater than that on the machine before the dampers were added, but without the dampers the field was killed more quickly by this enormoiis current, so quickly that apparently the breakers did not open tintil the current had fallen somewhat. However, with the heavy cage winding on the field structure, secondary currents were set up in this winding, tending to maintain the field strength, and thus the current rush was maintained at almost full value for possibly, 20 to 30 alternations. These oscillograph tests indicated very clearly that the armatures of these generators did not" have nearly so great internal self-induction as our calculations indicated. Meanwhile, the generators in the power house had been suf- fering from the tremendous shocks which accompanied short- circuits on the line. There is necessarily considerable local field around the end-windings of all these machines, and this stray field is especially large on machines with a small number of poles, and, in consequence, high ampere-turns per pole. These stray fields at the ends tend to exert a bending or distorting effect on the end-windings. In any given machine the distorting force varies as the square of the current carried by the coils. Our experience with the windings on these machines indicated that they were being subjected to enormous forces in the end- windings. The oscillograph tests gave an indication as to the amount of this force. As the machines cotild give about 15 times full-load cur- rent momentarily on short-circuit, the force acting on these end- windings woiold be 225 times normal; in this case, therefore, these forces were so great that it became a serious problem to devise a type of bracing on the end-windings sufficient to withstand such a force. It should also be borne in mind that probably as many short-circuits came, in one day, on these generators, as the or- dinary high-voltage power-house generator is called upon to sustain in one year. While ninety-nine shocks out of a hundred might not be sufficient to do damage, yet if the shocks occur fre- quently enough, the hundredth one will soon be reached. In our endeavors to support these windings against movement, probably the most complete system of bracing ever applied to alternating- ctirrent generators was developed and used on these machines. But in spite of this there was evidence of movement at times. It thus became evident that some method of limiting this short- circuit cvirrent to the value originally intended; namely, about 146 ELECTRICAL ENGINEERING PAPERS six times full-load current, would have to be applied. This was done by placing an unsaturated choke-coil, or impedance coil, on the trolley side of each machine. This coil takes up a comparatively small • voltage under normal operation, but in case of a short- circuit, the electromotive force generated in it is sufificient to limit the current rush to less than half the value it would attain without this coil. Thus as the shock on the end-windings of the generators varies as the square of the current, it is evident that cutting this current in half would cut the shock to one-quarter of its former value, which, with the method of bracing used on these machines, would mean the difference between good and bad. When these choke-coils were installed, the results on the power house were evident. The shocks on the machines were very greatly reduced, so reduced that we do not fear future trouble from this source. It is interesting to note that No. 4 machine; that is, the 4260 kilo volt-ampere generator, referred to before, was put in service a considerable time before the choke-coils were installed, and it went through the raost severe short-circuits ever encountered on this system. Its armature winding has never shown any distress. This is partly because, in the design of this machine, . the difificulties to be overcome were known, and the remedies could be applied in the most suitable manner. An iiiteresting point in connection with the use of the cage windings on these generators, is that the apparent regulation of the system has been improved. This was anticipated, but the actual result in practice was more pronounced than was expected. In installing new rotors for these machines with the heavier cage dampers, the inherent regulation of the generators was made somewhat poorer than before, partly in order to accommodate certain structural features in the rotor. It was anticipated that the cage winding with its damping effect wovild, to a certain extent, mask this poor regulation by making the machine sluggish as regards fluctuation in voltage with sudden variations in load. In practice it was found that, with the later rotors with their poorer inherent regulation, the average regulation of the system was considerabl}' better than before, thus indicating that most of the disturbances in the voltage, when the old rotors were used, were due to sudden changes in load, while the slow variations were taken care of by the automatic regulators. With the new rotors the voltage changes are so slow, that the Tirrill regulator DAMPERS ON SINGLE-PHASE GENERATORS 147 has plenty of time to act before any serious disturbance can take place. It must be borne in mind that in one way this New Haven power-house installation was more difficult than anything under- taken heretofore, and that is, in the use of 11,000 volt generators with one terminal connected directly to ground. Taking this condition into account, together with the enormous current rushes with consequent shocks on the winding, and the single- phase operation of units of such large capacity, it may reasonably be claimed that this was the most difficult case of alternating- current generation ever undertaken. 148 ELECTRICAL ENGINEERING PAPERS DEVELOPMENT OF A SUCCESSFUL DIRECT-CURRENT 2000 KW UNI-POLAR GENERATOR FOREWORD — In 1906, the Westinghouse Company obtained a contract for a 2000 kw uni-polar type generator direct coupled to a 1200 revolution steam turbine. Many diffictllties were encountered in shop tests on this machine, which were apparently corrected, but upon installation and operation on the customer's premises, many new and totally unexpected difficulties arose. This paper is a story of these troubles and their ultimate solution. It illus- trates very well how a responsible manufactiiring company will throw its whole engineering and manufacturing endeavors into correcting serious diffii- culties almost regardless of expense. It also serves to give student engineers a good idea of the practical side of manufacturing engineering. Fearing the results of the engineering efforts expended on this machine would eventually be lost, the author prepared them in their present form for presentation at the twenty-ninth annual convention of the American Institute of Electrical Engineers at Boston, June 1912. — (Ed.) This paper is not intended to be a theoretical discussion of the principles of unipolar machines; neither is it a purely descrip- tive article. It is rather a record of engineering experiences obtained, and difficulties overcome, in the practical development of a large machine of the unipolar type. For those who are in- terested in the designs and development of electrical machinery there may be many points of very considerable interest in this record. Some of the conditions of operation, with their attend- ant difficulties, proved to be so unusual that It is believed that a straightforward story of these troubles, and the methods for correcting them, will be of some value as a published record. Two theoretical questions of Unipolar design have come up frequently; (1) whether the magnetic flux rotates or travels with respect to the rotor of the stator; and (2) whether it is possible to generate e.m.fs. in two or more conductors in series in such a way that they can be combined in one direction, with- out the aid of a corresponding number of pairs of collector rings, to give higher e.m.fs. than a single conductor To the first question the answer may be made that in the machine in question, it makes no difference whether the flux rotates or is stationary; the result is the same on either assump- tion. To the second it may be said that when the theory of inter- linkages of the electric and magnetic circuits is properly con- sidered, it is obvious that the resultant e.m.f . is equivalent to that of one effective conductor, and therefore it is not practicable to obtain higher e.m.fs. than represented by one conductor, without the use of collector rings or some equivalent device. It VNI-POLAR GENERATOR 149 has been proposed in the past, by means of certain arrangjements of liquid conductors in insulating tubes, to add the e.m.fs. of several conductors in series, but such a scheme does not appear to be a practical device. Therefore, the theoretical considera- tions being largely eliminated, the author confines himself to the practical side only. In 1896 the writer designed a small unipolar generator of approximately three volts and 6000 amperes capacity at a speed of 1 500 rev. pfer min. This machine was built for meter test- ing and the occasion for its design lay in the continued trouble encountered with former machines of the commutator type designed for very heavy currents at low voltages. The general construction of this early machine is shown in Fig. 1. The rotating part of this machine consisted of a brass casting, cylindrical shaped, with a central web, very similar to a cast metal pulley. The two outer edges of this pulley or ring served as collector rings for col- lecting the current as indicated in the figure, while the body of the same ring served as the single conductor. The object of this construction, of rotor was to obtain a form which could be very quickly renewed in case of rapid wear, as this arrangement would allow a small casting to be made and simply turned up to form a new rotor. However, this renewal feature has not been of very great importance for the rotor of the first machine was replaced only after 12 years' service. This period of course did not represent continuous service, for this particular machine was used for meter testing purposes or where large currents were required only occasionally. A number of peculiar conditions were found in this machine. In the initial design the leads for carrying the current away from the brushes were purposely carried part way around the shaft in order to obtain the effect of a series winding by means of the leads themselves. In practice, they were found to act in this manner and, in fact, they over-compounded the machine possibly 30 to 40 per cent. In consequence, it was necessary to shunt them by means of copper shunts around the shaft in the opposite direction. 1 /^ ^^L=_ \ gxn I i J Fig. 1 150 ELECTRICAL ENGINEERING PAPERS Shortly after this machine was put in operation there was con- siderable cutting of -the brushes and rings, especially at very heavy currents. It was found that block graphite, used as a lubricant, gave satisfactory results. This machine was operated up to 10,000 to 12,000 amperes for short periods. The description of the above machine has been gone into rather fully, as it was a forerunner of the 2000-kw. machine which will be described in the following pages. The general principle of construction and the general arrangement of the two parts, or paths, of the magnetic circuit are practically the same in the two machines, as will be shown. In 1904, due to the rapidly increasing use of steam turbines^ the question of building a turbo-generator of the unipolar type was brought up, and an investigation was made by the writer to determine the possibilities. This study indicated that a commercial machine for direct connection to a steam turbine could be constructed, provided a very high peripheral speed was allowable at the collector rings or current collecting surfaces. It appeared that the velocity at such collector surfaces would have to be at least 200 to 250 feet per second, in order to keep the machine down to permissible proportions of the magnetic circuit, and to allow a reasonably high turbine speed. Con- trary to the usual idea, the very high speeds obtainable with steam turbines are not advantageous for unipolar machines. For example, while maintaining a given peripheral speed at the current collecting surface, if the revolutions per minute of the rotor are doubled, then the diameter of the .rotor collecting rings is halved, and the diameter of the magnetic core surrounded by the collector rings is more than halved, and the effective section of core is reduced to less than one-fourth. The e.m.f. generated per ring or conductor, therefore, on the basis of flux alone, would be reduced to less than one-fourth, but allowing for the doubled revolutions per minute, it becomes practically one-half. On the other hand, if the revolutions are reduced, while the speed of the collector ring is kept constant, then the e.m.f. per ring can be increased, as the cross section of the magnetic circuit increases rapidly with redtiction in the number of revo- lutions. But-at a materially reduced speed, the total material in the magnetic circuit becomes unduly heavy. In consequence, if the speed is reduced too much, then the machine becomes too large and expensive, while with too great an increase in speed, UNI-POLAR GENERATOR 151 the e.m.f. per ring becxjmes low or the peripheral speed of the rings must be very high. It is desirable to keep the number of collector rings as small as possible, for each pair of rings handles the full current of the machine, and therefore any increase in the number of rings means that the full current must be col- lected a correspondingly large number of times. Therefore, it works out that the range of speeds, within which the unipolar machine becomes commercially practicable, is rather narrow. In 1906, an order was taken for a2000-kw. 1200-rev. permin., 260-volt, 7700^ampere unipolar generator to be installed in a Portland cement works near Easton, Pa. The fact that it is a cement works should be emphazised, as having a considerable bearing on the history of the operation of this machine, as will be shown later. |U ' Ll ' U ' LlU"U"U'U ' ' ^/^^ •MSUUUUUUUU '-' Li ni l mS^iTi U -^ ^ Fig. 2 This 2000-kw. machine does not represent any theoretically radical features, being similar in type to the smaller machine already described, but modified somewhat in arrangement to allow the use of a large number of current paths and collector rings. The general construction of this machine is indicated in Fig. 2. The stator core and the rotor body are made of solid steel, the stator being cast, while the rotor is a forging. There are eight collector rings at each end of the rotor, the corresponding rings of the two ends being connected together by solid round conductors, there being six conductors per ring, or 48 con- ductors total. In each conductor is generated a normal e.m.f. of 32.5 volts, and with all the rings connected in series, the total voltage is 260. The stator core, at what might be called the pole face, is built 152 ELECTRICAL ENGINEERING PAPERS up of laminated iron, forming a ring around the rotor. This was laminated in order to furnish an easy method for obtaining the stator slots in which the conductors He which connect to- gether the brushes or brush holders for throwing the pairs of rings, in series. The slots in the stator laminations were made open, as'indicated in Fig. 3, in order to readily insert the stator conductors. There are 16 slots in this ring, and in each slot there is placed one large solid conductor. As first assembled, non-metallic wedges were used to close these slots, but later these were changed to cast iron for reasons which will be explained later. The rotor core consists of one large forging, as indicated in Fig. 2. Lengthwise of this rotor are 12 holes for ventilating Fig. 3 Fig. 4 purposes originally 2f in. diameter. Each of these holes con- nected to the external surface by means of nine If in. radial holes at each end of the rotor, these holes corresponding to mid- positions between the collector rings. It was intended to take air in at each end of the rotor aad feed it out between the collec- tor rings for cooling. In addition, as originally constructed, there was a large enclosed fan at each end, as indicated in Fig. 4. These fans took air in along the shaft and directed it over the collector rings parallel to the shaft. The object of this was to furnish an extra amount of air for cooling the surfaces of the rings, and the brushes and brush holders, as it was estimated, that the brushes and brush holders themselves could conduct away a considerable amount of heat from the rings by direct UNI-POLAR GENERATOR 153 contact, and that the cooling air from the fans, circulating among the brush holders, would carry away this heat. These fans were removed during the preliminary tests, for reasons whiich will be given later. The rotor collector rings consisted of eight large rings at each end, insulated from the core by sheet mica, and from each other by air spaces between them.. EJach ring has 48 holes parallel to the shaft. These holes are of slightly larger diameter thaii Fig. 5 :^:: the rotor conductors ovitside their insulation. Six holes in each ring were threaded to contain the ends of six of the conduc- tors which were joined to each ring. The six conductors con- nected to each ring were spaced symmetrically around the core. Fig. 5 shows this construction. The rotor conductors, 48 in number, consist of one in. copper rods, outside of which is placed an insulating tube of hard ma- terial. Each conductor, in fact,- consists of two lengths arranged for joining in the middle. The outer end of each conductor is upset to give a diameter larger than the insulating tubes, , and a thread is cut on this ex- " panded part. After th^ rings were installed on the core, the rods were inserted through the £ holes to the threaded part of a ring and were then screwed home. At the middle part of the rotor core, a groove is cut as shown in Fig. 6. Into this groove the two halves of each conductor project. These two ends are then connected together by strap conductors in such a way as to giye flexibility in case of expan- sion of the conductors lengthwise. This arrangement is also shown in Fig. 6. With this arrangement there is no possibility whatever of the conductors turning after once being connected. There is Fig. 6 154 ELECTRICAL ENGINEERING PAPERS a series of holes from the axial holes through the shaft to this central groove, for the purpose of allowing some ventilating air to flow over the central connections. As originally constructed, the conductors passed through com- pletely enclosed holes near the surface of the rotor core, as in- dicated in Fig. 7- This construction was afterwards modified to a certain extent. The face of the rotor at this point was also solid, as originally constructed. This was afterwards changed, as will be described later. The collector rings, as originally constructed, consisted of a base ring with a wearing ring on the outside, as shown in Fig. 8. Both rings were made of a special bronze, with high elastic limit and ultimate strength. On the preliminary tests these rings showed certain difficulties and required very considerable modifications, and several different designs were developed during the preliminary operation, as will be described. '■'///////////, Fig. 7 \ ,///////////////'//>^--J '} '^//////////////A Fig. 9 W/M'//MMm^ Fig. 8 The eight sets of brush holders at each end are carried by eight copper supporting rings. These supporting rings are insulated from the frame of the machine but are connected in series by means of the conductors through the stator slots. There are 16 brush holders studs per ring and two brush holders per stud, each capable of taking a copper leaf brush f in. wide by If in. thick. These. brush holders are spaced practically uniformly around the supporting rings. The supporting copper rings are continuous or complete circles, so that the current collected from the brushes are carried in both directions around the ring. There are two conductors carried from each ring through the stator slots to a ring on the opposite side of the machine, in order to connect the various brush holders in series. The arrangement is illustrated in Fig. 9. The above description represents the machine as originally UNI-POLAR GENERATOR 155 constructed and put on shop test. From this point on, the real story begins. Various unexpected troubles developed, each of which required some minor modification in the construction of the machine and, moreover, these troubles occurred in series, that is, each trouble required a certain length of time to de- velop, and each one was serious enough to require an immediate modification in the machine. In consequence, the machine would be operated until a certain difficulty would develop ; that is, that trouble would appear which took the least time to de- velop. After it was remedied, a continuation of the test would show a second trouble which required a remedy, and so on. Some of these troubles were of a more or less startling nature as will be described later. This machine, after being assembled according to its original design, was operated Over a period of several weeks in the testing room of the manufacturing company. It was operated both at no load and at full load, and a careful study was made of all the phenomena which were in evidence during these tests. The machine was first run at no-load without field charge to note the ventilation, balance, and general running conditions of the machine. The ventilation seemed to be extremely good, especially that due to the fans on the ends of the shaft. The noise, however, was excessive — so much so that anyone working around the machine had to keep his ears padded. At first it was difiicult to locate the exact source of this noiste, but it was determined that the end fans were responsible for a considerable part of it. On taking the saturation curve of the machine, it was found to be extremely sluggish in following any changes in the field current. The reason for this sluggishness is obvious from the construction of the machine, each magnetic circuit of the rotor core being surrounded by eight continuous collector rings of very heavy section, and also by eight brush holder supporting rings of copper of very low resistance. These rings, of course, formed heavy secondaries or dampers which opposed any change in the main flux. The total effective section of these rings was equivalent in resistance to a pure copper ring having a section of 49 sq. in. One can readily imagine that such a ring would be very effective in damping any sudden flux changes. This slug- gishness of the machine to changes in flux, however, was not an entirely unexpected result. The saturation curve showed that the machine could be carried 166 ELECTRICAL ENGINEERING PAPERS considerably higher in voltage than originally contemplated, for apparently the magnetic properties of the heavy steel parts were very good, and it was possible to force the inductions in these parts to much higher density than was considered prac- ticable in working out the design. This gave considerable. lee- way for changes which later were found to be necessary. In taking the saturation curve, the power for driving the machine was measured and it was found that there were prac^ tically no iron losses in the machine; that is, at full voltage at no-load the total measured losses were practically the same as without field charge. This apparently eliminated one pos- sible source of loss which was anticipated, namely, that due to the large open slots in the stator pole face, these slots being very wide compared with the clearance between the stator and rotor After completion of this test the machine was then run on short circuit. Apparently, as there was no iron loss shown in the no-load full voltage condition, the short circuit test with full load current should cover all the losses in the rotor which would be found with full load current at full voltage. Experience afterward proved this assumption to be correct, for in its final form the machine would operate under practically the same condition as regards temperatiire, etc., at full voltage as it would show at short circuit, carrying the same current, the principal difference being the temperature of the field coil. It was in this short circuit temperature run that the real troubles with the machine began. The measured losses, when running on short circuit, were somewhat higher than indicated - by the resistance between terminals times the square of the current. These extra losses were a function of the load and increased more rapidly with heavy currents. The measured power indicated that these excess losses were principally due to eddy currents. However, the total losses indicated in these preliminary tests, although somewhat higher than calculated, were still within allowable limits, as considerable margin had been allowed in the original proportions to take care of a certain amount of loss. It was therefore considered satisfactory to go ahead with the short circuit tests, and in making these it was the intention to operate long enough to determine the neces- sary running conditions as regards lubrication, heating, etc. As mentioned before, the original collector rings of the machine each consisted of a base ring upon which was mounted a second- ary or wearing ring, it being the intention to have this latter UNI-POLAR GENERATOR 157 ring replaceable after it was down to the lowest permissible thickness, as it would be rather expensive and difficult to replace the base ring which carried the rotor conductors. As the inner ring was shrunk on the core and the outer ring was. shrunk on over the base ring, with a very small shrinkage allowance, it was considered that the outer ring was in no danger of loosening on the inner ring, especially as both rings, being of bronze, and in good contact, should heat each other at about the same rate. This assumption, however, was wrong. The machine was put on short circuit load of about 8000 amperes early one evening and an experienced engineer was left in charge of it until about midnight. Up to that time the machine was working perfectly, with no under heating in the rings and no brush trouble, although vaseline lubrication was used. About midnight the engineer left the machine in charge of a night operator, and at about three o'clock in the morning this operator saw the brushes beginnin'g to spark and this very rapidly grew worse, so that in a very few minutes he found it necessary to shut the machine down. An examination then showed that several of the outer rings had shifted sideways on the base ring, as indicated in Fig. 10. One of these rings had even moved into contact with a neighboring ring so as to make Fig. 10 a dead short circuit on the machine. It was also noted that all the rings which loosened were on one side of the machine, and that the surfaces of the rings exposed to the brushes were very badly blistered. The brushes also were in bad shape, indicating that there had been excessive burning for a short time, An investigation of the loose rings showed that they had loosened on their seats on the inner or base rings. Investigation then showed that a temperature rise of 70 to 80 deg. cent., combined with the high centrifugal stresses, would allow the rings to loosen very materially. It was then assumed that as the ring had heated up, bad contact had resulted be- tween the inner and outer rings and this, in turn, had caused additional heating, so that the temperature rose rather suddenly after bad contact once formed. It developed later that this was probably not the true cause of the trouble, but at the time it was considered that the remedy for the trouble was in the use of rings which could be shrunk on with a greater tension. It was then decided to try steel outer rings iristead of bronze 158 ELECTRICAL ENGINEERING PAPERS on the end where the bronze rings had loosened. However, upon loading the machine, after applying the steel rings, a new diffi- culty was encountered. It was found that the loss was very greatly increased over that with the bronze rings. This loss was so excessive as to be prohibitive, as far as efficiency was concerned, and also the tests showed excessive heating of the rings and of the machine as a whole. Also, there were continual small sparks from the tips of the brushes, these sparks being from the iron itself, as indicated by their color and appearance. However, during the time these' rings were operated there did not seem to be any undue wear of either the brushes or the rings, but obviously there was continued burning, as indicated by the sparks. With thes.e steel rings it was found to be impossible to operate continuously at a current of 8000 amperes, due to the heating of the steel rings in particular and everything in general. At a load of 6000 amperes the loss was materially re- duced and it was possible to operate continuously but with very high temperatures. The tests showed that, with the steel rings, at full rated current, the loss was approximately 200 kw. greater than with the bronze rings, or about 10 per cent of the output. With both ends eqjiipped _ with steel rings, this would have been prac- pjg J J tically doubled. While this was recognized as an entirely unsatisfactory operating condition, yet it allowed the machine to be run for a long enough period to determine a number of other defects which did not develop in the former test. One of these defects was an undue heating of the rotor pole face. This was obviously not due directly to bunching of the flux in the air gap on account of the open^tator slots, for this heating did not appear when running with normal voltage without load. Further investi- gation showed that this was apparently due to some flux dis- torting effect of the stationary conductors in the stator slots, which carried about 4000 amperes each at rated load. On account of ample margjxi in the magnetizing coils the air gap was then materially increased, with some benefit. A further improvement resulted in the use of magnetic wedges, made of cast iron, in place of the non-magnetic wedges used before. These wedges are illustrated in Fig. 11. This produced a further beneficial effect,, but there was still some extra heating in the pole face. Cylindrical grooves alternating \ in. and 1 in. deep UNI-POLAR GENERATOR 159 and about i in.wide, with a j in. web of steel between, were then turned in the pole face. The resultant pole face was there- fore crudely larninated, as shown in Fig. 12. Also, on account of an apparent local heating of the metal bridge over the rotor slots, a narrow groove was cut in the closed bridge above each rotor slot, thus changing it to a partially open slot, as shown in the figure. This effectively eliminated the excess loss in the rotor pole face. This, however, led to another unexpected difficulty, which will be described later. After this trouble was cured, the short circuit test was con- tinued with a current of about 6000 amperes. After a con- siderable period of operation, a very serious difficulty in the operation of the machine began to show up, namely, trouble with lubrication. At first the lubrication was vaseline fed on to the rings by lubricating pads. This was apparently very effective for awhile, but eventually it was noted that slight sparking began, which, in some cases, would increase very rapidly and, in a comparatively short time, became so bad that the rings or brushes would be- come badly scored or blistered. Examination of the sparking ■brushes showed a coating of black " smudge " over the surface which seemed to have more or less in- sulating qualities. A series of tests then showed that when- ever sparking began, the contact drop between a brush and the collector ring was fairly high and this drop increased as the spark- ing increased. For instance, it was found that on good, clean surfaces, the voltage drop between the brushes and the ring might be 0.3 to 0.5 volt. As each brush carried about 250 amperes at full load, this represented 75 to 125 watts per brush. When this contact resistance rose to about one volt, noticeable sparking would begin-, the watts being, of course, proportionally higher, and when the contact drop became as high as two volts, representing about 500' watts per brush, very bad burning of the brushes and rings was liable to occur. A series of tests then showed that vaseline, or any other lubricating oil, would tend to form a coating over the brush contact and this coating would gradually burn, or be acted upon otherwise by the current, so that its resistance increased and the black smudge was formed which had more or less insulating qualities. Fig. 12 160 ELECTRICAL ENGINEERING PAPERS A great number of tests were then carried out with various kinds of lubricants and it was found that anything of an oil or grease nature was troublesome sooner or later, as the smudge was formed on the brush contact. Then graphite, formed into cakes or .brushes by means of high pessure, was tried on the rings and the results 'were very favorable compared with anything used before. In fact, the tests indicated that soft graphite blocks or brushes could furnish proper lubrication for the rings. The graphite is a conducting material, and a coating of it on the brush contact does not materially increase the resistance of the contact. This was supposed to have practically settled the question of lubrication and brush contact trouble, but ex- perience later gave an entirely new turn to this matter. While these tests were being carried on, a study of the ventila- tion of the machine was being made. Ths tests indicated that the end rings, that is, those next to the exciting coils, were con- siderably cooler than those near the center of the machine. However, as there were excessive losses and heating in the steel rings themselves, it was not possible to make any material im- provement until the rings were changed. The steel rings at one end of the rotor, and the bronze rings at the other end, were then removed and a second set of bronze rings was tried. These rings were specially treated in the manu- facture so that the elastic limit was very high, and they were put on much tighter than in the former case. The load tests were then continued and the excess losses were again measured at various loads. It was found that the losses were very small compared with those of the steel rings, thus verifying the former results. The temperatures of the rings were much lower than with the steel, but it was found that the heating of the rings waF unequal. It was finally determined that this unequal heating was due to the large external blowers which were driving the air over the rings in such a way as to heat those next to the center of the rotor to a much higher temperature than those at the outer ends. It was assumed at first that the air entering the axial holes through the core and blowing out between the rings as shown in Fig. 4, was more effective on the outer rings, and that this possibly caused the difference in temperatures. However, the radial holes at the outer ends were closed, and this made but little difference. The axial holes were then closed, and while the temperatures of the rings, as a whole, were increased, about the same difference as before was found between the end rings and the center ones. UNI-POLAR GENERATOR 161 It was then decided to remove the two large blowers to de- termine whether some other method of ventilation would be more effective. When this change was made the windage of the machine was greatly reduced and there was greater uni- formity in the temperatures and the average temperature of the rings was only about 10 deg. higher than with the fans. More- over, the windage loss was only about one-seventh as great as before, although the average temperature rise was not much higher, which indicated that the ventilation through the rotor holes was much more effective than that due to the blowers. In consequence, it was decided to increase the size of the axial holes through the rotor core from 2| in. to 3f in. diameter, and to " bell-mouth " them at their openings at the ends, in order to give a freer admission of air to the holes. When this was done it was found that the temperatures of the rings were lower than in any of the preceding tests, and moreover, they were fairly uniform. Also after the removal of the blowers, the objection- able noise, already referred to, was largely eliminated, so that it was not disagreeable to work arqund the machine. The graphite lubrication was continued with the bronze rings, on this test, and no difficulty was encountered, although the machine was ope;-ated for very considerable periods at approximately 8000 amperes. On the basis of these tests, the machine was shipped to its destination and put in service. Then the real difficulties began — difficulties which were riot encountered in the shop tests, princi- pally because the conditions under which the machine operated 'in service were radically different from those at tlje shop, and .also, because the shop test had not been continued long enough. This machine was operated in service, although not regularly, for a period of about two months, being shut down at times due to difficulties outside of the generating unit itself. However, this period of operation of the generator was suddenly ended by the stretching of one of the outer collector rings, which loosened it to such an extent that it ceased to rotate with the inner ring. This required the return of the rotor to the manu- facturer. This two- months' operation gave data of great practical value, and in consequence, a number of minor difficulties were eliminated in the repaired rotor. Upon the return of the rotor to the shop, an examination of the collector rings showed that the separate shrunk-on type of ring 162 ELECTRICAL ENGINEERING PAPERS was not practicable with any design of ring then at hand. There- fore, it was decided to make the collector rings in one solid piece with a very considerable wearing depth. This necessitated the removal of all the base rings and, in fact, it required a com- plete dismantling of the entire rotor winding. As the outer ring had loosened, there was a possibility of the base rings loosening in the "same way, and therefore it was considered necessary to apply some scheme for preventing this loosening in case of sudden heating and expansion of any of the collector rings. It was then decided to apply some form of spring support underneath these rings, which could follow up any expansion in such a way as to keep the rings tight under any temperature conditions liable to be met with in practice. The spring support used consisted of a number of flat steel plates arranged around the rotor core, as indicated in Fig. 13. These plates were of such length and stiff- ness that a very high pressure wa.s required to bend them down to conform with the rotor surface. These plates were arranged around the rotor core and drawn down with clamp rings until they fitted tightly against the mica. The collector ring was highly heated and slipped over the springs, the clamps being removed as the ring was slipped on. Tests were made to find at what temperature such a ring would loosen. While the best arrangement without springs would loosen at about 100 to 125 deg. cent., it was found that a ring supported, in the above manner, was still fairly tight at 180 deg. cent., which was far above any temperature which the machine would attain under any condition. It may be said here that, after several years' operation, this construction still appears to be first class, and no loosening of any sort has occurred. In removing the winding from the rotor, it wa's discovered that the insulating tubes over the rotor conductors had traveled back and forth along the rods a certain amount. This travel, if continued for a long enough period, would apparently have injured the insulation, although no trouble had yet developed. Apparently, during heating and cooling, the expansion and con- traction of the rods would carry the tubes with them lengthwise a very- small amount. The tubes would then seat themselves ift Fig. 13 UNI-POLAR GENERATOR 163 the supporting rings or core and would not return to their original positions. It was found that in the slotted pole face already described, the webs or laminations of metal overhanging the rotor slots would hold the tube when the rod was traveling in one direction, but would sometimes allow the tube to move slightly when the rod traveled in the other direction, so that there was a sort of extremely slow ratchet action taking place. It was evidently necessary to have the tubes fit rather tightly in the retaining or supporting holes in the rings and the core, and to have the rods fit rather loosely in the tubes. Also, it ap- peared that shellac or other " gummy " material on the inner surface of the insulating tubes, was harmful, for wherever shellac was, present the insulating tube always stuck to the rod and would tear at either side of such place. In consequence, the new set of tubes was made with a dry, hard finish on both the ouiside and the inside, and the inside surface was also paraf- FiG. 14 fined. This, when carried out properly, served to remedy this trouble. The reconstructed rotor, with the solid collector rings, was shipped to the customer and the service was continued. After operation for a considerable time, certain extremely serious dif- ficulties appeared. One of these was brush trouble, and another was undue wear of the rings. The brush trouble was a most discouraging one. The machine was located in an engine room adjacent to a rock-crushing build- ing. Fine dust was always floating around the machine and, this dust continuously passing through the machine tended to form a deposit immediately behind the brushes as shown in Fig. 14. This dust packed in rather solidly behind the brush, due to the high speed of the rings, and eventually it tended to lift the brushes away from the rings. It also showed a tendency to get under the brush contact, with consequent increased resistance of contact. Frequent removal and cleaning of the brushes was 164 ELECTRICAL ENGINEERING PAPERS impracticable, as they were not sufficiently accessible to do this readily. This rock dust, packed behind the brushes, also had a scouring or grinding action on the rings themselves. Accompanying this was an undue rate of wear of the rings. This, however, was not entirely mechanical wear, as it appeared also to be dependent upon the current carried and was, to some ex- tent, due to a burning action under the brush which tended to eat away the surface of the rings. However, while -the undue wear was not altogether due to dust back of the brushes, yet this accumulation of dust appeared to have a very harmful action on the machine. Various methods were considered for overcoming this collection of dust, one of which consisted of enclosed air inlets to the machine, fitted with screens for sifting out the dust. This lessened the trouble to some extent, but it was evident that it would not cure it entirely, as the entire machine was so located that dust could come in around the brush holders without going through the ventilating channels. The method finally adopted for overcoming the difficulty of accumulation of dirt was rather startling. It was casually sug- gested that the copper leaf brushes be turned around so that the rings would run against the brushes, so that the dirt or dust over the rings would be " skimmed off " by the forward edge of the brushes. This obviously would prevent the collection of dirt, but the question of running thin leaf copper brushes on a collector ring operated at a speed of about 220 feet per second (or 13,200 feet per minute) looked like an absurdity to any one with experience in electrical machinery, so that we all hesitated at first to consider the possibility of it. However, as something had to be done, the writer suggested to the engineer in charge, that he change the brushes on one of the rings so that they would be inclined against the direction of rotation. This gave no trouble and the other brushes were then changed to the same direction and the operation ever since has been carried on with this arrangement. To the writer this has always seemed an almost unbelievable condition of operation, but as there has not been a single case of trouble from this arrangement during several years of operation, one is forced to believe that it is all right. This change entirely overcame the trouble from accumu- lation of dirt. However, it did not entirely cure the burning of the brushes and rings above described, but rendered the matter of lubrication somewhat easier than at first. As to the other serious trouble, it was mentioned that there UNI-POLAR GENERATOR 165 was a burning action under the brushes which tended to " eat " or " wear " away the surface of the rings. This also tended to burn away the brush surfaces, the amount of burning in either case depending, to a considerable extent, upon the direction of the current. ' At one side of the machine the brushes would wear more rapidly, while at the other side the rings would wear faster. The polarity of the current was influential in this action. Particles of the metal appeared to travel in the direction of the current; that is, where the current was from the ring to the brushes, the ring would wear more rapidly, while the brush would show but little wear, while at the other end of the machine, the opposite effect would be found. How- ever, the particles of metal taken from the ring did not deposit, or " build up," on the brushes. During all this operation, graphite had been used for. lubri- cation. In the earlier stages, powdered graphite compressed into blocks, had been used. Later it was found that very soft graphite brushes in insulated holders would give ample lubri- cation for the rings. However, even with this lubrication and the removal of the dirt trouble, there was still an appreciable burning of the brushes and rings as indicated by the more rapid wear of the rings at one end of the rotor, and of the brushes at the other end. Extended tests showed that this burning was a function of the contact drop between the brushes and the rings. Neither the rings nor the brushes would burn appreciably if the contact drop between the brushes and the ring could be kept very low. When this drop became relatively high fabout one volt), the rings or brushes would show an undue rate of wear. It was found also that, after a considerable period of operation, it was very difficult to obtain a low brush contact drop, as the brush wearing surface became coated with a sort of " smudge," which seemed to have resisting qualities. An analysis of this coating showed a very considerable amount of zinc in it, and it was determined that the zinc in the collector rings was burning out and forming an insulating coating on the brush contacts. The remedy for this condition was the application of some clean- ing agent which would chemically act on the smudge and dis- solve it or destroy its insulating qualities. The right material for this purpose was found to be a weak solution of muriatic acid — about 4 per cent in water. When this was applied to the rings by means of a " wiper," at intervals, the brush contact drop could be reduced to a very low figure — frequently to 0.1 or 0.2 of a volt, 16C ELECTRICAL EXGIXEERING PAPERS and the rings would take on a very bright polish. Also, while this low contact drop was maintained it was found that the nngs showed an almost inappreciable rate of wear. However, one set of rings continued to wear somewhat faster than the other. This difficulty of unequal wear of the two sets of rings was over- come by arranging a switch so that the polarity of the two ends of the machine could be changed occasionally. The temperature of the machine was reduced by the above treatment of the rings. Obviously, part of the heat was due to the loss at the brush contacts, which, of course, was reduced directly as the contact drop was reduced. The machine was now running quite decently with compara tively heavy loads, from 7000 to 10,000 amperes, and the only trouble was in several minor difficulties which were then taken up, one at a time, in order to ascertain a suitable remedy. These difficulties, however, were not interfering with the regular operation of the machine. One of tke difficulties which finally developed was due to stray magnetic fluxes through the bearings. These fluxes, pass- ing out through the shaft to the shell of the bearing, consti- tuted, in themselves, the elements of a small unipolar machine, of which the bearing metal served as collecting brushes. The e.m.f . generated in the shaft was a maximum across the two ends of the bearing. Consequently the current collected from the shaft by the bearing metal "should have been greatest near the ends of the bearing, and least at the center. This was the case as . indicated by the appearance of the bearing itself, which showed evidence of pitting hear the ends but none at the center. To remedy this trouble, a small demagnetizing coil was placed outside the stator frame, at each end of the rotor, between the Totor core and the bearings. These coils were excited by direct current which was adjusted in value until practically zero e.m.f. was indicated on the shaft at the two ends of each bearing. This indicated that the unipolar action was practically eliminated. This arrangement has been in use ever since it was installed, and no more trouble of any sort has been encountered from local currents in the bearings or elsewhere. Some of the brushes did not show as good wearing qualities as desired and various experiments were made with different combinations of materials and various thicknesses and arrange- ment of the brush laminag. Brass leaf brushes were tried; also, mixtures of copper, brass, aluminum and various other leaf metals UNI-POLAR GENERATOR 167 in combination. None of these showed any better than the thin copper leaf brush. The tests finally showed that, such a brush, very soft and flexible, with a suitable spring tension, would give very satisfactory results. Also instead, of two brushes side by side, a single brush, covering the full width of a ring, was found to be more satisfactory. Some tests were also made with carbon brushes, consisting of a combination of carbon or graphite combined with some metal, such as copper, in a finely divided state. These brushes were claimed to have a very high carrying capacity and also to have a certain amount of self -lubrication. A set of these brushes was tried on one of the rings, but lasted only for a very short time. The apparent wear was rapid, but it is not known whether this was due to the very high speed of the collector rings, or rapid burning away of the brush or Ahe inability of this type of brush to quickly follow any inequalities of the collector rings. This test was abandoned in a comparatively short time. After getting rid of the old troubles, a new and unexpected one had to appear. For some unknown reason, the insulating tubes on the rotor conductors began to break down; also grounds oc- curred between the collector rings and the core. On account of the delay required in making any changes in the rings or rotor winding, the customer arranged with the manufacturer to have a new rotor built as a reserve, as it was obvious that sooner or later there would have to be considerable reconstruction of the insulation on the first rotor due to unex- plained short circuits and grounds. A new rotor was at once constructed, embodying all the good features of the first rotor, with some supposedly minor improvements. The old rotor was then "removed for investigation and repairs. The cause of the breakdowns of the insulation on the tubes was then discovered. The air entering through the axial rotor holes and passing out through the radial holes between the rings, carried fine particles of cement or crushed stone dust and this had " sand-blasted " the under side of the tubes. When the rotor had been operated during the preliminary two months' period, previously described, before the replacement of the rings, no evidence of this sand- blasting had been visible. Investigation showed that the in- sulating tubes in tlie former winding had been made with a fuller- board base, which is rather soft and fibrous in its construction. The tubes on the second winding had been made with " fish " paper instead of fullerboard, in order to give a hard finish on the 168 ELECTRICAL ENGINEERING PAPERS - \___ ^r - inside and outside. It was due to this hard material that the troubles from sand blasting occurred. However, fish paper tubes were superior to the fullerboard in strength and other qualities, and as they were inferior only, in this one character- istic, they were used again in rewinding the rotor, but where- ever the tubes were exposed in passing from one ring to the next, they were taped over with several layers of soft tape whch was also sewed. This gave a soft finish which would resist sand- blasting, and no trouble from this source has occurred for several years. From the breakdowns to ground, it was evident that an entire replacement of the rings was necessary in order to repair the mica bush or sleeve lying beneath the rings. This required the removal of the entire. rotor winding and rings. It was found that cement dust coming up through the radial holes had sifted in through various crevices- or openings around the holes and that, finally, con- ducting surfaces and paths were formed which allowed the cufrent to leak to ground sufficient -to eventually burn the insulation. Therefore, when replacing the mica sleeve over the rotor, extra care was taken to fit insulating bushings at the top of the radial holes in such a way as to seal or -close all- joints, thus allowing no leakage paths between collector rings and the body of the cofe. This is shown in Fig. 15. No further trouble has occurred at this point. In removing the collector rings for these repairs, it was found that the flat spring supports shown in Fig. 13 had been entirely effective and there was no evidence whatever of any disturbance of the rings on the core, and there was no injury to the mica, such as would be shown by any slight movement. The rings were also very tight so that it took a very considerable temper- ature to loosen them sufficiently for removal. In view of the delay and expense of repairing one of these rotors when the collector rings had to be removed, with the pos- sibility of damaging the insulating tubes over the conductors, and the insulating bush over the core, it was then decided that a movable wearing ring was practically necessary in order to make this machine a permanent success. Therefore, the problem of a separate outside wearing ring, as originally con- FiG. 15 UNI-POLAR GENERATOR 16C templated, was again taken up. The difficulty, already de scribed, of the zinc burning from the rings and forming a coatinj on the brushes, indicated that some other material, withou' such a large percentage of zinc, should give better results. Thi difficulty was to obtain such a material, with suitable charac teristics otherwise. All data at hand showed that rings, witl desirable characteristics electrically, did not have the propei elastic limits, or proper expansion properties when heated. Ii other words, when such rings were shrunk on the base or sup porting ring they would stretch to such an extent, when cooled that they would become loose again with very moderate in crease in temperature. The solution of this problem of a separati ring construction was found in the use of some spring arrange ment underneath the outer ring which would still keep it tigh on the inner ring even when hot. The spring arrangemen Fig. 16 used under the inner rings, as shown in Fig. 13 was then appliec with certain modifications. In order to get good contact be tween the inner and outer rings for carrying the current, eacl of these steel springs or plates was covered by a thin sheet o copper as shown in Fig. 16. While each copper she3t was o comparatively small section, the large number of springs usee gave sufficient total copper to carry the current from the oute to the inner or base ring without any danger of current passini through the spring plates themselves. This arrangement wa used in reconstructing this rotor and has proven entirel; successful. In order to determine the effects of various materials withou zinc, or with but a small quantity of it, a number of rings wer fitted up on a test rig and were operated for long periods witl currents, up to 12,000 amperes in some cases. In these tests 170 ELECTRICAL ENGINEERING PAPERS four different kinds of material were used, all of them representing different mixtures of copper with a small percentage of other materials but with little zinc in any of them. It was feared that copper brushes on the copper rings would not work satisfac- torily, but while there was apparently some difference between the action of the different rings, it was found that copper brushes running on copper were, in general, satisfactory. The brushes were in-clined against the rings, as in the actual machine, during this series of tests. These tests were carried through with various numbers of brushes, etc. It was found that the number of brushes could be reduced to about one-third the full number, and still collect the total rated current, but that any great reduction from the full number of brushes made the operation of the rings and brushes more sensitive, and more attention was required to keep them in perfect condition. It was also found that any hardness or undue "springiness" in the brushes, or brush material, would tend to give increased wear. Brushes of very thin leaf copper, eventually gave best results. It was also shown by these tests that if a very good polish could be maintained on the rings, the rate of wear from day to day was practically unmeasurable on account of its smallness. As a result of these ring tests, the rotor undergoing repair was equipped with outside copper wearing rings, spring sup- ported. The material in the rings was about 92 per cent pure copper, 2 per cent zinc and 6 per cent tin. The rotor was then installed in service and has been operating steadily for several years, with entire success. The other rotor, which had been operating while this rotor was being repaired, was then thoroughly examined after removal, to determine any possible defects. It was noted that the insulating tubes over the rotor conductors' were badly cracked or buckled in a number of places. Upon removal of the rods or conductors it was found that the insulating tubes were stuck so tightly to the copper rods that they would be torn in pieces in trying to remove them. As it had been intended that these tubes should move freely on the rods or conductors, as previously described, it was evident that there was something radically wrong. The true cause of the trouble was then discovered. In first fitting this set of tubes over the rods, they had been too tight, and, in order to make them fit easily, the men who assembled the machine had reamed them on the inside to enlarge UNI-POLAR GENERATOR them, and, in doing so, had cut away the inner hard shee fish paper which had formed the lining, thus exposing a shells surface. As soon as heated, this shellac stuck the tube to rod so that there could be no possible movement between two. In consequence, when the rods expanded or contrac the tubes moved backward and forward in the supporting he and wherever they stuck fast in the outer holes, something to give, so that eventually the tubes buckled or cracked or pu open. This was readily remedied by putting on new tubes pi erly constructed. As the rings on this rotor were in very g condition with but little worn away, the removable type ring was not added, as this would require turning off a h amount of effective material on the existing rings and replac it with new outer rings. It was decided that as there was sev years' wear in the old rings, it would be of no material advant to throw this away when it could be worn away in serv just as well as it could be turned off in a lathe. After the ri in this machine are worn down the permissible depth, they be refilled by the addition of the removable type. This unipolar generator has now been in service for qi a long period, with no difficulty whatever, and with an aver ring wear of less than 0.001 in. per day, or less than f in. year. This may seem like an undue rate of wear; but in rea it is an extremely low rate, if the high peripheral speed, and number of brushes, are considered. This machine open day and night, seven days in the week, and practically con uously during the entire year. Taking the peripheral sp into .account, the above rate of Wear represents a total trs of each ring of about 3.6 million miles for each inch depth of w< or about 150 times around the earth along a great circle. C sidering that there are brushes bearing on each ring at inter\ of about eight in., a wear of one in., for every 3.6 million m traveled, does not seem unduly large. If, at the same time is considered that the brushes are collecting from 7500 to 10,i amperes from each ring on a total ring surface of about 3i wide by 42 in. diameter, it is not surprising that there sho be more or less " wear " due to the collection of this currc In fact, the current collected averages from 16 to 20 ampe per square inch of the total ring wearing surface. This may compared with standard practice with large d-c. commutatc in which 1^° 2 amperes per square inch of commutator f is usual and 3 amperes is extreme. 172 ELECTRICAL ENGINEERING PAPERS On account of the final success of this machine, the story of its development is a more pleasant one to tell than is the case in some instances where entirely new types of apparatus are undertaken. It might be said, after reviewing the foregoing description, that many of the troubles encountered with this machine could have been foreseen; but such a statement would be open to question, for the engineers of the manufacturing company were in frequent session on all the various phases and difficulties which developed. The writer knows that in many cases, after any individual trouble was known, suggestion for remedies were not readily forthcoming. The writer does not know of any individual machine where more engineering and manufacturing skill was expended in endeavoring to bring about success, than was the case with this machine. As an example of engineering pertinacity, this machine is possibly , without a rival. A mere telling of the story cannot give more than a sHght idea of the actual fight to overcome the various difficulties encountered in the development of this machine. The results obtained were valuable in many ways. Many data were obtained which have since been of great use, both from a theoretical as well as a practical standpoint, in other classes of apparatus. Certain fundamental conditions encountered in this machine have led to the study of other allied principles which point toward possibilities in other lines of endeavor. Therefore this machine, which was very costly in its develop- ment, may eventually pay for itself through improvements and developments in other lines of design; The writer wishes to say a good word for the purchaser of this new apparatus. He was long-suffering, and was undoubtedly put to more or less trouble and inconvenience, but nevertheless he gave opportunity to correct difficulties. He recognized that the engineers were confronted with a new problem in this ma- :h)ne and he gave them an opportunity to carry it through to success. Apparatus of this type could only be developed to full succsss in commerical operation, as all the difficulties en- countered would never have been found on shop test. There- fore, the attitude of the customer was of prime importance in the development of such a machine. COMMUTATING POLES ON ROT ARIES COMMUTATING POLES IN SYNCHRONOUS CONVERTERS FOREWORD — ^About 1909, the use of commutating poles in synchronous converters was being studied. . Suggestions were made from time to time that our usual slow speed rotary converters should have interpoles. The author, therefore, prepared a short article, explaining wherein commutating poles would be of less value to rotary converters, of the then usual speeds and constructions, than they wotild be on direct-current generators. In the latter part of September, 1910, the Papers Coifimittee of the Amer- ican Institute of Electrical Engineers unexpectedly found themselves without a paper for the November meeting. Mr. P. M. Lincoln suggested to the Chairman of the Papers Committee that possibly Mr. Lamme might have some material on hand which could be gotten ready on short notice. In answer to a telegram, a rough draft of this paper was forwarded to the Meet- ings and Papers Committee , to determine whether it would be suitable maberial. It was at once approved and the writer was authorized to complete it for the November meeting. He called to his aid Mr. F. D. Newbury who helped to put it in better shape and who added about one-half more, covering a descrip- tion of existing types of rotaries, etc. Most of this latter part, which appeared in the original paper, has been omitted from this reprint. The author's dis- cussion has been added because it forms a technical ctwitinuation of the paper itself and brings out more clearly that the real need for commutating poles in rotary converters would come with higher speed, which has proven to be the case in later developjnents. Attention is called to the reference in the discussion to the effect of the dampers acting as short-circuited coils surrounding the commutating poles, thus rendering them sluggish in action. ~ Later experience has shown that this damping action, while possibly more or less harmful, is less serious than the effects of momentary hunting due to sudden changes in load. It has been found that the most complete form of damper, namely, the one shown in the illustration, in the discussion, is most effective in lessening the above effects of hunting. Consequently the construction eventually adopted has been a choiie between two evils, the sluggishness caused by the dampers being ac- cepted as the lesser. As this paper was written before the term "commutating pole" was adopted as standard, the term "interpole" has been used throughout. — (Ed.) Synchronous converters with interpoles have been used but little in this country, but they have been built to some extent in England and on the continent of Europe, principally by com- panies which are either directly connected with or very closely allied to the companies which have manufactured the great bulk of the converter apparatus installed in this country. Consider- ing that interpole generators and motors came into extensive use in this country at about the same time as in Europe, the question would naturally be raised why interpole converters have not come into similarly extensive use, especially as the principal designers of converters in this country are in direct touch with the designers of the commutating pole converters in Europe. The reply might be that the introduction of any new type of apparatus is a relatively slow process; but, on the other hand, interpoles on direct current generators and motors came into general use in a relatively short time, especially so in railway motors. This indicates that there has been a more or less pressing need for interpoles in certain classes of apparatus and the greater the need for the change the quicker was the change made. Any important change in design or type must be justified 174 ELECTRICAL ENGINEERING PAPERS by engineenng and commercial reasons, such as improved per- formance greater economy, or lower cost. In the railway motor, placed under the car, and more or less inaccessible, improved operation at the brushes and commutator, when equipped with interpoles, represented a pressing reason for the change in type, although the cost and efficiency were not appreciably changed. In the direct-current generator with the modern tendency toward higher speeds with lower cost, the interpoles represented a practical necessity. TThis has been recognized for several years and the change to the interpole type has been made as rapidly as circumstances will permit. Also, in variable-speed direct- current motors ijiterpoles have been in general use for a number of years, simply because the interpoles represent a very definite improvement in a number of ways. New types of apparatus should only be introduced where they represent some distinct improvement or advance over existing types. Where a new type does not represent such improvement and is simply introduced to gratify a personal whim of the purchaser, or desire on the part of a manufacturing company to produce something different from other companies, the new apparatus, as a rule, will not advance quickly into public favor since there is no real necessity for it. It is therefore a question whether the slowness in the introduction of interpoles in synchronous converters is due to lack of sufficient advan- tages, or American engineers do not sufficiently appreciate ^,^^S^^£^^ their advantages. There ap- pears to be room for wide differences in opinion on this subject. The synchronous converter and the direct- current generator are two quite different machines, in ' Fig. 1 their characteristics, and no one can say off hand, that interpoles will give the same results in both. In the following is given a partial analysis of the condi- tions occurring in the two classes of machines, which wilt indicate wherein interpoles are of greater advantage on direct-current generators than on converters. COMMUTATING POLES ON ROT ARIES 17! Taking up first, the direct current generator, it may be considered as containing two sets of magnetizing coils, namely, the armature and the field windings. Considering the armature winding alone, the magnetoniotive force of the armature winding has zero values at points midway between two adjacent brusli arms or points of collection of current and rises at a uniform rate to the point of the winding which is in contact with the brushes, T^his is illustrated in Fig. 1, Therefore the armature winding has Its maximum magnetizing effect or magnetomotive foi^ at that part of the core surface where the winding is directl; in contact with the brushes. However, the magnetic flux- se up by the armature winding will not necessarily be a maximun at this point, as this depends upon the arrangement of the mag netic or other material surrounding the armature. If this poin occurs midway between two field poles, then, while the mag netizing effect is greatest at this point, the presence of a larg air-gap at this same point may mean a relatively small magneti flux, while a much higher flux may be set up by the arinatur winding at the edges of the adjacent field poles. In the usua direct-current generator construction without interpoles, th position of commutation is almost midway between two adjacen poles and therefore the point of maximum magnetomotive force of the armature is also practically midway between poles The absence of good magnetic material over the armature a ■ this point serves to lessen the magnetic flux due to the armatur magnetizing effect, but even with the best possible proportion there will necessarily be a slight magnetic flux set up at thi point. While this field is usually of small value, yet unfor tunately it is of such a polarity as to have a harmful effect on th commutation of the machine. During the operation of commu tation, the coil which is being commutated has its two terminal short-circuited by the brushes. If this short circuited coil a this moment is moving across a magnetic flux or field, it wil have an e.m.f. set up in it which will tend to cause a local o short circuit current to flow. Such a current is set up by th flux due to the armature magnetomotive force described abov and unfortunately this current flows in such a way as to give th same effect as an increased external or working current to be re versed as the coil passes from under the brush. In other words the e.m.f. set up in the short circuited coil by the above fiel adds to the e.m.f. of self induction in the coil due to the reversE of the working current. 176 ELECTRICAL ENGINEERING PAPERS Another cause of difficulty in the commutation of a direct current machine is the self induction of the armature coils as they individually have the current reversed in them in passing from one side of the brush to the other. Each coil has a local magnetic field around itself, set up by current in itself and its n-eighboring coils. The value of this local magnetic field depends upon the arrangement of the winding, the disposition of the magnetic structtire around the coil, the ampere turns, etc. During the act of commutation, that part oi the local field due to the coil which is being commutated must be reversed in direction. It is therefore desirable to make the local field due to any individual coil as small as possible. This means that the number of turns per coil should be as low as possible, the amperes per coil also should be as small as possible, while the magnetic conditions sur- rounding the coil should be such as to give the highest reluctance. By the proper arrangement of the various parts, it is usually found that the e.m.f. of self induction, due to the reversal of the coil passing under the brush, can be made of comparatively small value so that, if no other conditions interfere, good commutation could be obtained under practically all commercial operating conditions. However, the magnetic field between the poles set up by the armature magnetomotive force as a whole, as described above, adds very greatly to the difficulties of commutation. If the armature magnetomotive force, or the field due to it, could be suppressed, then one of the principal limitations in the design and operation of direct-current generators would be removed, and the commutation limits would be greatly extended. Or, better still, if a magnetic flux in the reverse direction were estab- lished at the point of commutation, then the e.m.f. set up by this would be in opposition to the e.m.f. of self induction of the commutated coil and would actually assist in the commutation. This latter is what is accomplished by interpoles. When these are used the brushes on the commutator are so placed that, the short circuited or commutated coils are directly under the inter- pole. Consequently, the maximum magnetomotive force of the armature is in exact opposition to that of the interpoles. There- fore, the total ampere turns on the interpoles should be equal to the total ampere turns on the armature in order to produce zero magnetic flux under the interpole or at the point of commutation But, for best conditions there should not be zero field, but, a slight field in the opposite direction from that which the arma- ture winding alone would produce. Therefore, the magneto- COMMUTATING POLES ON ROT ARIES 17 motive force of the interpole must be greater than that of th armature by an amount sufficient to set up a local field- unde the interpole which will establish an e.m.f . in the short circuit& coils opposite to that set up by the commutated coils themselve and practically 'equal to it. The excess ampere turns require on the commutated poles is therefore for magnetizing purpose only and the amount of extra ampere turns will depend upon th value of the commutating field required, depth of air-gap unde the commutating pole, etc. The commutating field required i obviously a function of the self induction of the commutated co and evidently the lower the self induction the less commutatin field will be required. It is evident therefore that the commt tating field under the commutating pole bears no fixed relatio to the armature ampere turns or to the main field ampere turni but is, to a certain extent, dependent upon the proportions c each individual machine. It is evident that the magnetomotive force of a given arms ture varies directly with the current delivered, regardless of th voltage. Therefore, that part of the interpole magnetomoti'v force which neutralizes that of the armature should also var directly in proportion to the armature current. Also, the se induction of the commutated coils will vary in proportion to th armature current carried, and therefore the magnetic field unde the interpole for neutralizing this self induction should als vary in proportion to the armature current. It is therefor obvious that if the main armature current be put through th interpole winding, the magnetomotive force of this winding wi vary in the proper proportion, to give correct commutating cor ditions as the armature current varies, regardless of the voltag of the machine. This is on the assumption that the entii magnetomotive force of the interpole winding is effective at th air gap and armature, which implies an absence of saturation i the interpole magnetic circuit. In the usual construction, th interpole winding always carries the main armature currer as indicated above. One consequence of the use of the interpole is that somewhc less regard need be paid to keeping the self induction of th commutating coil at its lowest value. In consequence, there : somewhat more freedom in proportioning the armature windinj slots, etc., than in a non-interpole machine, and advantage can t taken of this in bettering the proportions for other characteristic 17S ELECTRICAL ENGINEERING PAPERS The conditions of design are therefore not as rigid in the interpole as in the non-interpole type. The above description of the interpole generator has been gone into rather fully, as many of the points mentioned tvill be re- ferred to again in connection with interpoles on synchronous converters. The synchronous converter differs from the direct current generator in one very important particular, namely, it may be considered as motor and generator combined. It receives cur- rent from a supply system the same as a motor and it delivers current to another system like a direct-current generator. The magnetomotive force of the armature winding as a motor acts in one direction, while the magnetomotive force of the armature winding as a generator acts in the opposite direction. As the input is practically equal to the output, it is evident that these two armature magnetomotive forces should practically neu- tralize each other, on the assumption that the armature mag- netomotive force, due to the polyphase current supplied has practically the same distribution as- that of the corresponding direct-current winding. Assuming that the two practically balance each other, then it is evident that one of the principal sources of commutation difficulty in direct current generators -43-75-15° Fig. 2 is absent in the converter and therefore the Umits in commuta- tion should be much higher than those of direct-current ma- chines. COMMUTATING POLES ON ROT ARIES 179 The following diagrams show the distribution of the alternating- current aiid direct-current magnetomotive forces on a six-phase rotary converter. The magnetomotive force distribution for the alternating-current input is plotted for several different positions of the armature. Three different positions are shown with the armatures displaced successively 15 electrical degrees. The general forins of these distributions repeat themselves for further similar displacements. These distributions are illustrated in Figs. 2^ 3 and 4. It is evident from these three figures that the peak value of the mag- netomotive force the armature varies as the armature is rotated as indicated by the heights of the center line in the three figures In Fig. 5, the magnetomotive force distribution of Fig. 2 anc the corresponding direct-current distribution of Fig. 1 are botl shown, but in opposition to each other. In this figure both are shown in proper proportion to each other, taking into accouni the alternating current amperes and the direct-current amperes output. The resultant of these two distributions is also indi' cated in these figures. In Fig. 6 the distributions correspond to Figs. 3 and 1 combined and the resultant is also shown. Fig. 7 combines Figs. 4 and 1. -^"-^"-o" h/ ^L i \ 1 \ J- w ■- Y^- --X- 1 1 \ y / V Fig. 4 Fig. 5 It is the resultant magnetomotive force in these three figurei which is important, as this is the effective magnetomotive forc( which tends to produce a flux or field over the commutatec 180 ELECTRICAL ENGINEERING PAPERS coil. It is evident from these figures, which are drawn to scale, that this resultant varies in height as the armature is rotated, but the maximum is only a relatively small per cent of the direct- current magnetomotive force. Therefore, it is obvious that one of the principal sources of difficulty in the commutation of the direct-current generator is practically absent in the converter, and it is also evident from this that the commutating conditions in the latter should be materially easier than in the former. This has proved to be true by wide experience in the construction and operation of converters. In the above figures the magnetomotive forces have been plotted to scale on the following basis: The six-phase converter winding is connected to three trans- formers with the so-caUed diametral arrangement; each of the three secondaries is connected across the diameter, or across 180 deg. points on the winding, the three diameters being dis- placed 60 deg. with resiJect to each other. Assuming the direct current in the winding as A, then the maximum value of the alternating current in. any one phase of the alternating-current end wUl be equal to f ^, or 0.667 A, assuming 100 per cent efficiency. However, as the alternating-current input must be somewhat greater than the direct-current output, due to certain Fig. 6 Fig. 7 losses in the machine, it is evident that the maximum alter- nating current in any one phase must be somewhat greater than 0.667 A . The field copper losses may be considered as part o£ COMMUTATING POLES ON ROTARIES 181 the output of the rotary. The armature copper losess maybe considered as due to an ohmic drop between the counter e.m.f. of the armature and the transformer e.m.f., and simply a higher transformer e.m.f. must be supplied to overcome this drop and therefore it does not effect the true current input of the rotary. However, the losses due to rotation, such as iron loss and the friction and windage are excess losses which represent extra current which must be supplied to the alternating-current end of the rotary. These rotational losses will usually be relatively small in a 25-cycle converter, being possibly 4 per cent or ,5 per ceiit in a small machine and 1^ per cent to 3 per cent in a large machine. In the 60-cycle converters, where the iron losses are relatively higher and the speeds are somewhat higher, giving greater friction and windage, the rotation losses may be con- siderably greater than on 25-cycle machines. Assuming these rotation losses will be 3 per cent, then the maximum alternating current per phase = — '^r-^ — = 0.687.4. The foregoing Figs. 5, 6 and 7 are worked out on this assumption of 97 per cent rota- tional efficiency and on this basis of rninimum value of the resultant magnetomotive force of the armature at the direct- current brush is about 7 per cent of the direct-current magneto- motive force of the same word- ing, while the maximum value is about 20 per cent. The lower the rotational efficiency the smaller would be these values, and with a rotational efficiency of about ;3 89 per cent, the minimum result- ant would fall to zero, while the maximum value would be about 13 per cent. The resultant magnetomotive force of a synchronous converter might be compared with that of a direct-current generator with pjg g compensating windings in the pole faces. It is' generally known that such direct-ciurent generators have much better com- mutating conditions than ordinary uncompensated machines. If such compensating winding on the field of a direct-current i_f a „^ — ,^^^t.^^^^^■,^ j-'Urt «ri^,^irt »«».»■(-.. „ £ 182 ELECTRICAL ENGINEERING PAPERS then the armature reaction could be completely annulled, which is not the case in the converter. But with compensating wind- ings located only in the pole faces, then the armature magneto- motive force midway between the poles could not be completely annulled, unless over-compensation is used, and the resultant would be as shown in Fig. 8, which is not quite as good as the average resultant in the converter. The commutating con- ditions in the converter can therefore be considered as at least as good as in a direct-current generator with a compensating winding of normal value located in the pole faces only. In the application of interpoles to the synchronous converter the same principles should hold as in a direct-current generator, namely, the interpole magnetomotive force should be sufficient to neutralize that of the armature winding and, in addition, should set up a small magnetic flux sufficient to overcome the self in- duction of the commutated coil. As the magnetomotive force the armature varies between 7 per cent and 20 per cent shown in the above figures, it is evident that perfect compensation of this cannot be obtained and that therefore only some average value can be applied. Assuming that 15 per cent will be required on the average to compensate for this, then in addition the inter- pole winding must carry ampere turns sufficient to set up the small magnetic field for commutation. Thus the total ampere turns on the interpole will be equal to 15 per cent of the armature direct-current ampere turns plus a small addition for setting up the useful or commutating field. In the direct-current gen- erator, the ampere turns on the interpoles must equal the total armature ampere turns plus a corresponding addition for the commutating field. It is therefore evident that an interpole winding on a converter will natiurally be very much smaller than on a direct-current generator, and in general it is between 25 per cent and 40 per cent of the direct-current. In the pulsating resultant magnetomotive force in the con- verter there lies one possible source of trouble with interpoles Assume, for example, the total ampere turns on the interpoles are equal to 30 per cent of the direct-current ampere turns on the rotary and that 15 per cent of this is for overcoming the average value of the resultant magnetomotive force, then an average of 15 per cent will be available for setting up a commutating field; but, according to the above diagrams,' the resultant mag- netomotive force of the armature varies from 7 per cent to 20 per cent. With a total interpole winding representing 30 per cent. COMMUTATING POLES ON ROTARIES 18! then the effective or magnetizing part will vary from 30-7 to 30-20; that is, from 23 per cent to 10 per cent. The effective magnetomotive force therefore tends to vary over qtiite a wide range so that the commutating field would also tend to vary up or down over a ve!ry considerable range, which is an undesirable thing for commutation. However, as this pulsation is at a fairly high frequency it tends to damp itself out by setting up eddy currents in the structure of the magnetic circuit. If a good conducting damper or closed circuit were placed around the interpole, it is probable that this pulsation would be almost completely eliminated, but such a damper possesses certain dis- advantages, as will be shown later. In practice this pulsation of the armature reaction under the interpoles is apparently not noticeably harmful in most cases, as evidenced by the fact that well-proportioned interpole converters in commercial service show no undue trouble at the commutator or brushes. Due to the relatively small number of ampere turns required on the interpole of a converter compared with those required on a direct-current generator, the design of the interpoles in the two cases presents quite different problems. In the direct-current generator the interpoles carry ampere turns, which in all cases are greater than the armature ampere turns, as explained before. As the field ampere turns on the main poles are, not infrequently, but little greater than the armature ampere turns, it is evident that the interpole winding may, in some cases, carry as many ampere turns as the main field windings. While but a small per cent of these interpole windings is effective in producing flux under the pole tip, yet they are all effective in producing leakage from the sides of the' poles. As the interpoles are gen- erally small in section compared with the main, poles, and as they may carry ampere turns equal to the main poles, it is evident that the effect of leakage may be relatively great on the interpole. For instance, if the leakage on the main poles is 15 per cent of the useful flux, then, with the same total leakage on the inter- poles, this may represent a very high 'value compared with the useful flux, due to the small section of the interpole and the relatively low useful interpole flux. In consequence, it is con- siderable of a problem to proportion the interpoles of a direct current generator so that the leakage flux will not saturate the interpoles at some part of the circuit. If they saturate, then 184 ELECTRICAL ENGINEERING PAPERS part of the ampere turns on the interpole are expended in such saturation and the part thus expended must be counted off from the extra or excess interpole ampere turns.' If, for example, the interpole winding requires 100 per cent for overcoming the armature and there is 20 per cent extra ampere turns for setting up a useful flux, then any saturation in the interpole circuit must represent additional ampere turns -on the field, as the above 120 per cent is necessary for useful flux and for neutralizing the armature. With reduced current, and consequent lower satura- tion, these additional interpole turns become effective in mag- netizing the gap and thus the commutating flux is too strong. At greatly increased load, more ampere turns are required for saturation, and the commutating flux is altogether too weak. It IS thus evident that a machine with highly saturated inter- poles will not commutate equally well for all loads. Herein lies a problem in the design of interpole generators, as it is difficult to maintain a relatively low saturation in the interpoles due to their small section and high ampere turns which cause leakage. It is well known that in the main poles of the generator, a leakage flux which is higher than the useful flux is objection- able, from the designer's standpoint; arid yet in the use of inter- poles this is a normal condition rather than an exception. In the synchronous converter the conditions are somewhat different due to the fact that the interpole ampere turns are usually only 25 per cent to 40 per cent as great as on a correspond- ing direct-current ge^ierator. The leakage at the -sides of the poles becomes relatively much less, while the useful induction remains about the same as on the direct-current generator. In consequence, saturation of the poles is not so difficult to avoid. Iri some cases, due to the smaller ampere turns on the interpole winding, the interpole coils, can be located nearer the pole tip and thus the leakage can be further reduced. However, the placing of the interpole coil over the whole length of the pole is not as objectionable in the converter interpole as it is on the direct-current generator as the ampere turns are less. It is those ampere turns which are located close to the yoke, or furthest away from the pole tip, which produce the highest leakage, while those close to the pole tip usually produce much less leakage, but in interpole generators with their high number of ampere turns on the interpoles it is often difficult to find space for the interpole winding, even if distributed over the whole pole length. In some cases, a direct-current machine may be larger than COMMUTATING POLES ON ROTARIES 18, would otherwise be required, simply to obtain space for the inter pole winding. This is not true" to the same extent in the appli cation of interpoles to converters. In the above the leakage is referred to as a function of th interpole winding as if the main winding had little or nothin to do with it. The reason for this may be given as follows: Fig. 9 represents two main poles and an interpole of a direct current generator or converter, with their windings in place The direction of current or polarity of each side of each coil i also indicated by + or — ; It is evident that between theintei pole and one main pole, the interpole winding and the mai field winding are of the same polarity, while on the opposite sid of the interpole, these two windings are in opposition. Let .. equal the ampere turns of the interpole and B the ampere turn in the main coil. Then, A+B will represent the leakage amper turns at one side of the interpole and A - B will represent th leakage ampere turns at the other side. Therefore, the leakag at the two sides of the poles is represented by {A+B) + {A - E = 2 A; that is, the leakage could be considered as due to th interpole winding entirely and may also be considered as due t double the interpole turns acting as one side of the Interpol only. Another way of looking at this is to consider that th windings on the main pole produce leakage in the interpoles but the leakage due to one main pole acts radially in one direc tion in the interpole, while that due to the other main pole i in the opposite direction. Considering therefore the interpole. leakage as being due t the interpole ampere turns only, it is evident that the syr chronous converter will not b ^ '^ ^~^ troubled with saturation of the ir terpoles to the same extent as direct-current generator. With th same size of interpole it is eviden + 1 PiQ. 9 that the converter should be abl to carry heavier overloads than th direct-current generator before saturation of the interpoles i reached. It was mentioned before that a closed conducting circui around the interpoles .would be objectionable. This has beei proved by experience with interpole generators. It is eviden from the preceding analysis that the ampere turns on the inter pole of a direct-current generator should always rise or fall ii 186 ELECTRICAL ENGINEERING PAPERS proportion to the armature ampere turns in order to give best commutation, assuming, of course, no saturation of the poles. If the interpole turns are directly in series with the armature winding, with no shunt across the interpole winding, it is evident that the interpole ampere turns must vary in direct proportion to the armature ampere turns. However, if anon-inductive shunt, for instance, were connected across the interpole winding in order to shunt part of the current, then in the event of a sudden change in load, the interpole winding being inductive due to its iron core and the shunt being non-inductive, the momentary division of current during a change in load would not be the same as under steady conditions. In other words, if the armature and interpole current were suddenly increased, then a large part of the increase would momentarily pass through the non-induc- tive interpole shunt until steady conditions were again attained. In consequence, the interpole ampere turns would not increase in proportion to the armature ampere turns just at the critical time when the proper commutating field should be obtained. The same condition is approximated when a separate con- ducting circuit is closed around the interpole. A sudden change in the current in the interpole winding, causes a change in the flux, and secondary currents are set up in the closed circuit, which always act in such a way as to oppose any change in the flux, whereas, the flux in reality should change directly with the current. The above described non-inductive shunt across the interpole winding might be considered also as completing a closed circuit with the interpole winding, and therefore retarding secondary currents would be set up in this closed circuit with any change in the flux in the interpole. In some cases it may be impracticable to get exactly the right number of turns on the interpole winding to give the correct interpole magnetomotive force. For example, on a heavy current machine, 1.8 turns carrying full current might be' re- quired on each interpole. If two turns were used, with the extra, current shunted, the right interpole strength would be obtained. A non-inductive shunt, however, is bad, as shown above. However, if an inductive shunt is used, instead of non- inductive, and the reactance in this shunt circuit is properly adjusted, then it is possible to get the right, interpole strength for normal conditions and still obtain satisfactory conditions with sudden changes in load. Also, by arranging the interpole winding so that a very considerable percentage of the current COMMUTATING POLES ON ROT ARIES IS is shunted normally by an inductive shunt having a relatival high reactance compared with the interpole, it should be possibl to force an excess current through the interpole winding in cas of a sudden increase in load, in case a stronger commutatin field were needed at this instant. On the interpole synchronous converter a non-inductive shun across the interpole winding should act very much as on an intei pole generator and therefore non-inductive shunts are inac visable. If any shunting is required it should be by means of a inductive shunt in those cases where the current from the cor verter is liable to sudden fluctuations, as in railway servic( "Where the service is practically steady, a non-inductive shur should prove satisfactory for the interpoles of converters c direct-current generators. Under extreme conditions of overload current, that is, i case of a short circuit across the terminals, it is questionable t what extent interpoles are effective. It is practicable to desig interpoles on direct-current generators which will not undul saturate up to possibly three or four times normal load. How ever, in case of a sudden short circuit the current delivered b the machine is liable momentarily to rise to a value anywhei from 15 to 30 times full load current. With this excessive cui rent the interpoles of the direct-current generator must 'neces sarily be more or less ineffective. On account of saturation, th commutating flux under the interpole cannot rise in proportio to the current. However, there should still be some commutatin field present, which condition is probably considerably bette than no field at all, or a strong field in the opposite directio as would be found without commutating poles. Therefore, i direct-current generators with well-proportioned interpoles the conditions on short circuit are generally less severe than i iion-interpole machines. If the pole is highly saturated by the heavy current rush oi short circuit, then it is evident that a highly inductive shun1 as described above, which would increase the interpole curren in a greater proportion than the armature current, would simpl mean higher saturation with little or no increase in the usefu flux under the interpole. In the synchronous converter at short circuit the condition may be somewhat different. When the converter is short cii cuited it can also give extremely high currents, possibly mucl greater than the corresponding direct-current generator can give 188 ELECTRICAL ENGINEERING PAPERS Both the armature winding tied to an alternating-current supply system, and the presence of the low resistance dampers on the field magnetic circuit, tend to make the short circuit conditions more severe in the converter. The worst condition, however, would appear to be in the relation of the interpole ampere turns to the armature ampere turns on short circuit. As shown before, the normal ampere turns on the interpole winding will be only 25 pJ. PF(Xlog2JV ■K D sin 6 N = number of slots per pole. Ic = current per conductor. Wi = total armature wires. D = diameter' of armature. Let a b = h, and c d = m. Then the flux cut by one conductor at one end is ,D(*+i) X 2.15-J. W^tXlog 2N TT D SVnd Therefore the e.m.f. per single turn of the armature winding. 210 ELECTRICAL ENGINEERING PAPERS due to the end flux, considering the end fluxes for both ends oi the core, becomes -^ ('■+1) ^ 2.15 /. Wt Xlog 2N ttD RsX2 Tc X D sin 6 10« Or, £. = - 7, 1^; r,i?. ,4.3 (2/i + ot) 108 X sin d Xlog 2 TV This formula is on the basis of non-magnetic paths around the end windings, that is, with no bands of magnetic material and nc magnetic supports under the coils. The effect of bands over the end winding is approximately equivalent to cutting the flu? path to half length for those parts of the end winding covered by the bands. There- fore, with bands, the diagram representing flux density in the commutating. zone of the end winding would be as indicated in Fig. 8. In this case the total flux corresponds to the total area of the curve including the dotted portion Of course the actual flux distribution would not be exactly as shown in this diagram for there would be some fringing in the neighborhood of the bands The diagram simply serves to illustrate the general effect of magnetic bands and an approximate method ol taking it into account. The effect of a magnetic coil support will be very similar tc that of a steel band in reducing the length of path and therefore increasing the flux in the neighborhood of the coil support However, in case of magnetic bands over the winding and coi supports under it the limit lies in saturation of the bands them selves. This usually represents a comparatively small total flux The coil support, however, would probably not saturate in anj case. The above formula for end flux can therefore be correctec for magnetic bands and coil supports by multiplying by a suit able constant to cover the increased flu3i. It is obvious that the determination of the end flux is, to i Fig. 8 certain extent, a, mipstinn nf iiidcnnpTit and pirnpripnof". N( THEORY OF COMMUTATION 211 fixed method or formula can be specified for all types of machines, for this flux would be iiifluenced very greatly by the bands, if of magnetic material, and by the material, size and location of the coil supports and their relation to the bands. Also, eddy cur- rents may be set up in the coil supports which will influence the distribution of the end flux in the zone of the commutated coil. However, in each individual case an approximation can be made which will, in general, be much closer than would be obtained from any empirical rule or by neglecting the effect of the end flux altogether. Chord Winding. The effect of chording the armature winding is to slightly diminish the flux density in the commutating zone which results in a slight reduction in the e.m.f. of the commu- tating coil. But a relatively much greater gain is obtained by the consequent shortening of the distance c d in Fig. 8 and the corresponding reduction of the total end flux. Due to the chording itself the flux density at h is reduced practically in the ratio of -^—-k-^ , where Ni = number of slots spanned by the log 2 iV ^ ■' coil. For example, if the full pitch is 20 slots and the coil spans 18 slots, then the density 'at b will be reduced in the ratio of , ,^ =0.971 due to the chording itself; and the flux log 40 18 along c d, Fig. 7, will be further reduced in the ratio of ^ due to the shorter end extension. The average flux along c d therefore will be reduced to 0.9X0.971=0.874, or about 87 per cent of that of a pitch winding. Effect of Brush Width. As in the case of the interpolar flux the width of the brush, or the number of armature coils short cir- cuited by the brush, has practically no influence on the e.m.f. generated per turn. However, the total effective armature iampere turns will be reduced slightly, if the average current in the short circuited turns is less than the normal current. This will have a very sUght effect on the e.m.f. Slot Flux By this is meant the magnetic flux across and over the arma- ture slots which does not extend to the yoke or field poles. Two general cases will be considered; first, that in which no local currents are present, which is the case in well designed interpole machines; and second, that in which there are local 212 ELECTRICAL ENGINEERING PAPERS currents set up in the short circuited coils, which is almost invariably the case in machines without interpoles or some other form of compensation. Also, pitch and chorded windings will be considered. Slot Flux with No Local Currents Pitch Winding. Let Fig. 9 represent an upper and a lower coil in the same slot, with equal turns anS currents. Then if there is no saturation in the adjacent teeth the flux density across the slot will be zero at the bottom of the lower coil and will rise to a maximum value at the tOp of the upper coil. There will also be a flux across the slot above the upper coil and also from the top of the tooth as indicated in Fig. 9. The total slot flux entering at the bottom of the teeth is therefore equal to the total flux which crosses the two adjacent slots, plus the flux crossing at the top of the slots. The interpolar flux which ex- -1 f -'-' / - d ^ Fig. 9 Fig. 10 tends from the armature surface to the poles or yoke is not in- cluded in this. As this slot flux is practically fixed in position the armature conductor in slot A, in passing from a to b must cut this flux. It is obvious that the flux which crosses above the uppermost conductor in the slot is cut equally by all the conductors in the slot, as the coil passes from position a to position b; but the flux crossing the slot below the uppermost conductor does not affect all the conductors equally, and therefore, for simplicity of cal- culation, an equivalent flux of lower value can be used which may be considered as cutting all the conductors equally. Let d Fig. 10, represent the depth of the conductors of one complete coil. / represent the distance between the upper and lower coils. n r/'t\rpypnt thp HiQtfl-nr'f* fi-rtm fVi n iiT^,- J- 3.19 IcTcCsLX2Xa THEORY OF COMMUTATION 213 5 represent the width of the slot, assuming parallel sides. M represent the ratio of width of armature tooth to the width of the armature slot, a/ the surface of the core. Tc represent turns per single coil, or per commutator bar. Cs represent the number of individual coils, or commu- tator bars, per complete coil. L represent the width of armature core, including ventilating spaces. Ic represent the current per armature conductor. Then, ampere turns per upper or lower coil = /c Tc Cs. Total flux across coil space = Flux across slot above coil = Flux- from tooth top across the slot is approximately, S.ig/.r. GLX2X0.54 Vw Total flux above -upper coil = 3.19 Ic Tc Cs L '- The. sum of the two fluxes represents the total fl.Ux across one slot which enters at the bottom of one tooth. As a similar flux passes across the slot at the other side of the tooth the total flux entering the tooth will be double the above and becomes ^ o,^ r T^ ^ 7- (2 ^0J33_dJ:_ 2a+1.0 8W.) 5 The average value of the effective flux for the upper and lower coils then becomes, 3 19 /. r. G Z. (2:^7^+^:40 + 2,16^^) (This average effective value is approximately 80 per cent of the total slot flux.) On the basis of a pitch winding and the assumption that only- one armature coil is short circuited, that is, with the brush covering the width of only one commutator bar, then the above slot flux is cut by all the coils in the slot in passing through one slot pitch. From this the e.m.f in the commutating coil due to the slot flux can be calculated directly and may be expressed as follows ^c = — TT^j-^— X number of slots ^3^9,^^^^^^(M7i+i+4a+2,16Wi) But CsX number of slots = No. of commutator bars _ total number of conductors Wi Therefore the above expression for e.m.f. may be changed to the form, ^ _ Z.\%IcWtTcRsL (2.67rf+4a+i+2.16 5 V«) ^'- 10^ ■ S K it is desired to comoare this exnression with a. np.rt.a.i-n well THEORY OF COMMUTATION 215 the quantity in the parenthesis in the above expression be repre- sented by Cx- The formula can then be changed to,, 17 /o^xo in^x ^ ^. IcTc^ Xnumhev commutator barsXi?sXl. E = (2X3.19Xci)X TQg It contains the same terms (except in the value of the constant) for the expression of the e.m.f. which has been used heretofore in determining the reactance of the commutated coil. Effect of Brush Width or Number of Commutator Bars Covered iy Brush. The above formulas are on the basis of the brush covering only the width of one commutator bar. Iii this case all the conductors of one slot cut across the entire slot flux in passing through one tooth pitch. However, if the brush covers more than one commutator bar, then the full slot flux is not cut in passing through one tooth pitch, and a movement greater than one tooth pitch is required for full cutting. For example, if there is one commutator bar per armature slot and the brush covers a width equal to two commutator bars, then the total -cutting of the slot flux will take place in two tooth pitches. Again, if there are three commutator bars per armature slot and the brush covers the width of one commutator bar, then the total cutting of the total slot flux would occur in one tooth pitch, while if the brush covered two bars, the total cutting would occur in I5 tooth pitches; and if it covered three bars If tooth pitches are required. In other words, the total cutting will occur in a period corresponding to the number of commutator bars per ■slot plus one less than the number of commutator bars covered by the brush. On this basis the correction factor for the slot e.m.f. should be expressed by the term „ „^ — r-, where Cs = number of com- mutator bars per slot, and 5i = number of commutator bars spanned by the brush. However, vt^ith several coils per slot, and with the brush spanning several bars, the rate of cutting of the tooth flux for the entire period is not quite the same as the rate for one tooth pitch. Taking this into account the correc- C tion factor should not be equal to ^ . ' — ^ , but is slightly Cs+ -Di—l greater. Up to four commutator bars per slot, and three bars *See Note i on page 246. 216 ELECTRICAL ENGINEERING PAPERS spanned by the brush the correction factor can be expressed by 1 1 * the term 1+ „ .^ — -p;—- Taking the lengthened period of reversal into account, it would appear that a wide brush covering a large number of commutator bars should be beneficial in reducing the e.m.f generated by the slot flux. This is true where the local currents are very small, or are absent, as is the case in a properly designed interpole machine. In a non-interpole machine where the local currents in the short circuited coils may be relatively high, this condition does not hold, as will be explained later. The above formtda for e.m.f. due to the slot flux should there- fore be modified by multiplying by a factor which takes into account the period of reversal as affected by brush width. Chord Winding. The armature winding may be chorded one or more slots and, in some instances, where there are several coils side by side there has been chord- ing of part of the conductors in the slot. In Fig. 11 is illustrated the conditions with one-slot chording. The total slot flux now occupies two teeth instead of one. Therefore the e.m.f. set up by cutting across this slot flux will be approximately one- half that which is obtained with a full pitch winding, on the basis of the brush covering the width of one bar only, for the e.m.f generated by cutting this fliix will be reduced in proportion as the period of cutting is^ increased There is one slight difference from the flux distribu- tion with a pitch winding, namely, that at the top of the teeth. "With a chorded winding this flux will be slightly greater than with a pitch winding, but the total effect -of this difference should be relatively so small that ordinarily the value need not be changed. Therefore equivalent fluxes used with chord windings can be taken the same as for pitch windings. In consequence, the e.m.f . due .to the slot flux, with one-tooth chording, may be taken as one-half that for a pitch winding, with the brush cover- ing one commutator bar in both cases. For two-slat chording the slot flux may be considered as oc- cupying the space of two teeth only, while there will be a mag- netically idle tooth at the center. The e.m.f. per coil actually generated by cutting the slot flux will be, for part of the period *See Note 2 on page 246. Fig 11 THEORY OF COMMUTATION 217 the same as for one-slot chording, but there will be an inter- mediate period where the slot e.m.f. is practically zero, which does not occur with a one-slot chording or with a pitch winding. The average results, however, should be practically the same as if the total slot flux were actually di^ributed over three teeth instead of two. Effect of Brush Width with Chord Winding. In the chord winding, when the brush covers two or more commutator bars, the period of cutting the slot flux will be lengthened just as with a pitch winding on the assumption of no local currents. For example, if there are three commutator bars per armature slot and the winding is chorded one slot, then with the' brush covering ^one .commutator bar, complete cutting Of the slot flux will occur in the space of six commutator bars. If the brush covers three commutator bars instead of one, then complete cutting will occur in the space of eight commutator bars, while in a corresponding full pitch winding it would occur in the space of five bars. There- fore, the wide brush represents an inaprovement with the chorded winding, but not to the same extent, relatively, as with the pitch winding. This is on the assumption of absence of local currents in the short circuited coils. * Bands on A rmature Core. By the preceding method of analysis the effect of bands of magnetic material on the armature core can be readily taken into account. This effect represents simply an addition to the total flux which can pass up the tooth and across the top of the slots. From the ampere turns per slot, the clear- ance between the bands and the iron core, the total section of the band, etc., the flux due to the band can be calculated. This flux can either be combined directly with the slot flii.'c already de- scribed and the resultant e.m.f. can then be calculated; or, the e.m.f. can be calculated independently for the baad flux alone Magnetic bands on the armature introduce a complication into the general e.m.f. formula due to the fact that in many cases the flux into the bands is such as to highly saturate the band material at relatively low armature currents. This flux therefore is usually not proportional to the armature ampere turns. If the e.m.f. due to the band flux is to be calculated separately, the following formula can be used: 4>n Tepresents the total magnetic flux in the band from the armature core considering both directions from the tooth, then _ 2t N pTcRs " 108 *See Note 3 on page 246. 218 ELECTRICAL ENGINEERING PAPERS This formula holds true for the band flux which passes throug the one tooth in the pitch winding. Proper allowance must be made for the effect of chord windings and brush width, which can be done by the methods already described. Slot Flux with Local Current? Pitch Winding. In the preceding analysis local currents have not been included, as the method would be greatly complicated by taking such currents into account. In the general methodi given below the effect of local currents in the short circuited coils can be most easily shown. As already explained, an armature coil, as it approaches the short-circuit condition, has an e.m.f . generated in it by the inter- polar and the end fluxes. After the coil is short circuited this e.m.f. is still generated by the coil and naturally a local or short circuit current tends to flow through the coil, brush contact Fig. 13 and brush.. In addition, the work, or supply, current is being furnished to the armature winding through the brushes. These two currents are superimposed in the short circuited winding in such a way as, to have a very pronounced influence in the distribution of the slot fluxes. This effect can be bast seen by first determining the distribution of the work current in the various parts of the short circuited winding on the assumption of no local current and second, determining the distribution of the local currents on the assumption of no work current, hut with the same armature magnetomotive force as in the first assump - tion. The two distributions can then be combined and the re- sultant currents in the various parts of the short circuited coils can be obtained. Let Fig. 12 represent the first assumption in which no local currents are present. In order to illustrate conditions to better advantage, four commutator bars are assumed to be covered by THEORY OF COMMUTATION 210 Tracing out the current in each short circuited coil in Fig. 12, it will be seen that the current decreases at a uniform rate and then rises in the opposite direction at the same rate until the short circuit is removed. The period of commutation is the longest possible with this number of commutator bars short circuited, and the brush conditions are ideal, as the current density at the brush contact is uniform at all parts The above are the conditions which the designer endeavors to obtain in the construction of good interpole machines, as will be shown later. In Fig 13 the same arrangement of winding and brushes is chosen as in Fig. 12 except that only the local currents are shown and the values of these are assumed as proportional to the e.m.fs. in the short circuited coils and the resistance in circuit. In this diagram the current is a minimum in the coils at the moment that short circuit occurs, and rises to a maximum value and then diminishes to zero value again at the end of the short circuit Fig 14 Fig 15 In Fig 14 the currents of Fig. 12 and 13 are superimposed The resultant currents in the various parts of the short circuited wmding are seen to rise after short circuit until a maximum value is reached and then decrease rapidly and reverse to normal value in the opposite direction. Therefore, the period from normal value of the current to normal in the opposite direction IS very much shorter than when no local currents are present It may therefore be considered that the period of reversal is much reduced by the presence of the local currents, so that the e m.f in the short circuited armature conductors generated by the slot flux is proportionately increased, compared with the value it would have in case the local currents were absent These conditions can be shown possibly m a somewhat better manner by curves a, b and c in Fig. 15 The curve a shows the distribution of current in the short circuited coils without any local currents Curve b shows the distribution of local currents 220 ELECTRICAL ENGINEERING PAPERS while curve c shows the resultant of the two. The distance be- tween d and / on curve c gives the period of reversal frorn normal current in one direction to normal current in the opposite direc- tion. This period is much shorter than the full period repre- sented by g/ which would be obtained without local currents. The period df, however, may not differ much from the period of commutation with the brush covering the width of only one bar, when the local current is high compared with the work cur- rent. In such case the gain in the period of commutation which should be obtained by means of the wider brush may be practi- cally offset by the effect of the local currents which also increase with the wider brush, so that over a considerable range the resultant of the two effects may be practically constant. This is one indication why, in non-interpole machines, the brush width may be varied over quite a range with relatively small noticeable difference in the commutation. This may be il- ^ ^~\ \ \ ~ll ^XA \ ^i, 1 \c \p \A \ \ d \ /•■ \ / Fig. 16 Fig. 17 lustra ted by Fig. 16, in which is shown the current conditions with two to five bars spanned. In this figure a b, b c,-c d, etc., each represent the width of one commutator bar. Therefore, curve A , extending over the width a c, represents two bars spanned. The period of reversal of the current from normal value m one direction to normal in the opposite direction is represented by g c for curve A, h d for curve B, i e for C and kfiorD. A comparison of these values Is interesting. Calling a b the period of reversal with the brush covering one bar only, then g c with two bars covered, is greater than ab. h d is also greater than a b, but less than g c, while j e is slightly less than ab, and ^/ is considerably less. However, the variation be- tween g c and ^/ is much less than between a c and af which would be the corresponding periods with no local currents. It should be borne in mind that the above curves are only relative, deoendine uoon the comparative values of the local THEORY OF COMMUTATION 221 which is not correct, but they serve, ta . illustrate the general principle. This method of presentation- is simply a skeleton of the problem of commutation when local ciirrents are present •in ths short circuited coils and it would be beyond the scope of this paper to attempt a fall solution. Effects of Field Distortion. One of the " bugaboos " of the designer of commutating machines has been the question of field distortion. It has usually been considered that, when the ma- chine is loaded the magnetic field is more or less distorted or shifted from its nprmal no-load position and that corrimutation is affected by this distorted field. To state the case plainly, the field distortion has practically nothing to do with the problem. The distorted field magnetism is simply a resultant of the no-load main field flux combined with that due to the armature winding. Therefore, the two components of the distorted full-load field are the no-load main- field, which is fixed in space and is usually practically constant, and the armature field, which is also fixed in space but varies with the load. If the brushes are set in a certain position with respect to the no-load field, then, as this component of the re- sultant full load field is practically fixed in space and in value, 'X has no variable influence on the commutating conditions. The true variable element which does affect the commutation is the armature field, or flux, and.it is in this very flux which is the basis of the preceding theory of commutation. Therefore, the distorted resultant field of a loaded machine does not present any new condition in the problem of commutation. One ex- ception, however, can be made to the above, namely, where there is any considerable saturation in the armature teeth or in the main field pole corners. The effect of the armature mag- netomotive force is to strengthen one corner or edge of the field pole and. to weaken the other edge, but when saturation is pro- nounced the strengthening action is much less than the weaken- ing action. The resultant of these actions is a decrease in the total value of the main field flux. If, now, this main field flux be brought back to its normal total value, or higher, a very con- siderable addition to the main field magnetomotive force will be necessary, which will be effective in increasing the field flux at the weaker pole corner to a much- greater extent than at the highly saturated pole corner. In consequence, with load, the main field distribution, or field form, may be considered as being changed from its no-load form A, to the form B, as indicated in Fig. 17. It is, in reality, strengthened at a point b, for 222 ELECTRICAL ENGINEERING PAPERS example. In such case the main field will have a var.able tn- fluence on the commutation, if the brush is set with a lead as at 6, and, to a slight extent, the effect of an interpole is thuf obtained. Effect of Brush Lead. Before taking up the problem of inter poles on direct current machines it might be well to consider th( effect of brush lead, as this gives a result intermediate bet weet true intsrpole and non-interpole commutating conditions. The preceding formulas apply to non-commutating poL machines without brush lead. However, except in case of re versing machines, such as street railway motors, or hoist motors etc., it is usual practice to give a forward lead to the brushe of direct current generators or a sHght backward lead to direc current motors. The effect of giving a lead at the brushes of i non-interpole direct current machine may be considered a being equivalent to the effect of an interpole with the exceptioi Fig. 18 Pig. 19 that correct flux conditions and proper commutation, with an given brush setting, are obtained only for one given load. As described before, with a non-interpole machine the arms ture winding sets tip a flux in the interpolar space. With nc lead at the brushes this flux is usually a minimum midway be tween the poles and rises toward the polar edges. The flu from the adjacent main poles has a zero value midway bet wee the poles and rises toward the polar edges, but has opposit polarities at the two sides of the midpoint. This is illustrated i Fig. 18. The resultant of the armature and field fluxes is ind cated by the dotted line A . This resultant falls to zero at on side of the midpoint and then rises in the direction opposite t that of the flux due to the armature ampere turns. At the othc side of the midpoint the two fluxes add, giving an increased re sultant flux m the same direction as the interpolar flux due to th THEORY OF COMMUTATION 223 commutation is shifted from a to the point of zero interpolar flux h, then commutation will occur without, any interpolar flux to be taken into account, that is, the e.m.f. generated by the short circuited armature conductors may be due to the slot and end winding fluxes only. If the brushes are shifted still further in the same direction to c, then, not only will the interpolar arma- ture flux be annulled but a flux in the opposite direction would be cut by the short circuited armature conductor, which will gen- erate an e.m.f. in opposition to that due to the armature fluxes in the slots and end windings. Consequently, the commutation can be materially assisted by such lead at the brushes. The difficulty in the use of this method of commutation lies in the fact that the commutating or reversing flux at c is the resultant of the main field flux and the armature interpolar flux at this point, and the latter flux varies with the load, while the former remains practically constant. Therefore the zero point of the resultant field shifts backwards" or forwards with change in load and the density of the commutating field beyond the zero point will therefore change with the armature current. In con- sequence, if the brushes are shifted into a suitable resultant field c at a given current, then with a different load the intensity of this field at c will be changed, and unfortunately the change will be in the opposite direction from that desired. In other words, the density of this resultant field will decrease with increase in load, whsreas just ths opposite effect is desired for good commu- tation over a wids rangs in load. In practice, however, an average condition is found which, in many cases, will give reasonably good commutation over a rela- tively wide range in load. The brushes may be shifted at no- load into an active field in such a way as to generate an e.m.f. in the armature coils of a comparatively high value. This e.m.f. will circulate considerable local current through the brush contacts and the amount of lead which can be given is dependent, to a certain extent, upon the amount of local current which can thus be handled without undue sparking. As the load is increased the strength of the resultant field, corresponding to this brush position, will be decreased, and with some value of the current this field will be reversed in direction. At this point the e.m.f. due to this field is added to the e.m.f. due to the slot and end winding fluxes. Obviously the limiting condition of commutation will be reached at a much higher cur- rent than would be the case if no load at all had been given. This condition is represented in Fig. 19, in which curves 1, 1, 2, 2, 3, 3, 224 ELECTRICAL ENGINEERING PAPERS etc., represent the armature and resultant flux distributions wit! various loads. In this figure the brushes are given a lead so thai commutation occurs at a point corresponding to b. It is obvious that at heavy load a still greater lead at th( brushes might give improved commutating conditions. How- ever, if the load were suddenly removed without moving the brushes toward a, then the short circuited coils would be cuttinj the main field at such density that serious sparking or flashing might occur. One serious objection to this method of commutation is thai the distribution of the resultant field is practically such thai equally good commutation cannot be obtained for all the coils in one slot when there are several coils or commutator bars pei slot. All the coils of one slot must pass under a given positior or value of the interpolar magnetic field at the same instant while the commutator bars to which these coils are connected must pass under the brush consecutively. If the field intensitj is just right for good commutation a^ the first coil per slol passes under the brush, then it may be entirely too great by tht time the last coil is commutated. For good commutation wit! a number of coils in one slot, the resultant interpolar flux shoalc have practically constant value over the whole range repre- sented by the period of commutation of all the coils in one slot This condition, however, is extremely difficult, or is frequentlj impracticable, to obtain with the ordinary non-interpole ma- chine. The above treatment of the problem of the effect of the brusl' lead has been based upon the armature interpolar magnetic field being located in the same position with lead as when then is no lead at the brushes. It has been assumed heretofore thai the non-interpolar flux due to the armature winding has a mini- mum value midway between the main poles and rises uniformh toward two adjacent pole corners. This, however, is only true when the point of commutation, or brush setting, is midwaiy be- tween the poles. When the brushes are shifted toward either pole the point of maximum armature magnetic potential is .shifted in the same way. This means that the distribution ol the armature interpolar flux will be modified directly by th( position of the brushes. Instead of rising uniformly toward th; two pole comers, with a minimum value midway between, i1 THEORY OF COMMUTATION 225 an increased value on the side toward which the brushes are shifted. This is illustrated in Fig. 20 in which A represents the armature interpolar flux distribution with the brush at a, while B represents it with the brush at b. This increased armature interpolar flttx due to the brush shifting means that the resultant interpolar flux due to both the armature and -main field fluxes will cross the zero line at a point further removed from the midpoint than in the case of no lead at the brushes. Consequently, in order to obtain a given useful commutating field the brushes must be given a greater amount of lead and this in turn shifts the zero point still further Thus, the act itself of shifting the brushes makes the commutating con- ditions more difficult. The calculation of the comtnutating conditions with any given lead therefore resolves itself into a determination of the re- FiG 20 sultant fluxes in which the coil is short circuited or commutated and the e.m fs generated by such fluxes. For the slot and end winding fluxes the calculation will be the same as for no-lead at the brushes The resultant flux in the interpolar space is the only condition which will introduce any variai;ion from the pre- ceding formulae and methods of calculation This part of the problem resolves itself simply into the determination of the resultant interpolar flux at the point of commutation for any given load The corresponding e.m.f can then be calculated. This, combined with the e.m.fs. due to the slot and end windings, gives the total short-circtiit e.m.f The method is, in principle, exactly the same as given before, except that the determination of the interpolar flux will be modified. Summation of Formulce In order to obtain the total voltage in the short circuited coil a summation should be made of the four separate voltages which have been derived for the interpolar. 226 ELECTRICAL ENGINEERING PAPERS end, slot and band fluxes. In reality it is the resultant fluxes which should be combined, but as the voltages to be derived from these fluxes represent somewhat different terms, a better procedure appears to be the summation of the voltages. Also, in practice it is thef e.m.fs. generated by the different fluxes, rather than the fluxes themselves, which are desired. The e.m.f. derived from the interpolar flux is P ^IcWtTcRs 2pTrDL 108 (0.25 /' + 0.5) {wD-Pp) where Ci is a correcting factor for chord winding, etc. The formula for the, end flux voltage is, £ =C2X r^^ X -. — a Xlog 2 N \\r sm V where Ci represents the correcting factor for chord windings, etc. The formula for the slot flux voltage is, P _ ^A^LWtT\RsL_ (2.666„ N p T, Rs Ec = Ci IQfi where d is the correcting factor for chord winding, brush width, etc. Therefore, 77 ^ . , Ic Wt Tc Rs r 2 Pit PL , /i, total- ^Q, yc, ( (0.25 />+0.5) {tD~PP))^ r^^^t-^^o,2N+ t! • ( 3 19 2. ^^-^^^^+^ a+t+2.16s >/») )"[ 2 4>NpT.Rs -t-C4X sni 10^ THEORY OF COMMUTATION 227 It is evident from this last equation that when there are no bands over the core the total e.m.f. in the short circuited coil is directly proportional to the current per armature coil or con- ductor. If the bands saturate, as would usually be the case with any considerable load, then the e.m.f. is no longer directly proportional to the current. Attention is called to this point as it has some bearing in the design of interpole machines. Condensed Approximate Formula. The above formula can be simplified very considerably by certain approximations which introduce but little error within the range of ordinary desigrt P First, the expression, /„ nr ^ , n rN / r, n .n does not seem (_U.zo p-f-U.o; [TV U — Jr p) to be capable of any general simplification. In fact, as shown from its derivation, it is not a general term, but applies only to certain constructions and may appear in a quite different form for other constructions. Therefore this expression must be used with judgment in any case. Moreover, this term appears only in non-interpole machines or in interpole machines only when the interpoles are narrower than the armature core or the number of interpoles is less than that of the main poles. There- fore this term may be neglected in many cases where interpoles are used. Second, the expression 4.3 -^H-. — sr— log 2 N can be changed as follows: 4.3 ^ - . „- = , with reasonable accuracy within the (sm 6) p ordinary limit of design, And log 2 iV = 0.9-1-0.035 N, with an error of about 4 per cent within the range of 6 to 24 slots. Therefore 4.3 4^-^ log 2iV=^^ (0.9+0.035 N), (sm 6) p approximately. This is simpler to handle, in practice, than the original term. , . , , . 2.666 rf+4 a+t+2.1 Q s Vn , . Third, the expression, ■ can be sim- plified very materially. Let the total depth of slot be represented by d*, which is equal to 2 d+a+1.5 t, approximately. 4:ds , 8 a - 3 / Then, the term, 2.666 d+i a+t can be changed to— r— H ^ — 228 ELECTRICAL ENGINEERING PAPERS Assuming a = 0.25 and < = 0.15, then ^ =0.52 approximately. o 2Md+4.a+t Us ,0.52 Therefore, Zs This is a very close approximation within the ordinary work- ing range of slot dimensions. Therefore, the above expression becomes, „ ^ H — ^ 1-2.16 Vw, which is much simpler to use in 6 S S practice. Fourth, in the simplified equation tt appears in the first and second terms, and 3.19 appears in the third term. These are so nearly equal that x may be used as a common factor for the three terms. The combined formulas for the total voltage per armature coil thus becomes, in approximate form, ^. I WtTrRs-n r 2pDL 108 L (0.25^+0.5)(7rZ?-P^) +C2 ^^ (0.9+0.035 N) + P J. (1.33 ds+0.52+2.16 s Vn)l 2 (p, Np Tc Rs 108 This appears to be about as simple a form as the equation can be put into when all the factors are to ,be included. It will be shortened for machines without magnetic bands on the core and in many interpole machines the term derived from the inter- polar flux may be omitted. For a given line of machines which are all of similar design, etc., it is probable that the terms can be further combined and simplified. Interfolar Machines In the' interpole machine a small pole is placed between two adjacent main poles for the purpose of setting up a local magnetic THEORY OF COMMUTATION 229 flux, in order to assist commutation, must be opposite in direction to the interpolar flux set up by the armature winding itself. To set up this flux in the opposite direction the magnetomotive force of the interpole winding obviously must be greater than that of the armature winding in the commutating zone. An armature coil, cutting across this interpole flux, generates an e.m.f. proportional to the flux, the speed and the number of conductors in series. This e.m.f. is in opposition to the e.m.f. in the short circuited coils, generated by the slot and end winding fluxes. For ideal commutation these e.m.fs. are not only in opposition, but they should also be of practically equal value. For perfect commutation the current in a short circuited coil should die down to zero value at about a uniform rate and should then rise to normal value in the opposite direction by the time the coil passes out from under the brush, as was illustrated in Fig. 12. This is the condition when no local currents are de- veloped in the short circuited coils and this can only be obtained when the interpole e.m.f. at all times, balances the armature e.m.fs. in the short circuited coils. Looking at the problem broadly, the resultant magnetic fluxes and e.m.fs. may be assumed as made up of two com- ponents which can be considered singly. One of these com- ponents is that which would be obtained with the armature magnetomotive force alone acting through the various flux paths, including the interpole. The other would be that which would be obtained with the full interpole magnetomotive force alone, the armature magnetomotive force being absent. Saturation is not considered in either case. Considering the first component, due to the armature mag- netomotive force alone, there would be the slot and the end fluxes with their short circuit e.m.fs., as already described, and in addition, there would be a relatively high flux, and short- circuit e.m.f. due to the good magnetic path furnished by the interpole core. In case the interpole does not cover the full width of the armature, or the number of inter poles is less than the main poles, there will also be some interpolar flux and e.m.f., as already described. Considering the second component, the entire interpole mag- netomotive force would set up a relatively high flux through the interpole magnetic circuit and a. correspondingly high e.m.f. would be generated in a short dreuited armature coil cutting this flux. 230 ELECTRICAL ENGINEERING PAPERS When these two components are superimposed, it is seen that the interpole flux due to the armature magnetomotive force is in direct opposition to that due to the interpole magneto- motive force and therefore only the e.m.f. due to their dif- ference need be considered. As the interpole winding has the higher magnetomotive force, the resultant interpole e.m.f. is in opposite direction to the armature e.m.fs., and should be sufficient to neutralize them. This way of considering the prob- lem avoids a number of confusing elements which would com- plicate the explanation, if given in detail. In practice it is difficult to obtain exact equality between the interpole and armature e.m.fs. That due to the armature fluxes is generated in all parts of the coil including the end winding, while the e.m.f. due to the interpole flux is generated only in that part of the coil which lies in the armature, slots. However, it makes no difference in what part of the coil the e.m.f. due to the interpole is generated provided it is of such value that it properly opposes and neutralizes the various e.m.fs., due to the armature fluxes. Therefore, in practice the interpoles need not have the same width as the armature core and, where space and magnetic conditions will permit, the number of interpoles can be made half that of the main poles. According to the method outlined, the whole problem of the design of the interpole depends, first, upon the determination of the s.m.fs. due to the armature fluxes, and, second, upon the determination of such interpole flux as will generate an e.m.f. in the short circuited armature coils which wQl equal, or slightly exceed, the armature e.m.fs. Interpole Calculations. Assuming that all the armature fluxes, except the interpolar, are unaffected by the presence of interpoles, the armature e.m.f. to be balanced by the interpole would be represented by the formula los ic.^L u) (0.25^+0.5) \FD^:p^ +Ci (^) (0.9-1-0.035 N) + r. 7. l-33NpRsTc +c>L l-C3-(I.-ii)X2.16 Vw J+C4X ■ ^\^ — - In this formula L represents the width of the armature core. Li represents the effective width of interpole at the gap on the basis of the full number of interpoles. L—Li is the difference between the width of the armature core and the interpc5le face. This term enters when the interpole is narrower than the armature core. When alternate interpoles are omitted and the remaining interpoles are of the same width as the armature core the conditions are practically the same as when the full number of interpoles are used but with their width equal to half the core width. Other combinations should be treated in the same way so that the above formula can be taken to represent the general conditions. In practice it is desired that the resultant interpole e.m.f., and therefore the interpole flux, vary in proportion to the arma- ture short-circuit e.m.f. which is to be neutralized. As shown by the last equation, this e.m.f. is proportional to the armature 232 ELECTRICAL ENGINEERING PAPERS current, except where there is saturation in the armature flu; path, as in the case of magnetic bands over the core. Therefori the interpole magnetomotive force should vary in proportioi to the armature current, neglecting core bands. In consequence in practice the interpole winding is always connected in serie: with the armature winding. The interpole magnetomotive force can be considered as madf up of two components, one of which neutralizes the armatun magnetomotive force, and the other component represents the ampere turns which set up the actual interpole flux. The firs1 component will be referred to as the neutralizing ampere turn; or neutralizing turns, and thejDther as the magnetizing ampert turns or magnetizing turns. Let T represent the total interpole turns for one interpole T, represent the total magnetizing interpole turns for one interpole. Ta represent the total effective " armature turns pei ,,_ total eff. ampere turns of armature j " number poles X total current ' / represent the amperes per interpole coil. Then I Ti = IT-I Ta, or, T= Ta+Ti. Let g = effective air gap per interpole. 5i = flux density under the interpole, and -Ei = e.m.f. in an armature coil of turns Tc due to the interpole flux Then, „ 3.19 I T i ^i— :: — The e.m.f. due to one interpole is equal to Bj IT D LiT^Rs Or, for two interpoles E,= 3.19 /r* TT DLiX2T,Rs THEORY OF COMMUTATION 233 This e.m.f . should be equal to the e.m.f. generated m the same coils by the armature Hux, or Ei = Ec. Therefore, dAQITiir DLiX2Tc Rs ~IcW,_TcRsTC j^cuJ. 1.1) (0.25^+0.5) iwD-Pp) +,,xi#- (0.9+0.035 iV)+.3L (1.3WM) " Mr n 7-^oirV-1-l. 24,NpRsTc +C3 (L— Li)2.16 Vn \+Ci ^^ In the second term of this equation /, Wi = IXTa'X 2p, where To' = total armature turns per pole, as distinguished from effec- 7~"' R 7"" tive turns per pole Ta, and Ta =-^ — rr. where h = ~~-^ as will i — op Wi be shown later under the subject of " Effective Arinature Am- pere Turns." Therefore, neglecting magnetic bands on the core, the above expression becomes, T 7-.j>g r y {L~L,)2D p ' 3.1QDLi{l-bp) L (0.25 ^+0.5) {ivD-Pp) +C2X— r- (0.9+0.035 Nj+CiL p s +C3(Z--Li)2.16\/reJ (ci (L-Li) 3.19 DZi {l-bp) 2Dp (0.25^+0.5) (7r£»-P^) +..xi# (0.9+0.025 iV)+C3L(M3A±0:L> +cs (L-ii) 2.16 V^J] 234 ELECTRICAL ENGINEERING PAPERS If the full number of interpoles is used, and each covers the full width of the armature, then L—Li = Q, and -n[ ^+ 3.19PL?(l-M) {'-''^ ^'■'+'-''' ^> +C3-L (1.33 j.+o.y) ^ Therefore the total interpole turns for one pole are equal to the effective armature turns, per pole multiplied by a constant which is a function of the- proportions of the machine. How- ever, this holds true only for the condition of no saturated path for the armature flux, such as magnetic bands. The above formula gives the interpole turns for two inter- poles acting on each armature coil. With but one interpole per coil the number of conductors per armature coil generating the interpole e.m.f. is halved so that the flux density must be at least doubled, and the effect of the armature flux in the inter- polar space over the other half of the armature coil must also be taken into account. This can be done in the preceding formula by using the equivalent value of ii. * With half the number of interpoles the effective gap length, g, will not be the same as with the full number of interpoles with the same mechanical gap, for the flux from the interpole maybe considered as returning across the gap of the two adjacent main poles and the value of g must be increased to represent the total resultant gap. Let ge represent the effective resultant gap, gm represent the effective gap under the main poles, A i represent the area of the interpole gap, and Am represent the area of one main pole gap. These areas can be derived from the field distribution or " field form " of the main and the interpoles. A ■ Then, the effective resultant gap ge = g+ ^ / gm, and this should be used instead of g. With half the number of interpoles and on the basis of the interpole flux returning through the two adjacent main poles, it may be assumed that this flux weakens the total flux in one THEORY OF COMMUTATION 235 there is no saturation in the main jwles or armature teeth under them, then no additional ampere turns, other than for the increased gap, will be required on account of the main poles carrying the interpolar fluxes. However, where there is much saturation of the main poles or teeth, then additional interpole ampere turns will be required, as will be described later in con- nection with effects of saturation. Chord Windings with Interpoles. Chorded armature windings can be used with interpoles with satisfactory results provided the interpoles are suitably proportioned. There are apparently some advantages with such an arrangement, but there are also disadvantages of such a nature that it is questionable whether it is advisable to use chord windings with such machines, except possibly in special cases. When chorded windings are used with interpoles, the e.m.f. due to the armature flux is usually much smaller than with a pitch winding and thus fewer interpole magnetizing turns are required. Also, the effective armature turns which must be neutralized by the interpole are reduced somewhat, which also means a sUght reduction in interpole turns. Against these advantages must be charged the disad- vantage of a wider interpole face. This in itself would not be objectionable where there is space for such wider pole face, but if the space between the main poles must be increased it may lead to sacrifice in the proportions of the main poles or changes in the general dimensions, such that the result as a whole is less economical than with a pitch winding. Effective Armature Ampere Turns. The term Ta representing the effective armature ampere turns should be considered, as the "value of this term is influenced by a number of conditions, such as the number of bars covered by the brush, the amount of chording in the armature winding, etc. With a full pitch wind- ing and neglecting the reduction in current in the short circuited coils, the magnetomotive force of the total armature winding is represented by the expression, " ^ ' ■ and per pole it is, J' ^' However, when the brush spans several coils, so that 2p a number of armature coils are short circuited at the same time, the average current in these short circuit coils should be con- siderably less than the normal value so that the effective ampere turns per pole is correspondingly reduced. Allowance must be 236 ELECTRICAL ENGINEERING PAPERS made for this reduction as it has considerable influence in de- termining the correct number of interpole turns. On the basis of no local currents, the average value of the current in the short circuited coils is just half that of the wort currents per conductor. Let B represent the total number of commutator bars, -Bi represent the number of bars spanned by the brush pi represent number of current paths, and p number of poles. Then, — ='total number of armature turns per pole, and pip —^ — - = number of turns by which the total armature turns pei 2 pi pole must be reduced to obtain the effective turns per pole, or. Ta = B Tc Bi Tc ppi 2pi ^^ Wt „ Wt Bi Tc 2 ' " " 2ppi 2p Let Bi Tc be represented as a percentage of Wi, ov BiTc = bW Then, r.= -^^ {l-hp) I pip IcWt {Ic pi) W, ^ IWt „, Wt Therefore, Ta = 2p 2ppi 2ppi' '" 2ppi {i-bp) Chorded windings also have an influence on the effective arma ture ampere turns per pole. When the winding is chorded on slot, for example, then, in one slot per pole, the upper and lowe coils will be carrying current in opposite directions and tKei magnetizing effects will be neutralized. In consequence, th total effective arxtiature ampere turns are correspondingly r€ duced and this must b.e allowed for in determining the Interpol THEORY OF COMMUTATION 237 Conditions Affecting Interpole Proportions. The foregoing formulas have been based upon the use of interpoles of such pro- portions that the interpole flux varies directly as the magnetizing current and its distribution over the ccimmutating zone is such as will give the proper opposing e.m.f . at all times. However', the proportionality of flux to current can only be true as long as there is no saturation in the interpole magnetic circuit. Such saturation is liable to be found in practice and not infrequently it is quite a problem of design to avoid it within the working range of the machine. Also, another difficult problem lies in so designing the interpole face that the flux distribution in the commutating zone is such that its e.m.f. will properly balance the armature e,m;fs. in the short circuited coils, especially as the latter are generated by cutting fluxes which may be distribifted in a quite different man- ner from the interpole flux. Fig. 21 Shape and Proportion of Interpole Face. As alrea;dy shown, the effective interpole flux under the pole face is theTesultaiit of the total interpole magnetomotive force and the opposing armature magnetomotive force. As the armature winding is distributed over a surface and the interpole winding is of the polar or concentrated type, the resultant magnetomotive force would normally be such as would not tend to give a uniform flux distribution under the interpole unless the interpole face is properly shaped or proportioned for such distribution. The conditions may be illustrated in Fig. 21. In this figure the lines A A represent the armature magnetomotive force, with a full pitch winding, and the brush covering one commutator" bar. The heavier part of the lines ab c a,t the peak of the magneto- motive force diagram, represents the armature magnetomotive force which would be obtained under the interpole face, and also the ilux distribution which would be obtained, with ho interpole 238 ELECTRICAL ENGINEERING PAPERS magnetomotive force, and with uniform gap under the pole faces. In opposition is shown the interpole magnetomotive force and flux distributions def for corresponding conditions. The resultant magnetomotive force is represented hy g h i, and with a uniform gap under the pole, the resultant interpole flux would have a similar distribution. Instead of this, either a flat or, in some cases, the reverse distribution is required, that is, with a slight " hump " in the middle instead of a depression- By pr6perly shaping the pole face so as to give an increased air ■gap toward the edges, the flux distribution can be made practi- cally anything desired. In some cases a relatively narrow pole tip with a very large air gap will give a close approximation to the desired- flux distribution. However, in practice the above distribution of the armature magnetomotive force is rarely found. The use of brushes which cover more than one conimutator bar serves to cut off or flatten Out the pointed top of the armature magnetomotive force diagram, as shown by the dotted line B, in Fig. 21, and thus lessen the depression at the center of the resultant magneto- motive force distribution. As intimated before, this problem of proportioning the inter- pole face turns upon the determination of the armature e.m.fs. in the short circuited coils which have to be balanced by the interpole. If the different armature e.m.fs. are determined for the whole period of commutation and then superimposed, the resultant e.m.f. indicates the flux distribution required under the interpole. Usually the e.m.fs. due to the end winding, and to the interpolar flux, if any, will be practically constant during the whole period of commutation. If no local currents are present the e.m.f. due to the slot flux will also be practically constant, although it may be slightly reduced near the beginning and end of the commutation period. The sum of these e.m.fs. should therefore be practically Constant over the whole commutation period and therefore, in a well designed machine, the interpole flux- density should be practically constant over the whole commutation zone. As explained before, special shaping of the poles and pole face will be necessary, in most cases, to obtain exactly this proper flux distribution. .Large interpole air gaps are obviously advantageoiis in obtaining such distribution. In fact, a very small interpole gap makes the determination of the proper _ interpole face dimensions very difficult in. many caseis. Dn account of the internole usuallv covering- less l-.han +wn THEORY OF COMMUTATION 239 armature teeth, the ordinarily accepted methods of determining the effective length of air gap under a pole will not apply, in many cases, which may lead to a slight error in the results. Practically the effective gap under the narrow interpole will usually be longer than determined by the ordinary methods. This partly explains the fact that, in some cases, an increase in mechanical clearance between the interpole face and the armature core does not require anything like a corresponding increase in the interpole magnetizing ampere turns. The effec- tive interpole air gap increases, but at a much less fate than the mechanical gap. The brush setting in relation to the interpole is of great im- portance. The point of maximum armature magnetomotive force is definitely fixed by the brush setting. With the interpole fixed in position, any shifting of the brushes backward or forward will obviously change the shape of the- resultant magnetomotive forc3 distribution under the interpole face and in consequence the flux distribution will be changed. With but one armature coil per slot and the brush covering but one commutator bar, good commutating conditions might be found over a considerable range of brush adjustment, by suitably varying the interpole ampere turns. However, with two or more coils per slot and with the brush short circuiting several bars, any marked change in the resultant interpole magnetomotive force and flux distribu- tion will mean improper commutation for some of the coils. Proper brush setting is therefore of first importance. It has been "assumed in the foregoing treatment, that an exact balance between the interpole and armature e.m.fs. will give the best conditions. From certain standpoints, this is true, but in practice usually a slight excess in the interpole strength, or " over-compensation " of the interpole, as it is frequently called, is advantageous. Reference to Fig. 14 shows that in a machine without interpoles, and therefore without compensation, the current flowing between the brush contact and the commutator is crowded tpward one brush edge, this being the edge at which the commutation of a coil is completed, that is, at the so-called forward brush edge. With over-compensation the opposite effect occurs — that is, the brush current density is below the average at the forward edge. This is, to a certain extent, a de- . sirable condition. Also, if there is any saturation of the inter- pole circuit at overloads, the over excitation of the interpole winding can take care of the saturation ampere turns, so that 240 ELECTRICAL ENGINEERING PAPERS normal compensation can be obtained at considerably higher load than in a machine with no over compensation. Furthermore, over compensation is desirable on account of the effect of the resistance of the coils undergoing commutation, which heretofore has been neglected as being of minor importance. Such re- sistance tends to lower the current density at the middle of the brush contact, and increase it toward the brush edges. Over compensation will oppose this at the forward edge, but increase it at the back edge, which is less objectionable. Also, as shown in Fig. 21, there is liable to be a depression at the center of the interpole flux distribution, if the pole face is not properly shaped. This depression tends to cause higher current densities at the brush edges. Over compensation again tends to reduce this density at the forward brush edge. Thus there are several good reasons for slight over compensation, and practical opera- tion bears this out, especially on high voltage machines, where the short circuit e.m.fs. average higher than in other machines. Balanced Circuits. It has been assumed that the armature ampere turns per pole have been the same for all poles. This will be true for the usual two-circuit or series type of winding, or its allied combinations, but is not necessarily true of the parallel type of armature winding. In such a winding a number of circuits are connected in parallel at the brushes, and, unless ample provision be made for equalizing the different circuits, they may not carry equal currents at all times. As the re- sultant interpole flux and e.m.f. is directly dependent upon the opposing armature ampere turns, it is obvious that any ine- qualities in the armature currents would lead at once to incorrect interpole conditions. A poorly equalized parallel-wound arma- ture might furnish conditions such that the interpoles cannot be adjusted for satisfactory operation. Also paralleling of the interpole windings, unless care be taken to insure equal current division among the circuits, is liable to lead to trouble. Saturation of the Interpole Circuit. Heretofore the interpole turns T, as determined, have been only those required for forcing the resultant interpole flux across the effective interpole air gap, and nothing has been allowed for any turns required for magnet- izing the parts of the interpole circuit other than the gap, Where such additional turns are required they must be added to the turns T, already determined. Saturation in the interpole magnetic path is the principle cause for such additional turns, but saturation in the various THEORY OF COMMUTATION 241 flux paths may occur in such a way as to be either harmful or beneficial, depending upon where it is located. Beneficial saturation may be assumed to be such as will reduce the arma- ture short circuit e.m.fs., while harmful saturation tends to re- duce the interpole e.m.f. While the useful interpole flux passing into the armature may be relatively low — say one-fifth that required for saturation of the interpole material — ^the leakage flux between the interpole and the two adjacent main poles is often very much greater than the useful flux so that the interpole at the part where it carries the highest total flux may be worked up to possibly half saturation, or higher, with normal load on the machine. The interpole leakage flux is due to the total ampere turns on the inter- pole, while the useful interpole flux is due only to the magne- tizing component of the interpole ampere turns, which may be as low as 15 per cent to 25 per cent of the total interpole ampere turns. The leakage flux is thus liable to be a high percentage of the total interpole flux. While the ampere turns on the interpole will rise in direct proportion to the current, the effective magnetizing component will rise in direct proportion only below saturation of the inter- pole circuit. Any ampere turns required for saturating this circuit will be taken from the magnetizing component of the interpole winding. Therefore, when any appreciable saturation occurs, the effective magnetizing component will not vary in proportion to the current, and the interpole e.m.f. will not vary in proportion to the armature e.m.fs. As the magnetizing com- ponent of the interpole winding usually represents a relatively small number of ampere turns per pole a comparatively slight saturation in the interpole circuit may have an appreciable effect. It is therefore advisable to work at as low a saturation as possible in the interpole circuit so that practically no saturation occurs within the ordinary working range of the machine. Where saturation occurs in any of the armature flux paths, as, for instance, with saturated bands over the armature core, the result of such saturation will serve to neutralize the effect of saturation in the interpole magnetic circuit. In other words, the armature e.m.f. will not rise in proportion to" the current and thei'efore the opposing interpole e.m.f. does not need to increase in proportion either. The principal source of saturation in the interpole circuit lies in the magnetic leakage from the interpole to the adjacent main 242 ELECTRICAL ENGINEERING PAPERS poles. Serious trouble has often been encountered by not making due allowance for such leakage. However, there may be other causes for saturation. When the full number of inter- poles is used the interpole magnetic path or circuit is independent of the main pole magnetic circuit, except in the yoke and in the armature core below the slots, as indicated in Fig. 22. In the yoke it may be seen that the interpole flux' is in the same direction as the main flux at one side of the main pole and is in opposition to the main flux at the other side. The same is true in the armature core. Therefore the interpolar flux tends to reduce the flux in one part of the yoke and tends to increase it in the other part. If the saturation in these parts is relatively low, then the magnetomotive force required for forcing the low and the high fluxes through the yoke will be but little greater than if these fluxes were equal. However, if the yoke is highly saturated the increase in ampere turns required for the high part much more than offset the decrease in ampere turns for the low part, so that, as a result, additional am- pere turns are required for sending \-"'~r;'-^"^—^'--^-''"'i the interpole flux through this path. The interpole ampere turns therefore i N 1 must be increased on this account, [,. . -i--t-j;-->— — ^^-,t---^^ j when the saturation is high. The same ^ . „„ condition holds for the armature core. A similar condition occurs vyhere half the number of interpoles is used and when there is much saturation of the main pole and the armature teeth under it, as already referred to. This con- dition requires additional interpole ampere turns. In practice, with the ordinary compact designs of direct cur- rent machines, it is usually difficult to keep the total interpole flux as low as one-third that which gives any material saturation and, not infrequently, it is much higher than this. Therefore, by direct proportion it might be assumed that such machines could carry only double to treble load without sparking badly. However, the resistance of the brushes, etc., will be of such as- sistance that relatively higher loads may bs commutated rea- sonably well. For instance, with the interpole worked at about half saturation at normal load, the machine may be able to commutate considerably more than double load without undue sparking. It is also of material assistance, where heavy over- loads are to be carried, to over-excite the interpole "^winding, THEORY OF COMMUTATION 243 than required at normal load, as described before. In this case, at light loads, the interpole e.m.f. exceeds the armature e.m.f. a certain amount which is taken care of by the brush resistance as local currents will be less harmful when the work current is low. As partial saturation is obtained at overload, the two e.m.fs. become equal but at a higher load than would be the case without over-excitation of the interpole. Commutating Conditions on Short Circuit. When a direct current generator is short circuited across its terminals, either through a low external resistance or without such resistance, a current rush will occur which will rise to a value represented approximately by the generated e.m.f. divided by the resistance in circuit. This current rush is only of short duration as the excessive armature current will react to demagnetiza or " kill " the field.. If the short circuit is without external resistance the current rush may reach an enormous value as the internal re- sistance on large machines is usually very low. This means that currents from 25 to 40 times full load may be obtained on " dead " short circuit. Experience shows that under such current rushes, any kind of direct current machine will tend to flash viciously at the brushes. By the preceding theory and analysis a rough approximation to the commutating conditions on short circuit can readily be obtained. Assuming an interpole machine, the following con- ditions will be found: 1. The interpole will be highly saturated so that it is of little or no dir.^ct benefit. 2. The slot flux will rise to such a value that the armature teeth in the commutating zone are practically saturated. 3. There may be some interpolar flux from the armature, as the hjgli interpole saturation may allow this. 4. The armature end flux, with the exception of that part due to magnetic bands, will rise practically in proportion to the current. The following short circuit e.m.f. conditions will be obtained; 1. There will be possibly a slight e.m.f. due to the armature interpolar flux. 2. There will be an e.m.f. due to the tooth flux which is almost as high, per conductor, as could be obtained by a conductor cutting the flux under the main field at no load, for saturation of the armature teeth may be assumed to be the limit in both cases. 3. There will be an e.m.f. due to the end flux which may be 10 to 20 times larger than at normal full load. 244 ELECTRICAL ENGINEERING PAPERS Therefore, the total e.m.f. in the short circnited coil due to cutting the armature flux on dead short circuit may be higher than would be obtained if the brushes were shifted at no load until the commutated coil lies under the strongest part of the main field. As very few machines of large capacity would stand this latter condition without flashing, it may be assumed that they would be no more able to stand a dead short circuit without flashing. In fact, 8 to 10 times full load current will make an interpolar machine of normally good design flash badly, as it is impracticable to make an interpoie of the usual type which will not saturate highly at 8 to 10 times normal current. If, however, the interpoie is combined with compensating windings in the main poles, the interpoie leakage may be made so small that comparatively low saturation is obtained normally in the interpoie circuit. In such case the interpoie may be effec- tive with heavier currents and the flashing load may be very much higher than with the usual type of interpoie machines. Conclusion The foregoing is a general presentation of the problem of commutation, which is admittedly cnide and incomplete in some points. In particular may be mentioned the part describing the action of local currents. Also, the method of considering the resultant action in interpoie machines as the superposition of two components does not tell the whole story, but the actual analysis, in detail, of a number of these phenomena would be so confusing and complicated, that a general physical conception of what takes place during commutation would be lost. In the ultimate analysis it will be found that a number of the methods, described are, in reality, simply illustrations of the conditions of commutation rather than an analysis of the conditions them- selves. However, the method as given throws Ught on many things which take plade during commutation. It also includes a nimiber of conditions which are not -covered in the usual methods of dealing with this problem. For example, the number of commutator bars spanned by the brush is an important ele- ment in this method of handling the problem, whereas, in many former methods, this point was either omitted, or treated in an empirical manner. In this method the results obtained would be very greatly in error if the brush span were not included. Any theory or method of calculation is open to question until THEORY OF COMMUTATION 245 method has been tried on a very large number of direct current machines, including high speed, direct current generators, direct current turbo-generators, direct current railway motors of all sizes, moderate- and low-speed generators of all capacities, industrial motors of various designs including adjustable speed motors and machines with half the number of interpoles. In those cases where the actual test data of the machines was very accurately obtained, the agreement between the tests and the calculated results by the above method was found to be close. In fact, the method in some cases indicated errors or inaccuracies in the test results. In a number of cases of early interpole machines there was considerable disagreement between the' results of the calculation and the actual test, but, in many of these cases, later experience showed definitely -that the proper interpole field strength or proportions had not been obtained in the actual test or that the proper brush setting had not been used. These cases were thus, to a certain extent, a verification of the method, for in general the greatest discrepancies between the calculated and the test results corresponded to the ma- chines which eventually proved to have the poorest proportions or adjustment. This theory of commutation looks complicated and cumber- some in its practical application, but it should be understood that it is, in reality, an exposition of a general method from which special and simpler methods may be derived for different types and designs of machines. It indicates plainly that the problem is so complicated that no simple formulae or methods of calculation can be devised which will cover more than individual cases, and that such formulse, if appHed generally, will lead to errcir sooner or later. If, however, the general derivation of such simplified formulae is well understood, then they may be used with proper judgment and with much less danger of errorin the results'. It is evident, from the general analysis, that the whole problem must be handled with judgment, for new or different conditions are encountered in almost every type of inachitie. A great many problems, closely allied to that of commutation in interpole machines, have not been considered, because some of them represent special cases of the general theory, while others are somewhat outside the subject of this paper. Of the former class may be mentioned, commiitation of synchronous converters, nlachines with distributed or true compensating 246 ELECTRICAL ENGINEERING PAPERS windings, the so-called " split-pole " converter, and the commu- tator type alternating current motors, etc. In the latter clasa may be included such problenls as the effect on commutation of closed circuits around the interpoles, losses due to commutation^ current distribution at the brush contact, etc. Some of these subjects were included in this paper as originally prepared, but on account of its undue length they had to be omitted. NOTES Cs 1. Page 215 — B\' applying this correction factor, - — — -^ — the aA'crage slot e. m. f. for the period of commutation is ob- tained. 2. Page 216 — General use of this factor, 1 + -^5 r; — — — should be avoided. It is not applicable to all types or com- binations of windings. Use instead the factor giving average e. m. f. (See Note 1). 3. Page 217 — Considering chording, the correction factor C Cs becomes — — — ;:; — —, — ; — t; where K = slots Cs +Bi—1 —^-- Cs+Bi—1+ K chorded X Cs- For the general case, where there are P' circuits. Cs andPpoles, the correction factor becomes ht Cs + Bi —y + K. 4. Page 228 — For ordinary working range of slot dimensions,. 2.16 s\/n= 1.07 X tooth pitch at armature surface. This formula may be further simplified by substituting 1.07 Ft for 2.16 s^n, Pt being the tooth pitch at the armature surface. 5. Pape 232 — If average slot e. m. f. is used in calculating Ecr ' (See Note 1). this expression should be multiplied by a factor Cp to obtain average value of Ei. Cp is the ratio of the average flux density in the commutating zone, to the maximum density, Bi. PHYSICAL LIMITATIONS IN D.C. MACHINES 247 PHYSICAL LIMITATIONS IN DIRECT-CURRENT COMMUTATING MACHINERY ^FOREWORD — This paper was presented before the American Institute of Electrical Engineers in San Francisco, September 16, 1915, at the Electrical Congress at the Panama- Pacific International Exposition. It gives the results of the author's work on determination of commutating limits covering the period of many years of work. As regards commutation, it is, in reality, a supplement to the paper, ' 'Theory of Commutation and Its Application to Commutating Pole Mg,chines." On the subject of flashing, it covers some very -interesting limitations, based upon experience and special testa. The subjects of noise, limitations in peripheral speed, variations in voltage, etc., are also treated in a general manner. This paper is, in reality, more or less of a general stunmary of the author's experience in direct-current machinery. Although usually looked upon as an "alternating-current man," he has probably spent as much total time on direct- current work as on alternating. Many of the early more or less radical devel- opments and improvements in direct -current machinery resulted directly from his work. A description of some of these is covered in his historical papers, entitled, "The Development of the Direct- Current Generator in America," and "The Development of the Street Railway Motor in America," which ap- pear in the latter part of this volume. Many of the limiting conditions in direct-current machinery as described in this paper were determined by the " author himself, either by direct experiment or by analysis from mmierous tests on a large variety of machines. Although this paper was prepared about four years ago, more recent experience has not modified any of the data, to any great extent. — (Ed.) IN DIRECT-current commutating machinery there are many limitations in practical design which cannot be exceeded without undue risk in operating characteristics. 248 - ELECTRICAL ENGINEERING PAPERS Many of these limits are not sharply defined in practise, due, in many cases, to the impossibility of taking advantage of all the helpful conditions and of avoiding the objectionable ones. There are many minor conditions which affect the permissible limits of operation, which are practically beyond the scope of reliable calculation. Usually, such conditions are recognized, and al- lowance is made for them. It is the purpose of this paper to treat of some of the major, as well as minor, conditions which must be taken into account in advanced direct-current design. These are so numerous, and are so interwoven, that it is difficult to present them in any consecutive order. Probably the most serious limitation encountered in direct- current electric machinery is that of commutation. This is an electric!al problem primarily, but in carrying any design of direct- current machine to the utmost, certain limitations are found which are, to a certain extent, dependent upon the physical characteristics of materials, constructions, etc. A second limitation which is usually considered as primarily an electrical one, namely, flashing, (and bucking) is in reality fixed as much by physical as by purely electrical conditions. A third limitation is found in blackening and burning of com- mutators, burning and honeycombing of brushes, etc. These actions are, to a certain extent, electrical, but are partly physical and mechanical, as distinguished from purely electrical. There are many other limiting conditions dependent upon speed, voltage, output per pole, quality or kind of materials used, etc. As indicated before, these cannot all be treated separately and individually, as they are too closely related to- other characteristics and limitations. Commutation and Commutation Limits In dealing with the limits of commutation, it is unnecessary to go into the theory of comrhutation, except to indicate the general idea upon which the following treatment is based. This has been given more fully elsewhere,* and therefore the following brief treatment will probably be sufficient for all that is required in this paper. In this theory it is considered that the armature winding as a whole tends to set up a magnetic field when carrying current, and that the armature conductors cutting this magnetic field *A Theory of Commutation and Its Application to Interpolar Machines. ■ by B. G. Lamme, A. I. E. E. October, 1911. PHYSICAL LIMITATIONS IN B.C. MACHINES 249 will generate e.m.fs. just as when cutting any magnetic field. From consideration of the armature magnetomotive force alone, the flux or field set up by this winding would have a maximum value over those armature conductors which are connected to the brushes. If the magnetic conditions or paths surrounding the armature were equally good at all points, this would be true. However, with the usual interpolar spaces iri direct-current machines, the magnetic paths above the commutated coils are usually of higher reluctance than elsewhere. However, what- ever the magnetic conditions, the tendency of the armature •magnetomotive force is to establish magnetic fluxes, and, if any field is established in the commutating zone by the armature vsrinding, then those armature coils cutting this field will have e.m.fs. generated in them proportional to the field which is cut. As part of this armature flux is across the armature slots them- selves, and part is around the end windings, both of which are practically unaffected by the magnetic path in the interpolar space above referred to, obviously, then no matter how poor the ■magnetic paths in the interpolar space above the core may be made, there will- always be e.m.fs. generated on account of that part of the armature flux which is not affected by those paths. In the coils short circuited by the brushes, these e.m.fs. will naturally tend to set up local or short circuit currents during the interval of short circuit. In good commutation, as the commutator bars connected to the two ends of an armature coil which is carrying current in a given direction, pass under the brush, the current in the coil itself should die down at practically a uniform rate, to zero value at a point corresponding to the middle of the brush, and it should then increase at a uniform rate to its normal value in the opposite direction by the time that the short circuit is opened as the coil passes from under the brush. This may be considered as the ideal or straight line reversal or commutation which, however, is only approximated in actual practice. This gives uniform current distribution over the brush face. If no corrective actions are present, then the coil while under the brush tends to carry current in the same direction as before its terminals were short circuited. In addition, the short circuit current in the coil, due to cutting the armature flux, tends to add to the normal or work current before reversal occurs. The resultant current in the coil is thus not only continued in the same direction as before, but tends to have an increased value. 250 ELECTRICAL ENGINEERING PAPERS Thus the conditions at the moment that the coil passes out from under the short circuiting brush are much worse than if no short circuit current were generated. The reversal of the current would thus be almost instantaneous instead of being gradual as called for by the ideal commutation, and the resultant current reversed much greater than the work current alone. However, the introduction of resistance into the local circuit will greatly assist in the reversal as will be illustrated. later. The ideal condi- tion however, is obtained by the introduction of an opposing eim.f. into the local short circuited path, thus neutralizing the tendency of the work current to continue in its former direction. As this opposing e.m.f. must be in the reverse direction ta the short circuit e.m.f. which would set up by cutting the arma- ture magnetic field , it follows that where commutation is accomp- lished by means of such an e.m.f. it is necessary to provide a magnetic field opposite in direction to the armature field for setting up the commutating current. This may be obtained in various ways, such as shifting the brushes forward (or backward) until the commutated coil comes under an external field of the right direction and value, which is the usual practise in non- commutating pole machines; or a special commutating field of the right direction and value may be provided, this being the practise in commutating pole and in some types of compensated field machines. When the commutating e.m.f. is obtained by shifting the commutated coil under the main field, only average conditions may be obtained for different loads; whereas, with suitable commutating poles or compensating windings, suffi- ciently correct commutating e.m.fs. can be obtained over a very wide range of operation. In practise, it is difficult to obtain magnetic conditions such that an ideal neutralizing e.m.f. is generated. However, the use, of a relatively high resistance in the short circuited path of the commutated coil very greatly simplifies the problem. If the resistance of the coil itself were the only limit, then a relatively low magnetic field cut by the short circuited coil would generate sufficient e.m.f. to circulate an excessively large local current. Since such current might be from 10 to 50 times as great as the normal work ciurent, depending upon the size of machine, it would necessarily add enormously to the difficulties of commuta- tion whether it is in the same direction as the work current or is in opposition. To illustrate the effect of resistance, assume, for exariiple, a short circuit e.m.f. in the commutated coil of two PHYSICAL LIMITATIONS IN B.C. MACHINES 2.51 volts, and also assume that a copper brush of negligible resist- ance short circuits the coil, so that the resistance of the short circuited coil itself practically limits the current to a value 20 times as large as the work current. Now replace this copper brush with one giving about 20 times as large a resistance (some form of graphite or carbon brush) then the total resistance in circuit is such that the short circuit current is cut down to a "value about equal to that of the work current. This at once gives a much easier condition of commutation, even without any reversing field; while with such field, it is evident that extreme accuracy in proportioning is not necessary. Thus a relatively liigh resistance brush — or brush contact, rather — is of very great help in commutation; especially so in large capacity machines where the coil resistance is necessarily very low. In very small Machines, the resistance of the individual armature coils has quite an influence in limiting the short circuit current. It is in its high contact resistance that the carbon brush is such an important factor in the commutating machine. Usually, it is the resistance of the brush that is referred* to as an important factor in assisting commutation. In reality, it is the resistance . of the contact between the brush and commutator face which must be considered, and not that of the brush itself, which usually is of ^'ery much lower resistance, relatively. As this contact re- sistance or drop will be referred to very frequently in the fol- lowing, and as the brush resistance itself will be -considered in but a few instances, the terms " brush resistance '' and " brush drop " will mean contact resistance and contact drop respectively, unless otherwise specified. Short Circuit Volts per Commutator Bar. As stated before the armature short circuit e.m.f. per coil, or per commutator bar, is due to cutting a number of different magnetic fluxes, such as those of the end windings-, those of the armature slots, and those over the armature core adjacent to the commutating zone. Each of these fluxes represent different, conditions and distri- butions, and therefore the individual e.m.fs. .generated by them may not be coincident in time phase. Therefore, the resultant e.m.'f. usually may not be represented by any simple graphical or mathematical expression.' When an external flux or field is superimposed on the armature in the commutating zone, it may be considered as setting up an additional e.m.f. which may be added to, or - subtracted from, the resultant short circuit e.m.f. due to the armature fluxes. 252 ELECTRICAL ENGINEERING PAPERS These component e.m.fs. are not really generated separately in the armature coils, for the external flux combines with part of the armature flux, so that the armature coil simply generates an .e.m.f. due to the resultant flux. However, as part of the armature short circuit e.m.f. is generated by fluxes which do not combine with any external flux, as in the end winding, for in- stance, it follows that, to a certain extent, separate e.m.fs. are actually generated in the armature winding in different parts of the coil. For purposes of analysis, there are advantages in considering that all the e.m.fs. in the short circuited armature coil are generated separately by the various fluxes. A better quantitative idea of the actions which are taking place is thus obtained, and the permissible limitations are more easily seen. In the following treatment, these component e.m.fs. will be considered separately. As that component, due to cutting thfe various armature fluxes, will be referred to very frequently hereafter, it- will be called the " apparent " armature short circuit e.m.f. per coil, or in abbreviated form, "the apparent short circuit e.m.f!" In practise, on account of the complexity of the separate elements which make up the apparent short circuit e.m.f., it is very difficult, or in many cases, impossible, to entirely neutralize or balance it at all instants by means of an e.m.f. generated by an extraneous field or flux of a definite distri- bution. Therefore, it should be borne in mind that, in practise, only an approximate or average balance between the two com- ponent e.m.fs. is possible. With such average balance there are liable to be all sorts of minor pulsations in e.m.f. which tend to produce local currents and which must be taken care of by means of the brush resistance. Pulsations or variations in either of the component e.m.fs. are due to various minor causes, such as the varying magnetic conditions which result from a rotating open slot armature, from cross magnetizing and other distorting effects under the commutating poles, variations in air-gap reluctance under the commutating poles, pulsations in the main field reluc- tance causing development of- secondary e.m.fs. in the short circuited coils, etc. Some of these conditions are liable to be present in every machine; some which would otherwise tend to give favorable conditions as regards commutation, are partic- ularly liable to -set up minor pulsations in the short circuit e.in.f. Therefore, brushes of high enough resistance to take care of the short circuit e.m.f. pulsations are a requisite of the present types of d-c. machines, and it may be assumed that there is but little PHYSICAL LIMITATIONS IN D.C. MACHINES 253 prospect of so improving the conditions in general that relatively high resistance brushes, or their equivalent, may be discarded. It is only on very special types of machines that low resistance brushes can be used. With ideal or perfect commutation, the two component e.m.fs. in the short circuited coil should balance each other at all times. However, as stated before, this condition is never actually ob- tained, and the brush resistance must do the rest: With ideal commutation, the current distribution over the brush contact face should be practically uniform, and a series of voltage read- ings between the brush tip and commutator face should show uniform drops over the whole brush face. In most cases in practise however, such voltage readings \v\\\ be only averages. For example, instead of a contact drop of one volt at a given point, the actual voltage may be varying from zero to two volts, or possibly from minus one volt to plus three volts These pulsating e.m.fs. will result in high frequency local currents, which have only a harmful influence on the commutation and commutator and brushes. These pulsations may bc assumed to be roughly related in value to the apparent short circuit volts generated by the. armature conductor. In other words, the higher the apparent short circuit volts per conductor, the larger these pulsations are liable to be. As the currents set up by these pulsations must be limited largely by the brush contact resistance, it is obvious' that there is a limit to the pulsations- in voltage, beyond which the current set up by them ma}' be harmful. A very crude practise, and yet possibly, the only fairly safe one, has been to set an upper limit to the apparent short circuit volts per bar, this limit varying to some extent with the conditions of service, .such as high peak loads of short dura- tion, overloads of considerable period, continuous operation, etc. Experience has shown that in commutating pole machines, the apparent short circuit voltages per turn may be as high as four to four-and-one-half volts, with usually but small evidence of local high frequency currents, as indicated by the condition of the brush face. If this polishes brightly, and the commutator face does not tend to " smut," then apparently the local currents are not excessive. However, in individual cases, the above limits have been very considerably exceeded in continuous opera- tion, while, in exceptional cases, even with apparently well proportioned commutating poles, there has been evidence of considerable local current at less than four volts per bar. 254 ELECTRICAL ENGINEERING PAPERS The contact drop- between brush and commutator with the usual brushes is about 1 to 1.25 volts As is well known, this drop is not directly proportional to the current, but increases only slowly with very considerable increases in current density at the brush -contact. For instance, with 20 amperes per sq. in. in a given brush, the contact drop may be one volt; at 40 amperes per square inch, it may be 1 25 volts, while at 100 amperes per square inch, it may be 1 4 volts, and, with materially higher currents, it may increase but little further. This peculiar prop- erty of the brush contact is, in some ways, very much of a dis- advantage For instance, if the local currents are to be limited to a comparatively low density, then necessarily the voltages generating such currents must be kept comparatively low. With the above brush contact characteristics, two volts would allow a local current of 20 amperes per square inch to flow, (there being one volt drop from brush to commutator and one volt back to the brush). If, however, the local voltage is three volts instead of two, or only 50 per cent higher, then a local current of possibly 150 to 200 amperes per square inch may flow, and this excessive current density may destroy the brush contact, as will be de- scribed later. It may be assumed in general that the lower the apparent short circuit voltage per armatur£ conductor, the lower the pulsations in this voltage are liable to be. Assuming therefore, as a rough approximation a 50 per cent pulsation as liable to occur, then, from the standpoint of brush contact drop, the total apparent voltage of the commutated coil in continuous service machines should not be more than 4 to 45 volts, which accords pretty well with practise. For intermittent services, such as railway, materially higher voltages are not unusual As the main advantage pf the carbon brush is that it determines or limits the amount of short circuit current, it might be ques- tioned whether such advantage might not be carried much fur- ther by using higher short circuit voltages and proportionately greater resistance. However, there are reasons why this cannot be done. The carbon brush is a resistance in the path of the local current, but it is also in the path of the work current. As the brush resistance is increased, the greater is the short circuit voltage which can be taken care of with a given limit in short circuit current, but at the same time, the loss due to the work current is increased. Decreasing the resistance of the brush contact in- creases the loss due to the short circuit current, but decreases PHYSICAL LIMITATIONS IN D.C. MACHINES 255 that due to the work current. Thus, in each individual case, there is some particular brush resistance which gives minimum loss However, this- may not always be the resistance desired for best commutation, from the operating standpoint, but these two conditions of resistance appear to lie fairly close together. Practise is a continual compromise on this question of brush con- tact resistance In some machines, a low resistance brush is practicable, with consequent low loss due to work current. In other cases, which, to the layman, would appear to be exactly similar, higher resistance brushes give better average results. Thus one grade of carbon brush is not the most suitable for dif- ferent machines unless they have similar commutating condi- tions However, it is impracticalDle to design all machines of different speeds, types, or capacities so that they will have equal commutating -characteristics In non-commutating pole ma- chines where only average commutating fluxes are obtainable, the resistance of the brush is usually of more importance than m the commutating pole type, for, in the latter, a means is pro- vided for controlling the value of the short circuit current How- ever, advantage has been taken of this latter fact to such an extent in modern commutating pole machines, that the critical or best brush resistance has again become a very important condition of design and operation " Apparent " Short Circuit e m.f per Brush. The preceding considerations lead up to another limitation, namely, the total e.m.f . short circuited by the brush. This again may be considered as being made up of two components, — the apparent short circuit e.m f per bar times the average number of bars covered by the brush, hereafter called " The apparent short circuit e.m.f. per brush " ; and the e.m.f. per bar generated by the com- mutating field, times the average number of bars covered by the brush As has been shown, ordinary carbon brushes can short circuit 2 to 25 volts without excessive local current. Obviously, if the resultant e.m.f. generated in all the coils short circuited by the brush, — that is, the resultant of the short-circuit e.m.fs , due to both the armature and the commutating field is much larger than 2| volts, large local currents will flow. Therefore, in a commutating pole machine, for instance, the strength of the commutating pole field should always be such that it also neutralizes the total short circuit e.m.f. across the brush within a limit represented by the brush contact drop, in order to keep •256 ELECTRICAL ENGINEERING PAPERS within the Umits of permissible local currents. With -very low resistance brushes, the proportioning of the commutating field for neutralization of the apparent brush e.m.f. would have to be much closer than with higher resistance brushes. Moreover, not only should this e.m.f. generated by the com- mutating flux balance the total short circuit voltage across the brush within these prescribed limits, but these limits should not be exceeded anywhere under the brush. It might be assumed that if there is a pulsation of two volts per coil, for instance, then the total pulsation would be equal to this value times the average number of coils short circuited. However, this in general is not correct, as the e.m.f. pulsations for the different coils are not in phase, and their resultant may be but little larger than for a single coil Based upon the foregoing considerations, the limiting values of the apparent brush e.m.f. may be approximated as follows: Assume ordinary carbon brushes with 1 to IJ volts drop with permissible current densities — that is, with 2 to 2\ volts opposr ing action as regards local currents. Also, assume, for example, an apparent brush short circuit e.m.f. of 5 volts, with brush resistance sufficient to take care of 2\ volts. Then the total e.m.f. due to the commutating flux need not be closer than 50 per cent of the theoretically correct value, with permissible local currents. This is a comparatively easy condition, for it is a relatively poor design of machine in which the commutating pole strength cannot be brought within 50 per cent of the right value. Assuming next, an apparent brush e.m.f. of 10 volts, then the commutating pole must ■ be proportioned within 25 per cent of the right value. In practise, this also appears to be feasible, without undue care and refinement in proportion- ing the commutating field. If this machine never carried any overload, this 25 per cent approximation would represent a relatively easy condition, for experience has shown that pro- portioning within 10 per cent is obtainable in some cases, which should allow an apparent brush e.m.f. of 25 volts as a limit. However, experience al'o shows that this latter is a compara- tively sensitive condition, which, while permissible on short peak loads, is not satisfactory for normal conditions. Where such close adjustment is necessary to keep within the brush correcting limits, any rapid changes in load are liable to result in sensitive commutating conditions, for the commutating pole flux does not always rise and fall exactly in time with the arma- PHYSICAL LIMITATIONS IN D.C. MACHINES 257 ture flux, and thus momentary unbalanced conditions of pos- sibly as high as 10 or 12 volts might occur with an apparent brush e.m.f. of 25 volts. Also, very slight saturation in the commutating pole magnetic circuit may have an unduly large influence on unbalancing the e.m.f. conditions. In other words, the apparent brush short circuit and neutralizing e.m.fs. must not be unduly high compared with the permissible corrective drop of the brushes Experience shows that an apparent e.m.f. of 10 volts across the brush in well designed commutating pole machines is usually very satisfactory, while, in occasional cases, 12 to 13 volts allow fair results on large machines, and, in rare cases, as high as 1.5 to 18 volts has been allowed on small ma- chines at normal rating. However, overloads, in some cases, limit this permissible apparent brush voltage. As a rule, 30 volts across the brush on extreme overload is permissible, but, usually this is accompanied by some sparking, usually not of a very harmful nature if not of too long duration. Under such overload conditions, doubtless unbalancing of three volts or more may be permissible, and thus, with 30 volts to be neutralized, this means about 90 per cent theoretically correct proportioning of the commutating pole ftu'x. Cases have been noted where as high as 35 to 40 apparent brush volts have been corrected by the commutating pole on heavy o\'erloads with practically no sparking. This, however, is an abnormally good result, and is not often possible of attainment. Obviously, with such high voltages to be corrected, any little discrepancies, in the balancing action between the various" e.m.fs. are liable to cause excessive local current flow.- Incidentally, the above indicates pretty ' clearly why d-c generators are liable to -flash viciously when dead short cir- cuited. The ordinary large capacity machine can give 20 to 30 times rated full load current 'on short circuit. If. this large current flows, then, neglecting saturation, the armature short circuit e.m.f. across the brush will be excessive. Assuming, for instance, a 10-volt limit for normal rating, then with only ten times full load current, the apparent short circuit e.m f would be 100 volts. The commutating pole, in the normal con- struction, does not have flux margin of 10 times before high saturation is reached, and in consequence, it may neutralize only 50 to 60 volts of the 100. Therefore a resultant actual e.m.f of possibly 40 volts must be taken care of by the brushes This means an enormous short circuit current in addition to 258 ELECTRICAL ENGINEERING PAPERS the 10 times work current. Vaporization of the copper and brushes occurs and flashing results, as will be described more fully in the treatment of flashing limits. Brush contact drops of 1 to 1.5 volts have been assumed in the preceding, and certain limits in the apparent short cir- cuit e.m.f. based on these drops, have been discussed. How- ever, the conditions may be modified to a considerable extent by effects of temperature upon the brush contact resistance. Usually it has been assumed that the well known decrease in contact resistance of carbon and graphite brushes with increase in temperature, is in some ways related to the negative tem- perature coefficient of carbon and graphite. The writer has been among those who advanced this idea, but later experience, based upon tests, has shown that the reduced drop with increase in temperature does ■ not necessarily hold any relation to the negative temperature coefficient of the carbon brush itself, for similar changes in the contact drop have been found with ma- terials, other than carbon, which actually had, in themselves, positive temperature coefficients. Moreover, in some tests, the changes in contact resistance with increase in temperature have proved to be much greater in proportion than occurs in the carbons themselves. In some cases, the measured drops with temperature increases of less than 100 deg. cent, decreased to one-half or one-third of the drops measured cold. Obviously, these decreased contact resistances or drops may have a very considerable effect on the amount of local current which can flow and, therefore, in such case the foregoing general deductions, should be modified -accordingly However, the results are so aft'ected by the oxidation of the copper commutator face, and other conditions also more or less dependent upon temperature, that, as yet, no definite statement can be made regarding the practical effects of increase in temperature except the general one that the resistance is usually lowered to a con- siderable extent. Apparently, oxidation of the copper face tends toward higher contact resistance. Ofttimes, " sanding off " the glaze tends to give poorer commutation. The above points to one explanation of this. Assuming any desired limits for the apparent e.m fs., such as 4 to 4| volts per commutator bar, it is possible to approximate by calculation the limiting capacities of generators or motors in terms of speed, etc. Appendix I shows one method of domg this In the writer's experience, a number of machines have been PHYSICAL LIMITATIONS IN D.C. MACHINES 259 earned up to about the limits derived in the appendix, and the prartical results were in fair accord with the calculations. In general, it may be said that in large machines, the upper limits of capacity in terms of speed, etc. are so high that they do not indicate any great handicap on future practise. In the foregoing, the limits for the apparent short circuit e.m.f. per bar and per brush have been based upon the brush contact resistance. However, it may be suggested that something other than the brush contact resistance might be used for limiting the local current, and thus the commutating limits might be raised. For instance, an armature winding could be completely closed on itself, with high resistance leads carried from the winding to the commutator bars. Each of such leads would be in circuit only where the brushes touched the commutator bars. Thus there could be very con- siderable resistance in each lead with- out greatly increasing the total losses ; and, unlike the brushes, each lead would be in circuit only for a very small proportion of the time. About 10 years ago, the writer de- signed a non- commutating pole d-c. turbo-generator with such resistance leads connected between the winding and the commutator. The leads were placed in the arrriature slots below the . main armature winding. The idea was to have enough resistance in circuit with the short circuited coils that the brushes at no load could be thrown well forward into a field flux sufficient to produce good commutation at heavy load, even if very low resistance brushes were used. Tests of this machine showed that the non-sparking range, with the brushes shifted either forward or back of the neutral point was very much greater than in an ordinary machine. In this case, it developed that the leads were of too high resistance for practical purposes, as the armature ran too hot, the heat-dis- sipating conditions in a small d-c. turbo-armature not being any too good at best. These tests however, indicate one possibility in the way of increasing the present limits of voltage per bar and volts across the brush. Moreover, such resistances can have a positive temperature coefficient of resistance, instead of the I III I 11 I Fig. 1 260 ELECTRICAL ENGINEERING PAPERS negative one of the carbon brushes and contacts. Also, the corrective action in Hmiting local currents would vary directly with the current over any range, and not reach a limit, as in car- bon brushes. Considerable experience with resistance leads in d-c. operation has also been obtained in large a-c. commutator type railway motors, designed for operation on both a-c^ and d-c. circuits. Apparently these leads have a very appreciable balancing action as regards division of current between brush arms in parallel. With but few brushes per arm, it appears that very high current densities in the brushes can be used without undue glowing or honeycombing. Presumably the reduction in short circuit current, when operating on d-c, also has much to do with, this. Some special tests were made along this line, and it was found that a very low resistance in the leads, compared with that which was best foi: a-c. operation, was sufficient to exert quite a decided balancing between the brush arms. With properly proportioned resistance leads it should be pos- sible to use very low resistance brushes, and relatively high current densities. Advantage of this might be taken in Various ■ways. There may prove to be serious mechanical objections to such arrangements. However, if the objections are not too serious, the use of resistance leads in this manner may be prac- tised at some future time as we approach more extreme designs. Flashing One of the limits in commutating machinery is flashing. This may be of several kinds. There may be a large arc or flash from the front edge of the brush, which may increase in volume until it becomes a flash-over to some other part of the machine. Again, a flash may originate between two adjacent bars at some point between the brush arms, and may not extend further, or it may grow into a general flashover. Different kinds of flashes may arise from radically different causes, some of which may be normally present in the machine, while others may be of an accidental nature. Whatever the initial cause, the flash itself means vaporized conducting material. If the heat developed by or in this vapor arc is sufficient to vaporize more conducting material — that is, generate more conducting vapor — then the arc or flash will grow or continue. Thus, true flashing should be associated with vaporization, .and, in many cases, in order to get at the initial .PHYSICAL LIMITATIONS IN B.C. MACHINES 261 cause of flashing, it is only necessary to find the initial cause of vaporization. Arcs Between Adjacent Coptmulator Bars. This being one of the easiest conditions to analyze, it will be treated first, especially as certain flashing conditions are dependent upon this. A not uncommon condition on commutators in operation is a belt of incandescent material aroUnd the commutator, usually known as "ring fire." This is really, incandescent material between adjacent bars, such as carbon or graphite, scraped off the brush faces usually by the mica between bars. As the mica tends to stand slightly above the copper, due to less rapid "wear," its natural action is . to scrape carbon particles off the brush. These particles are conducting and if there is sufficient voltage, and current to bring them up to incandescence, this shows as a streak of fire around the commutat-or. In many cases, by its. different intensities around the commutator, this ring fire shows plainly the density of the field flux, or e.m.f. distribution around the machine. It is practically zero in the commutating or neutral zone, and shows plainly under the main field. In loaded machines,- this often indicates roughly the flux distortion. In machines which act alternately as motors and generators, as in reversing mill work, the point of highest incandescence shifts forward or backward over the commutator, depending upon the direction of field distortion. In undercut commutators (those with mica cut below the cop- per surface) this ring fire is also observable at times, due to con- ducting particles in the slots between bars. Usually such particles consist o"f- carbon or graphite, as already stated, but particles of copper may also be present. Also, oil or grease, mixed with carbon, will carbonize under incandescence, and will thus add to the ring fire. Often when a commutator is rubbed with an oiled cloth or wiper, ring fire will show very plainly, and then gradually die down. The burning oil exaggerates the action, and also, the oil itself may enable a conducting coating to adhere to the mica edges, thus starting the action, which disappears when the oil film is burned away. However, when the oil can penetrate the mica, the incandescence may continue in spots and at intervals, the mica being calcined or burned away so that it .gradually disppears in spots. This is the action usually called " pitting ", which experience has shown to be almost invariably caused by conducting material in the mica, such as carbonized oil, carbonized binding material, copper and carbon particles which have been carried in with the oil, etc. 262 ELECTRICAL ENGINEERING PAPERS TTTTT This ring fire is not always a direct function of the voltage between bars, although, under exactly equivalent conditions of speed, grade of brushes, etc., it is closely allied with voltage condi- tions. In high voltage machines, usually hard high-resistance brushes are used, which tend to give off the least carbon in the form of particles; while in low voltage machines, soft, low-re- sistance brushes, with a good percentage of graphite in them, are common, and these naturally tend to coat the mica to a greater extent. Under extreme conditions, this ring fire may become so intense locally that there is an actual arc formed between two adjacent bars, due to vaporization of the copper. This may show in the form of minute copper beads at the edge of the bar, or minute " pits " or " pockets " may be burned in the copper next to the mica. In extreme cases, where the voltage between bars is sufficient to maintain an arc, conical shaped cavities or holes may be burned in the copper. In such cases, the arc is usually explosive, resembling somewhat a small "buck-over." An examination of the com- mutator will show melted-out places, as in Fig. 2. Part of the missing copper has been vaporized by the arc, while part may have become so softened or fused that it is thrown off by centrifugal force. Exper- ience shows that sometimes these explosives arcs grow into general flashes, while at other times, they are purely local. An extended study was made of such arcs to determine the conditions which produced them. Also, numerous tests were made, the results of which are given below. It was determined first that these explosives arcs between adjacent bars were dependent, in practically all cases,- upon a fairly high voltage between bars. This was reasonable to expect, but it was found that the voltage between bars which would produce arcs in one case, would not do so in another. Apparently there were other limiting or controlling conditions. It developed that the resistance of the armature winding between two adja- cent bars has much to do with the arc. Apparently an excessive current is necessary to melt a small chunk out of a mass of good heat-conducting material like a large copper commutator; and also, a certain amount of time is required to bring it up to the o Fig. 2 PHYSICAL LIMITATIONS IN B.C. MACHINES • 263 melting point Therefore, both time and current are in\-olvcd, as well as voltage. A series of tests was made to determine some of the limiting conditions. The com.mutator of a small machine (about 20 kw., high speed) was sprinkled with iron filings, "fine dust, etc. during several days' operation under various conditions of load, field distortion, etc. Such dust, whether conducting or not; apparently would not cause arcing between bars. Graphite was finally applied with a special " wiper," and with this, ^mall arcs or flashes could be produced at 50 to 60 volts maximum between commutator bars. It soon became evident that this was too small a machine from which to draw conclusions. Then numerous other much larger gen- erators were tested. A slow -speed engine type generator of 200-kw. capacity at 250 volts, was speeded up to about double speed, in order to obtain sufficiently high e.m.f. between commutator bars. With a clean commutator nothing was obtained at 40 volts maximum per bar. The commutator was then wiped with a piece of oily waste which had been used to wipe off other com- mutators. Arcs then occurred repeatedly between commutator bars, although all such arcs were confined to adjacent bars and there were no actual flashovers from brush holder to brush holder. Moreover, the arcs always appeared to start about midway between brush arms or neutral points^ and lasted only until the next neutral point was reached. Quite large pits or cavities were burned in the bars next to the mica, as shown in Fig. 2. some of these being possibly ; inch in width, and 1/16 inch deep or more at the center. This indicated excessively large currents. These arcs would develop at about 32 to 34 volts between bars, and they were very vicious (explosive) above 35 volts Still larger machines were tested with various speeds, voltage between bars, etc It was found that, as a rule, the larger the machine — or rather, the lower the resistance of the armature winding per bar — the lower would be the voltage at which serious arcing would develop. In these tests, it was found that graphite mixed with grease gave the most sensitive arcing conditions.. In these various tests, no arcing between bars was developed in any case at less than 28 volts maximum, while 30 volts was approximately the limit on many machines. However, the results varied with the speed Apparently it took a certain time to raise the incandescent material to the arcing point and to build up a big .arc. Therefore, the duration of the possible arcing period appeared to be involved. If this were so, then a higher 264 • ELECTRICAL ENGINEERING PAPERS voltage limit for a shorter time should be possible with the same arcing tendency. Also, if this were the case, then with 30 volts maximum, for instance, between commutator bars with an un- distorted field flux, the arcing should be the same as with a some- what higher voltage with a highly distorted narrow peaked field. In other words, the limiting voltage between bars on a loaded machine might be somewhat higher than on an unloaded machine. This was actually found to be the case, the difference being from 10 per cent to 15 per cent in several instances. This, however, depended upon various limiting conditions such as the actual period within which the arc could build up to a destructive point, etc. One very interesting case developed which apparently illus- trated very beautifully the effects of lengthening or shorten- ing the period during which the arc could occur. A high-speed, 600-volt generator of a motor-generator set was speeded up about 60 per cent above normal. Even at normal speed this was a rather high-frequency machine, so that the period of time for a commutator bar to pass from neutral point to neutral point was very short. At the highest speed the graphite-grease was used liberally on the commutator, but without causing arc- ing, even when the voltage was raised considerably higher than usually required for producing arcs between bars in other machines of similar size. Neither was there much ring-fire at the highest speed with normal voltage. Finally, after an application of graphite, without forming arcs or unusual ring- fire, the speed was reduced gradually, with normal voltage maintained. The ring-fire increased with decrease in speed, until at about normal speed, it was so excessive that the on-lookers expected an explosion of some sort. However, the voltage was now below the normal arcing point and nothing happened. At still lower speed, but with reduced voltage on account of saturation, the ring-fire gradually decreased. Apparently at the very high speeds, the time was too short for the ring-fire to reach its maximum; while with reduction in speed, even with somewhat reduced voltage, the, ring-fire increased to a maxi- mum and then decreased. This test was continued sufficiently to be sure that it was not an accidental case. Only a certain combination of speed, frequency, voltage, etc. could develop this pecioliar condition, and it was purely by accident that this combination was obtained, for the result was not foreseen in selecting the particular machine used. PHYSICAL LIMITATIONS IN D.C. MACHINES 265 A summation of these and other tests led to th6 conclusion that there were pretty definite limits to the maximum volts per bar, beyond which it was not safe to go. These limits however, involved such a number of conditions that no fixed rule could be established, and apparently, the designer has to use his judgment and experience to a certain extent, if he works very close to the limits. The grades and materials of the brushes, the thickness of the mica, flux distortion from over- loads, etc. must be taken into account. For instance, the above tests were made on machines with 1/32-inch mica between bars. This thickness is fixed, to a great extent, in non-undercut commutators, by conditions of mica wear, as will be referred to later. But with undercut commutators, thicker mica can be used, and, while the gain in permissible safe voltage between bars is not at all in proportion to the mica thickness, yet it is enough to deserve consideration. The general conclusions were that with 1/32-inch mica, large current machines would very rarely flash with 28 volts maximum between bars; while with moderate capacities, 30 volts is about the^ lower limit; and with still smaller machines, 100 kw. for example, this might be as high as 33 to 35 volts, the limit rising to 50 or 60 volts with very small machines. Of course, the brush conditions have something to do with the above limits, and many exceptions to these figures will be found in actual practise. Many machines are in daily service which are subject to more or less ring-fire, but which have never developed trouble of any sort, and doubtless never will. Ap- parently, ring-fire in itself is not hannful, as a rule. It is only where it starts some other trouble that it may be considered as actually objectionable. The above limiting figures are interesting when compared with the voltages necessary to establish arcs in general. An alternating arc through air will not usually maintain itself at less than some limiting voltage such as 20 to 25 volts, corres- ponding to peak values of 28 to 35 volts. Moreover, an arc formed between the edges of two insulated bodies, such as ad- jacent commutator bars, will naturally tend to rupture itself due to the shape of its path. Furthermore, the resistance and reactance of the short circuited path, while comparatively low in large machines, will tend to. limit the voltage which main- tains the arc. In small machines with relatively high internal drops in the short circuited coils, the current will not reach a 266 ELECTRICAL ENGINEERING PAPERS commutator vaporizing value unless the initial voltage between bars is comparatively high, and usually the explosive actions are relatively small, and, in many cases, no serious arcs will develop at all. Obviously, the less the local current can in- crease in the case of short circuits between adjacent bars, the higher the voltage between bars can be, without danger. In machines having inherent constant current characteristics, very high voltages between, adjacent commutator bars are possible without serious flashing or burning. In consequence, from the flashing standpoint, constant current machines can be built for enormously high terminal voltages, compared with constant potential machines. This is a point which is very commonly overlooked in discussing high-voltage d-c. machines. Coming back to the subject of arcs between commutator bars, these are more common than is usually supposed, for, in many cases, the operating conditions are such that these arcs, if very small, or limited, will show no visible evidence. Only very minute •particles of copper may be vaporized. How- ever, these minute arcs may sometimes lead directly to more serious flashing. If, for instance, they occur in proximity to some live part of the machine, such as an over -hanging brush holder which is at a considerable difference of potential from the arcing part of the commutator, the conducting vapor rtiay bridge across and start a big arc or flash. In one instance, which the writer has in mind, a very serious case of trouble occurred in this way. This was , a very large capacity- 250- volt, low-speed, generator, in which the maximum volts per bar were not unduly high. When taking the saturation curve in the shop test, this machine " bucked" viciously several times, apparently without reason. An investigation of the burning indicated a possible source of trouble. The brush holder arms or supports to which the individual holders were attached, were located over the commutator about midway between neutral points, and, about one inch from the com- mutator face. This was not the normal position of the brush arms, as a temporary set of holders was being used for this test. It was noted that just before the flashovers occurred, con- siderable ring-fire developed. The conclusion was drawn from all the evidence that could be obtained, that a small arc had formed between bars that had reached to the brush arms, thus short circuiting a high enough voltage to draw a real flash. This happened not once but several times. The proper holders PHYSICAL LIMITATIONS IN D.C. MACHINES 267 were then applied, which put the brush arms in a much less exposed position, and not a single flashovcr occurred in all the subsequent tests and operation. In another case, a large syn- chronous converter carrying full load on shoj) test flashed over a number of times, apparently without sufficient cause. The- commutation was perfect, as evidenced by the fact that there was no perceptible sparking. The maximum voltage between bars was comparatively low. At first the flashovers were blamed on drops of water from the roof of the building, but this theory was soon disproved.. An examination of the brush holders showed that certain live parts, fairly close to the com- mutator, were at a considerable difference of potential from the nearest part of the commutator. There was but little ring- fire on the commutator, and therefore, minute arcs at first were not blamed for the trouble. A modified brush holder was tried however, with a view to decreasing the high difference of poten- tial between the live parts. All flashing then disappeared and no trouble of this sort was ever encountered in a large number of duplicate machines brought through afterwards. Both the above cases should be considered as abnormal, and they have been selected simply as examples of what small arcs between bars may do. These two cases do not in themselves constitute a proof of this action, but they serve to verify other evidences which have been obtained. In view of the fact that small arcs of a ■ non-explosive sort may form at voltages considerably lower than the limits given in the preceding part of this paper,- it should be considered whether such small arcs can cause any trouble if no other live parts of the machine are in close proximity. One case should be Considered, namely, thart of other commutator bars adjacent to the arc. When conducting vapor is- fornied by the first minute arc, this vapor in spreading out may bridge across a number of commutator bars having a much higher total differ- ■ ence of potential across them than that which caused the initial arc. Assume, for instance, a very crowded design of high- voltage commutator. In some cases, in order to use high rota- tive speeds, without unduly high commutator peripheral speed, the commutator bars are sometimes made very thin and the volts per bar very high, possibly up almost to the limit. As- suming a thickness of bar and mica of 0.2 inch (or 5 bars per inch) and a maximum volts per bar of 25, then there is an e.m.f. of 125 volts per inch circumference of the commutator. In such 268 ELECTRICAL ENGINEERING PAPERS case, a small arc between two bars may result in bridging across a comparatively high voltage through the resulting copper vapor. Therefore, when considering the possible harmful effects of minute arcs, the volts per inch circumference of the commuta- tor should be taken into consideration. The writer observed one high-voltage commutator which flashed viciously at times, apparently without " provocation." The only explanation he could find was that the vapor from little arcs resulting from ringfire was sufficient to spread all over the commutator, the bars being very thin and the voltage per bar very high. How-; ever, difficuH ies from this cause have not yet become serious, probably because no one has yet carried such constructions to the ektreme, in practical work. High voltage between commutator bars may result in flash- ing due to other than normal operating conditions. Excessive overloads may give such high voltages per armature coil or per cornmutator bar, immediately under the brush, that the terrific current rush will develop conducting vapors under the brush, which appear immediately in front of the brushes, as such vapors naturally are carried forward by rotation of the commutator. This short circuit condition under the brush has already been referred to when treating of commutation limits. It was shown then that an inherent short circuit voltage of 4 to 4| volts" is permissible in good practise. Immediately under the com- mutating pole this voltage is practically neutralized by the commutating pole field, but immediately ahead or behind the pole it is not neutralized usually, except to the extent of the commutating pole flux fringe. Thus, the resultant voltage between two bars a little distance ahead of the brush, is liable to be considerably higher than under the brush Assuming, for instance, 85 volts per bar, due to cutting the resultant field just ahead of the' brush, then with 10 times full load current, for example, there would be 35 volts between bars, and this is liable to be accompanied by highly conducting vapor formed by the excessive current at the brush contact, this vapor bein^ carried forward by rotation of the commutator. Here are the conditions for a flash, which may or may not bridge across to some other live part. If the current rush is not too great, this flash will usually appear only as a momentary blaze just in front of the brush. In many cases, if this blaze or heavy arc were not allowed to come in contact with, or bridge between, any. parts having high difference of potential, it would not be PHYSICAL LIMITATIONS IN B.C. MACHINES 269 particularly harmful. In case of " dead short circuiting" of large moderately high-voltage machines where the current can rise to 25 or 30 times normal, it is astonishing how large such arcs or flashes may Jiecome, and to what distances they will reach. The arc will sometimes go in unanticipated directions. The conducting vapor may be deflected by magnetic action and by air drafts. Shields or partitions will sometimes pro- duce unexpected results, not necessarily beneficial. Unless such shields actually touch the commutator face so that con- ducting vapor cannot pass underneath them, the vapor that does pass underneath may produce just as harmful results as if the shields were not used. Trying to suppress such arcs by covers or shields is very much the case of damming a river at the wrong end in order to prevent high water. From the preceding considerations it would appear that a compensated direct-current machine should have some ad- vantages over the straight commutating-pole type in case of a severe short circuit. With the lesser saturation in the com- mutating pole circuit due to the lower leakage, the apparent armature short circuit e.m.f. will usually be better neutralized under extreme load conditions, and thus there will be lower local currents in the brush contacts. In addition, the armature flux will be' practically as well neutralized behind and ahead of the brush, as it is under the brush, so that, with ten times current as in the former example, there may be only a low e.m.f. per bar ahead of the brush, instead of the 35 volts for the former case. Obviously, the initial flashing cause, and the tendency to continue it ahead of the brush, will be materially reduced. The compensating winding is therefore particularly advantageous in very high voltage generators,, in which the bars are usually very thin and the maximum volts per bar are high. There is a prevailing opmion that when a circuit breaker opens on a very heavy overload or a short circuit, flashing is liable to follow from such. interruption of the current. In some cases, this may be true. However, when a breaker opens on a short circuit, it is difficult for the observer to say whether both the opening of the breaker and the flash are due to the excessive momentary current, or one is consequent to the other. The short circuit, if severe, will most certainly cause more or less of a flash at the brush contacts by the time the breaker is opened, and if this flash is carried around the commutator, or bridges 270 ELECTRICAL ENGINEERING PAPERS across two points of widely different potentials, then it is liable to continue after the breaker opens, and thus s,ives the im- pression that the flashing followed the interruption of the cir- cuit. In railway and in mine work in particular, a great many flashes which are credited to overloads are primarily caused by partial short circuits on the system, or " arcing shorts," which are extinguished as soon as the main breakers are opened, so that but little or no evidence of any short circuit remains. Such a partial short circuit however, may be sufficient to open the generator circuit and to cause a flash at the same time. Not infrequently, such flashes are simply credited to opening of the breakers. There are other conditions, however, where a flash is liable to result directly from opening the breaker on heavy overload. If as referred to before, the apparent short circuit e.m.f. per brush on heavy overload is from 25 to 35 volts, then if the armature magnetomotive force could be interrupted suddenly, with a correspondingly rapid reduction in the armature flux, while the commutating field flux does not die down at an equally rapid rate, then momentarily, there will be an actual short circuit voltage of a considerable amount under the brushes which may be sufficient to circulate large enough local currents to start flashing.. With commutating pole machines, this con- dition may result from the use of solid poles and solid field yokes. Laminated commutating poles are sometimes very much of ail improvement. However, the yokes of practically all direct current machines are of solid material, and thus tend to give sluggishness in flux changes. The above explains why non- inductive shunts, or any closed circuits whatever, are usually objectionable on commutating poles or their windings. Iii non-commutating pole machines, where the brushes are liable to be shifted under the main field magnetic fringe in order to commutate heavy loads, flashing sometimes results, when such heavy overload is interrupted. Also, if the rupture of the current is very sudden, there \\ill be an inductive " kick " from the collapse of the armature magnetic field. This rise in voltage sometimes is sufficient to start a flash, especially in those cases where flashing limits are already almost reached. In synchronous converters, the conditions are materially different from d-c. generators as regards flashing when the load is suddenly broken. In such machines, the flash is liable PHYSICAL LIMITATIONS IN D.C. MACHINES 271 to follow the opening of the breaker, if simply a heavy over- load is interrupted. This is possibly more pronounced in the commutating pole machine than in the non-commutating pole type.. In a commutating pole converter, the commutating pole magnetomotive force is considerably larger than the re- sultant armature magnetomotive force, under normal opera- ting conditions, but is much smaller than the armature magneto- motive force considered as a straight d-c. or a-c. machine. Normally the commutating pole establishes a commutating field or flux in the proper direction in the armature. However, if, for any reason, the converter becomes a motor or a generator, even momentarily, the increased magnetomotive force of the armature may greatly exceed that of the commutating pole, so that the commutating pole flux will be greatly increased, or it may be greatly reduced, or even reversed, depending upon which armature magnetomotive force predominates. The above is what happens when a synchronous converter hunts, and under the accompanying condition of variable armature magnetomotive force, the commtxtating pole con- verter, with iron directly over the commutating zone, is liable to show greater variations in the flux in the commutating zone than is the case in the non-commutating pole converter. Ex- perience has shown that when a synchronous converter carry- ing a heavy overload has its direct-current circuit suddenly interrupted, it is liable to hunt considerably for a very short period, depending upon the hunting constants of the individual machine and circuit. Apparently, all converters hunt to some extent with such change in load. This hunting means wide variations in the commutating pole flux with Corresponding sparking tendencies. For a " swing " or two, this sparking may be so bad as to develop into a flash. Thus the flash follows the interruption of the circuit. Curiously, the most efl!ective remedy for this condition is one which has pro^•ed most objectionable in d-c. machines, namely, a low-resistance closed electric circuit surrounding the commutating pole. The primary object of this remedy is not to form a closed circuit around the commutating field, but to obtain a more effective damper in order to minimize hunting. In a paper presented before the Institute several years ago,* the writer showed that the ideal type of cage winding for damp- *Interpoles in Synchronous Converters, by B. G. Lamme and F D Newbury, A. I. E. E., Trans. 1910. 272 ELECTRICAL ENGINEERING PAPERS ing synchronous converters, namely, that in which all circuits- are tied together by common end rings, was not suitable for commutating pole converters due to the fact that the various sections of this cage winding form low-resistance closed- circuits around the commutating poles. This was in accord with all evidence available to that time, and no one took exception to it. However, later experience has shown that this was incor- rect, for, in later practise, it was found that the use of a complete cage damper of low resistance which decreases the hunting tendency, also greatly decreases the flashing tendency, so that- today most converters of the commutating pole type are being made with complete cage dampers. Apparently, the flashing tendencies in converters due to hunting are much worse than those due to flux sluggishness. Therefore, a sacrifice can be made in on© for the benefit of the other. In the case of a dead short circuit on the d-c. side of a syn- chronous converter, there is liable to be flashing, just as in the d-c. machine, and the flash and the breaker opening are liable to occur so closely together that an observer cannot say which is first. In d-c. railway motors, flashing at the commutator is not an uncommon occurrence. One rather comrhon cause of flash- ing, especially at high speed, is due to jolting the brushes away from the commutator, due to rough track, etc. This is espe- cially the case with light spring tension on the brushes. The carbon breaks contact with the copper, forming an arc which is carried around. Another prolific source of flashing is due to opening and closing the motor circuit in passing over a gap or dead section in a trolley circuit. Here the motor current is entirely interrupted, and, after a short interval, it comes on again, without any resistance in circuit except that of the motor itself. If the current rush at the first moment of closing is not too large, and if the armature and field magnetic fluxes build up at the same rate, then there is usually but small danger of a flash, except under very abnormal conditions. The rapidly changing field flux however generates heavy currents under the brushes, thus tending toward flashing. The reactance of the motor, especially of the field windings, limits the first cur- rent rush to a great extent. According to this, closed second- ary circuits of low resistance around either the main poles or the commutating poles, should be objectionable, and experience^ bears this out. PHYSICAL LIMITATIONS IN D.C. MACHINES 273 In railway armatures, as a rule, fewer commutator bars per pole are used on the average than in stationary machines of corresponding capacity, except possibly, in large capacity motors. This is due largely to certain design limitations in such apparatus, but this has doubtless been responsible for a certain amount of flashing in such apparatus. Average e.m.f. and " Field Form." A rather common prac- tise has been to specify the average volts per bar in a given machine. This, in itself, does not mean anything, except in a very general way; for the limit is really fixed by the maximum volts per bar, as already shown, and there is no fixed relation between the average and the maxirhum volts per bar. The ratio be- tween these two voltages is dependent upon the field flux dis- tribution, — that is, the " field form." In practise, this ratio varies over a wide range, depending upon the preferences of the designer, upon limitations of pole space available, etc. Also, with load, it depends upon the amount of flux distortion of the field, which, in turn varies greatly in practise. In well proportioned modern machines, where space and other limita- tions permit, the average e.m.f. per bar is about 70 per cent of the maximum at no load, and about 55 per cent to 60 per cent with heavy load. This means that about 15 volts per bar, average, is the maximum permissible, in large machines with considerable - field distortion, if a maximum of 28 volts per bar is not to be exceeded. On this basis, a 600-volt machine should therefore have not less than 40 comniutator bars per pole. However, this is with considerable field distortion. If this distortion is reduced or eliminated, the average volts can be considerably higher, as in machines with high saturation in the pole faces, pole horns and armature teeth, or with com- pensated fields. Synchronous converters are practically self- compensated and can therefore have higher limits than the above, if the normal rated e.m.f. is never to be exceeded. How- ever, in 600-volt converter work, in particular, wide variations sometimes momentarily occur, up to 700 to 750 volts, and such machines should have some margin for such voltage swings. The ordinary 600-volt d-c. generator also attains materially higher voltages at times, which would be taken into account in the limiting voltage per commutator bar and the total number of commutator bars per pole. Obviously, the " fatter " the field form, the nearer the aver- age voltage can approach the maximum. With an 80 per cent- 274 ELECTRICAL ENGINEERING PAPERS field form, instead of 70 per cent, for instance, the number of bars per pole can be reduced directly as the polar percentage is increased; and 35 bars per pole with 80 per cent would be as good as 40 bars with 70 per cent assuming the same percentage of field distortion in both cases. An increase in the polar arc will tend toward increased distortion, but the reduced number of turns per pole should practically balance this, so that, other things being unchanged, the flux distortion should have prac- tically the same percentage as before. In large machines of very high speeds, large polar percentages, — that is, large " field form constants," are very advantageous, but are not always obtainable, due to the space required for the commutating pole winding. In compensated field machines, with their smaller commutating pole windings, the conditions are probably best for high field form constants, and high aver- age volts per bar; and thus this type often lends itself very well to those classes of ma- chines where the minimum / ^"""^^^ possible number of commu- j tator bars is necessary. This m is the case with 'very high . . Jv^j speeds, and also for very high /i^ voltage machines. / \ Usually it is considered that the commutating conditions of a machine are practically the same with the same current, whether it be operated as a generator or motor. However, when it comes to fiashing conditions, there is one very consider- able difference between the two operations. In the d-c. gen- erator, the field flux distortion by the armature is such as to crowd the highest field density, and thus the highest volts per bar, away from the forward edg6 of the brushes. In the motor, the opposite is the case, and therefore there is a steeply rising field, and a corresponding e.m.f. distribution in front of the brushes. As the flash is carried in the direction of rotation it may be seen that, in this particular, the generator and motor are different. Blackening and Burning — High Mica — " Picking Up " Copper In the preceding, certain limitations of commutation and flashing have been treated. There are, in addition, a number Fig. 3 PHYSICAL LIMITATIONS IN B.C. MACHINES 275 of other conditions which are related closely to commutation, and which have already been touched upon to a limited extent. One of these is the permissible current density in the brushes or brush contacts. As brought out before, there are two currents to be con- sidered, namely, the work current which flows to or from the outside circuit, and the local or short circuit current which is purely local to the short circuited coils and the brush. The true current density is that due to the actual resultant current in the brush tip or face, which is very seldom uniform over the whole brush tip. The " apparent " current density is that due to the work current alone — assumed to be uniform over the brush tip. The current density very commonly has been as- sumed as the total work current, in and out, divided by the total brush section, and, moreover, this has been considered as the true current density, the local or short circuit currents being neglected altogether. This method of considering the matter has been very misleading, resulting in many cases, in a wrong or unsuitable size of brush being used just to meet some specified current density. In many of the old, non-com- mutating pole machines, the local currents were predominant under certain conditions of load, for the brushes, as a rule, had to be set at the best average position, .so that at some average load, the commutating conditions would be best. At higher and lower loads, the short circuit currents were usually com- paratively large. The wider the brush contact circumferen- tially, the greater would be the short circuit currents and the higher the actual current density at one edge of the brush, while the apparent density would be reduced. Thus, in at- tempting to meet a low specified current density, the true den- sity would be greatly increased. The fallacy of this procedure was shown in many cases in which the brush contact was very greatly reduced' by grinding off one edge of the brush. Very often, a reduction in circumferential width of contact to one- half resulted in less burning of the brush face. The apparent; density was doubled but the actual maximum density was actually reduced. Many of these instances showed very conclusively that much higher true current densities were prac- ticable, provided the true and apparent densities could be brought more nearly together. This is what has been accom- plished to a considerable extent in the modern well designed commutating pole machine. In such machines, the current dis- 276 ELECTRICAL ENGINEERING PAPERS tribution at the brush face is nearly uniform under all condi- tions of load. It is not really uniform, even in the best machines; but the variations from uniformity, while possibly as much as 50 per cent in good machines, is yet very small compared with the variation in some of the old non-commutating pole machines. In consequence, it has been possible to increase the apparent current densities in the brushes in modern commutating pole machines very considerably above former practise, while still retaining comparatively wide brush faces. In fact, the width of the brush contact circumferentially is not particularly limited if the commutating field flux can be suitably proportioned; that is, where a suitable width and shape of commutating field can be obtained. In many of the old time machines, an ap- parent density of 40 amperes per square inch under normal loads was considered as amply high, while at the present time, with well proportioned commutating poles, 50 per cent higher apparent densities are not uncommon. However, experience shows that the same brushes, with perfectly uniform distribu- tion of current at the brush face, can carry still higher currents. Therefore, in modern commutating pole machines, the actual upper limit of brush capacity is not yet attained. But there are reasons why this upper linit is not practicable. One reason is that already given, that uniform current distribution over the brush face is seldom found. This neans that a certain margin must be allowed for variations. A second reason lies in the unequal division of current between the various brushes and brush arms. This may be due initially to a number of different causes. However, when a difference in current once occurs, it tends to accentuate itself, due to the negative co- efficient of resistance of the carbon brushes and brush contacts. If one of the brushes, for instance, takes more than its share of current for a pei4od long enough to heat the brush more than the others, then its resistance is lowered and it tends to take still more current. If there were no other resistance in the current path, it is presumable that the parallel operation of carbon brushes would be more or less unsatisfactory. In the practical case, however, instead of the operation being im- practicable,, it is merely somewhat unstable. Unequal division of current between the brushes on the same brush arms, is to some extent, dependent upon the total current per arm. Where there are many brushes in parallel and the total current to be carried is very large, it is obvious that one brush may take PHYSICAL LIMITATIONS IN B.C. MACHINES 277 an excessively large current without materially decreasing the current carried by the other brushes. As a rule, the larger the current per arm, the more difficult is the problem of prop- erly balancing or distributing the current among all the brushes. Schemes have been proposed, and patented, for forcing equal division, but, as a rule, they have not proved very practicable, although some comparatively simple expedients have been tried out with a certain degree of success. In the same way, the division of current among brush arms of the same polarity is not always satisfactory. 50 per cent variation of current between different arms is not very unusual, and the writer has seen a number of instances where the varia- tion has been 100 per cent, or even much more. Obviously, with such variation, it is not practicable to work the brushes up to the maximum density possible, for some margin must be allowed for such unbalancing. Experience has shown that when current passes through a moving contact, as from a brush to the commutator copper, or vice versa, a certain action take place which resembles elec- trolytic action to some extent, although it is not really electro- lytic. It might also be said to resemble some of the actions which takes place in an arc. Minute particles appear to be eaten or burned away from one contact surface, and these are sometimes deposited mechanically upon the opposing surface. The particles appear to be carried in the direction of current flow, so that if the current is from the carbon brush to the copper, the commutator face will tend to darken somewhat, evidently from depositation of carbon. If the current is from the copper to the' carbon, the brush face will sometimes tend to take a coating of copper, while the commutator face will take a clean, and sometimes raw, copper appearance. As the current is in both directions on the ordinary commutator, this action is more or less averaged, and therefore is not usually noticed. With one polarity or direction of current, the com- mutator face eats away, while with the other direction, the brush face is eaten away and may lose its gloss. The above action of the current gives rise to a number of limiting conditions in directive urrent practise. Experience shows that this "'eating away " action occurs with all kinds of brushes, and with various materials in the commutator. It appears to be dependent, to a considerable extent, upon the losses at the ■contact surface. In other words, it is dependent upon both the 278 ELECTRICAL ENGINEERING PAPERS current and the contact drop. With reduction in contact drop, this burning action apparently is decreased, but in commutating machinery, this reduction cannot be carried vcr>' far, in most cases, on account of increase in short circuit current, which nullifies the gain in contact drop. In fact, in each individual machine, there is some critical resistance which gives least loss and least burning at the contact surfaces. Practise has shown that this burning action is very slow at moderate current densities in carbon and graphite brushes — so slow as usually to be' unnoted. However, if the actual current density in the brush face is carried too high, the burn- ing of the brushes may become very pronounced. With the actual work current per brush usual in present practise, the burning of the brush face may usually be credited to local cur- rents in the brushes. This is one pretty good indication of the presence of excessive local currents. It also indicates the location and direction of such currents, but is not a very exact quantitative measure of them. It is not uncommon, in exam- ining the brushes of a generator or motor, to find a dull black area under one edge of the brush, which obviously has been burned, while the remainder of the brush face is brightly polished. In severe cases, practically as good results will be obtained if the burned area is entirely cut away by beveling the edge of the brush. This eating away of either the brush face or the commutator,, and the deposit upon the opposing face, leads to certain very harmful conditions in direct-current machinery. As stated before, if the true current density is kept sufficiently low in the contact face, the burning is negligibly small in most cases. However, where the current passes from the commutator to the brush, it is the commutator copper which eats away, while the mica between commutator bars doe? not eat away, but must be worn away at the same rate that the copper is burnt, if good contact is to be maintained. Let the burning of the copper- gain ever so little on the wear of the mica, then trouble begins. The brush begins to " ride " on the mica edges and docs not make true contact with the copper. This increases the burn- ing action very rapidly, so that eventually the mica stands well above the copper face. This is the trouble usually known as " high mica." It is frequently credited to unequal rates of wear of copper and mica. This idea of unequal wear has- been partly fostered by the fact that with relatively thick. PHYSICAL LIMITATIONS IN D.C. MACHINES 279 mica, the action is greatly increased,, or, with very thin com- mutator bars, with the usual thickness of the mica, the high mica trouble becomes more serious. In both these latter cases, it is the higher percentage of mica, — that is, the relatively poorer wearing characteristics of the mica itself, which is at fault. But the commutator copper does not wear away. In fact, it is not physically possible for it to wear below the mica. It is " eaten away " or burned, as described above. In some special cases, where this burning is unusually severe, the mica apparently wears down about as fast as the copper, so that the commutator remains fairly clean and has no particularly burnt appearance, but grooves or ridges, showing undue wear. But this rapid apparent wear is a pretty good indication that exqessive burn- ing action is ])rescnt at times, usually due to excessive local currents. In some cases, this burning action may be present only during heavy or peak loads which ma}' be so interspersed with periods of light running that the true wear of the mica catches up with the burning of the copper. In such eases, the commutator may have a beautiful glossy appearance normally, but may wear in grooves and ridges On account of this burn- ing action, practise has changed somewhat in regard to stagger- ing of brushes on commutators to prevent ridging between the brushes. Formerly, it was common practise to displace all the positive brushes one direction axialh', and the negatives in the other direction, in order to have the brushes overlap. This, however, did not entirely prevent ridging, for the burning of the copper occurred only under one polarity. It is now con- sidered better practise to stagger the arms in pairs. With commutating pole machines, the true current densities in the brushes are carried up to as high a point as the non- burning requirements will permit. Reduction in local currents has been accompanied by increase in the work current density. Therefore, conditions for burning and high mica are still exist- ent, as in non-commiitating ]jole machines. In recent years, a new practise, or rather an extension of an old practise, has been very generally adopted, namely, undercutting the mica between bars. In early times, such undercutting was practised to a certain extent, usually however, to overcome mica troubles principally. In the newer practise, such undercutting is pri- marily for other reasons, although the mica problem is partly concerned in it. Diiring the last few years, extended experi •ence has shown that graphite brushes, or carbon brushes with 280 ELECTRICAL ENGINEERING PAPERS considerable graphite in them, are extremely good for collect- ing current, but on the other hand, are very poor when it comes to wearing down the mica, due to their softness or lack of ab- rasive qualities. Due to the graphite constituent, such brushes- are largely self-lubricating, and therefore, "ride" more smoothly on the commutator than the ordinary carbon brush. They are therefore much quieter, and this is a very important point with the present high speeds which are becoming very much the practise. However,' by undercutting the mica, all difficulty from lack of abrasive qualities in the brush is overcome, and thus the good qualities of such brushes could be utilized. The advantage of self-lubricating brushes should be apparent to anyone who has had difficulties from chattering and vibration of brushes, due to lack of lubrication. Such chattering may put a commutator " to the bad " in a short time, and the con- ditions become cumulatively worse. Chattering means bad contact between the brush and commutator, which in turn, means sparking and burning, which means increased chatter-' ing or vibration. The above refers to burning of the commutator face. But such burning also may have a bad effect on the brushes. When the commutator copper burns away to any extent, it may de- posit on the brush face following the direction of the current. This coating on the brush face sometimes leads to serious trouble, by lowering the resistance of the contact surface. This not only allows larger short circuit current and greater heating of the brush, but it makes the resistance of that particular path lower than that of other parallel brush paths. In con- sequence, the coated brush takes an undue share of the total current, as well as an unduly large local current. The result- ant heating may be such that the brush actually becomes red hot or glows. This heating further reduces the resistance,- and tends to maintain the high temperatures. This glowing, or overheating very frequently causes disintegration of the bind- ing or other material in the brush, so that it gradually honey- combs at or near its tip. This action may keep up until the brush makes bad contact. It may be that a similar action may occur coincidently on other brushes, but, there is no uniformity about it. This action of transferring copper to the brush is sometimes known as " picking up copper." It is not limited to brushes of one polarity, except where the metallic coating is caused primarily by the work current. Where it results from PHYSICAL LIMITATIONS IN B.C. MACHINES 281 high local currents, it may be on the brushes of either polarity, ior the local currents go in and out at each brush. However, according to the writer's experience, this coating is more com- mon on the one polarity. Glowing and honeycombing of brushes is not necessarily ■dependent upon the metallic coating on the brushes, although this latter increases the action. Anything that will unduly in- ■creas^ the amount of current in any brush contact for a period long enough to result in heating and lower contact resistance, with brushes in parallel, may start this glowing and honey- combing. It is not as common an, action in modern machines as in old time ones. As an evidence that poor contact or high contact drop tends to produce burning, may be cited the fact that, in many cases ■of apparent rapid wear of the commutators, such' wear has been practically overcome by simply undercutting the mica and thus allowing more intimate contact between brush and •copper. In some instances, this also lessened or eliminated the tendency to pick up copper. Thus undercutting has been very beneficial in quite a number of ways. Number of Slots, Conductors Per Slot, Etc. There arc certain limitations in direct-current machines, de- pending upon the minimum number of slots per pole which can be used. Provided satisfactory commutating conditions can be obtained, it is in the direction of economy of design to use a relatively low number of slots per pole, with a correspond- ingly large number of coils per slot. This is effective in several ways. In the first place, insulating space is saved, thus allow- ing an increase in copper or iron sections, either of which al- lows greater output. In the second place, wider slots are favor- able to commutation. Thus the natural tendency of d-c. de- sign is toward a minimum number of slots per pole. But if this is carried too far, certain objections or disadvantages arise or become more prominent, so that a,t some point they over- balance the advantageous features. As the slots are widened and the number of teeth dimiaished,, variations in the reluct- ance of the air gap under the main poles, with corresponding pulsations in the main field flux become more and more pro- nounced. These may effect commutation, as the short cir- cuited armature coils form secondary circuits in the path of these pulsations. But before this condition becomes objec- 282 ELECTRICAL ENGINEERING PAPERS tionable, other troubles are liable to become prominent, such as " magnetic noises," etc. If the machine is of the commutat- ing pole type, there are liable to be variations in the commutat- ing pole air gap reluctance, so that it may be difficult to obtain proper conditions for commutation. A relatively wide com- mutating zone is required if there are many coils per slot; also, all the conductors per slot usually will not commutate under equal conditions, which may result in blackening or spotting of individual commutator bars symmetrically ^aced around the commutator, corresponding to the number of slots. Innon- commutating pole machines, it may be difficult to find a suit- able field or magnetic fringe in which to commutate, and thus the first and last coil in each slot will have quite different fluxes in which to commutate. Depending upon the relative weight of the various advant- ages and disadvantages of a small number of slots per pole, practise varies greatly in different apparatus. In small and medium capacity railway motors, where maximum output in minimum space is of first importance, and where noise, vibra- tions, etc. are not very objectionable, the number of slots per pole used is probably lower than in any other line of d-c. ma- chines, six to eight per pole being rather common. In the smaller and medium size stationary motors, where noise must be avoided, a somewhat larger number of slots is used in gen- eral, depending- somewhat upon the size of the machine. On still larger apparatus, excepting .possibly, small low-speed en- gine type generators, 10 slots or more per pole are used in rfiost cases, and, in general, more than 12 are preferred. In the large 600-volt machines, the number is fixed partly by the minimum number of commutator bars per pole, and the num- ber of coils per slot. Assuming three coils per slot, then with a minimum number of commutator bars of about 40 per pole, the minimum number of slots per pole will be 14, and with two bars per slot, will.be correspondingly larger. This there- fore represents one of the limits in present practise. Noise, Vibration, etc. Mention has been made of limita- tions of noise and vibration being reached, in considering the minimum number of slots. This is a very positive limitation in design, especially so in recent years, when everything is being carried as close as possible to all limits in economies in materials and constructions. All the various conditions which cause undue noises in electrical apparatus are not yet well known, PHYSICAL LIMITATIONS IN D.C. MACHINES 283 and the application of remedies is more or less a question of " cut-and-try." A fundamental cause of noise in direct-current machines lies in very rapid pulsations or fluctuations in magnetic conditions. This has been well known for years, and many solutions of the problem of preventing such variations in magnetic conditions from setting up vibrations and consequent noise, have been proposed, but many of them appear to hold only for the particu- lar machine, or line of machines, for which they were devised. A perfectly good remedy in one machine not infrequently proves an utter failure on the next one. There are certain remedies for noise in direct-current machines which apply pretty generally to all machines, but, as a rule, such remedies mean more expensive constructions. In general, large air gaps and gradual tapering of the flux at the pole edges tend toward quiet opera- tion. A large number of slots per pole tends toward quietness. However, the trend of design has been toward very small air gaps, especially in recent designs of small and moderate size d-c. motors; also, the aim has been to use as few armature slots as possible. Moreover, newer designs with steel or wrought iron frames, as a rule, have the magnetic material in the frames reduced to the lowest limit that magnetic conditions will per- mit. Also, with the general use of commutating poles, the tendency has been toward " strong " armatures and corres- pondingly weak fields, so that the total field fluxes and field frames arc relati\'ely small compared with the practise of a few years ago. With these small frames, resonant conditions not infrequently are encountered, especially in those machines which are designed to operate over a very wide range in speed. There is liable to be some point in the speed range where the poles or frame, or some other part, is properly tuned to some pulsating torque or " magnetic pull " in the machine. In such case, a very slight disturbance of a periodic nature may act cumulatively to give a very considerable vibration and conse- quent noise. The pulsations in magnetic conditions which produce vibra- tion may be due to various causes, but, as a rule, the slotted •armature construction is at the bottom of all of them. Open type armature slots usually are. much worse than partially closed slots. Such open slots produce " tufting " or " bunch- ing " of the magnetic flux between the field and armature, and it is this bunching of flux which usually, in one form or another, 284 ELECTRICAL ENGINEERING PAPERS produces a magnetic pulsation or pull which sets up vibration. This bunching of lines may be such as to set up pulsating mag- netic pulls at no-load as well as full load In other cases, the ampere turns in the armature slots tend to exaggerate or accen- tuate the bunching so that the vibration varies with the load. This bunching of the flux may act in various ways. The total, air gap reluctance between the armature and main poles may vary or pulsate, so that the radial magnetic pull between any main pole and the armature will pulsate in value. If the re- luctances under all the poles are varying alike, then these pulsating radial pulls will tend to balance each other at all instants. However, if the reluctances under the different poles do not vary simultaneously, then there are liable to be un- balanced » radial magnetic pulls of high frequency, depending upon the number of armature teeth, speed of rotation, etc. If this frequency is so nearly in tune with the natural period of vibration of some part of the machine, such as the yoke,, poles or pole horns, armature core and shaft, that a resonant condition is approximated, then vibration and noise are almost sure to occur. Radial unbalanced pulls, as described, are liable to occur when the number of armature teeth is other than a multiple of the num- ber of poles; and the smaller the number of teeth per pole, the larger will be the unbalancing m general As a remedy, it might be suggested that the number of armature slots always be made a multiple of the number of poles However, there are several objections to this One serious objection is that, on small and moderate size d-c machines, the two-circuit type of armature winding is very generally used, and, with this type of winding, the number of armature coils and commutator bars must always be one more or less in number than some multiple of the number of pairs of poles. Mathematically therefore, with a two- circuit winding, the number of slots can never be a multiple of the number of poles unless an unsymmetrical winding is used, that is, one with a " dummy " coil A second objection to using a number of slots which is a multiple of the number of poles, is that there are pulsating magnetic pulls \t. hich may be exag- gerated by this very construction There are two kinds of mag- netic pulls, a radial, which has already been considered, and a circumferential, due to the* tendency of the armature core to set itself where it will enclose the maximum amount of field flux. Obviously, if the arrangement of slots is such that when PHYSICAL LIMITATIONS IN B.C. MACHINES 285 one pole has a maximum flux into the teeth, another pole has a minimum, then the circumferential puslations in torque will be less than if all poles enclosed the maximum or the mini- mum flux simultaneously. This latter condition will be produced when the number of armature slots is a multiple of the number of poles. Therefore, in dodging unbalanced radial magnetic pulls by using a number of armature slots which is a multiple of the number of poles, the designer is liable to exaggerate the circumferential variations in torque or pull, so that he is no better off than before. This circumferential pulsating magnetic pull may act in various ways to set up vibration, and if there is any resonant condition in the machine, vibration and noise will result. Several years ago, the writer made some very interesting tests on a number of d-c. machines to discover the nature of the vi- brations which were producing noise. These machines had very light frames and were noisy, although not exces- sively so. The following results were noted: In .certain four-pole machines, it was noted that the frames vibrated in a radial direction, as could be easily determined by feeling. How- ever, upon tracing around the frame circumferentially, nodal points were „ . noted. In some cases, there were rIG. 4 points of practically no vibration midway between the pt)les, as at A in Fig. 4. In other cases the point of least vibration was at B, directly over the main poles. Apparently, minimum vibration at A and maximum at B occurred when the pulsating magnetic pulls were in a radial direction, while, with circumferential pulls, the maximum vibra- tion was at A. It was also noted in some instances that a varia- tion in the width of the contact face of the pole against the yoke produced vibrations and noise, and nodal points in the yoke, the vibrations being a maximum at A . In still other cases in commutating pole machines, vibrations and noise were apparently set up by either radial or circumferen- tial magnetic pulsations under the commutating poles themselves, as indicated by the -fact that removal of the commutating poles, or a considerable increase in their air gaps, tended to overcome the noise. In such cases, the noise usually increased with the load, in constant speed machines. 286 ELECTRICAL ENGINEERING PAPERS Skewing of the armature slots, or of the pole faces, has proven quite effective in some cases of vibration and noise. Tapered air gaps at the pole edges have also proven effective in many individual cases. However, the causes of the trouble and the remedies to be applied in specific cases are so numerous and so varied that at present it is useless to attempt to give any liinita- tions in design as fixed by noise and vibration due to magnetic conditions. " Flickering" of Voltage, and " Winking " of Lights From time to time, cases have come up where noticeable " winking " of incandescent lights occur, this being either of a periodic or non-periodic character, the two actions being due to quite different causes In either case, the primary cause of the difficulty may be in the generator itself, or it may be in the prime mover. The characteristics of the incandescent lamp itself tends, in some cases, to exaggerate this winking To. be observable when periodic, the period must be rather long, cor- responding to a very low frequency Periodic flickering of voltage may be considered as equivalent to a constant d-c voltage with a low-frequency small-amplitude alternating e.m.f. superimposed upon it In view of the fact that incandescent lamps of practically all kinds give satisfactory service without flicker at 40 cycles with the impressed e.m.f. varying from zero to 40 per ce'nt above the effective value, one would think that a relatively small variation of voltage, of 3 per cent or 4 per cent for instance, would not be noticeable at frequencies of 5 to 10 cycles per second. However, careful tests have shown that commercial incandescent lamps do show pronounced flicker at much lower percentage variations in voltage, de- pending upon the thermal capacity of the lamp filament. Based on such thermal capacity, low candle power 110-volt lamps, for example, should show more flicker than high candle power lamps. Also, tungsten lamps for same candle power should be more sen- sitive than carbon lamps, due to their less massive films. In fact, trouble from winking of lights has become much more pro- nounced since the general introduction of the lower-candle power, higher-efficiency incandescent lamps. In view of the fact that winking has been encountered with machines in Which no pronounced pulsations in voltage appear to be possible, a series of tests was made some years ago to determine what periodic variation was noticeable on ordinary PHYSICAL LIMITATIONS IN D.C. MACHINES 287 low-candle power Tungsten and carbon lamps. A lamp circuit was connected across a source, of constant direct e m.f , and in series with this circuit was placed a small resistance which could be varied at different rates and over varying range The results were rather surprising in the very low pulsations in volt- age which showed_ flickering of the light when reflected from a white surface. With the ordinary frequencies corresponding to small engine type generators — that is, from 5 to 10 cycles — peri- odic variations in voltage of J per cent above or below the mean value were sufficient to produce a visible wink, with IG-candle power carbon lamps; while 1 per cent variation above and below was quite pronounced. With corresponding tungsten lamps, only about half this variation is sufficient to produce a similar wink. These tests were continued sufficiently to show that sucli periodic fluctuations in voltage must be limited to extremely small and unsuspected limits This condition therefore imposes upon the designer of such apparatus a degree of refinement in his designs which is almost a limitation in some cases. It is probable that non-periodic fluctuations in voltage do hot have as pronounced an effect in regard to winking of lights as is the case with periodic fluctuations, if they do not follow each other at too frequent intervals, unless each individual pulsation is of greater amplitude, or is of longer duration Possibly a momentary variation in voltage of several per cent will not be noted, except by the trained observer, unless such variation. has an appreciable duration A brief discussion of the two classes of voltage \'ariations may be of interest, and is given below. Periodic Fluctuations. As stated before, these may be due to conditions inside the machine itself, or may be caused by speed conditions in the prime mover. Not infrequently, the two act together. Variations in prime mover speed can act in two ways; first, by vsirying the voltage directly in proportion to the speed, and second, by varying the voltage indirectly through the excitation, the action being more or less cumulative in some cases. Such speed variations usually set up pulsations corresponding directly to the revolutions per minute and in- dependent of the number of poles on the machines. In the machine itself, periodic pulsations of frequency lower than normal frequency of the machine itself, may be caused by magnetic dissymmetry of some sort, or b\- unsymmetrical windings. Usually, such dissymmetries give voltage fluctua- 288 ELECTRICAL ENGINEERING PAPERS tions at a frequency corresponding to the normal frequency of the machine, and therefore will have no visible effect unless such normal frequency is comparatively low, which is usually the case in engine type d-c. generators. In other cases, these ■dissymmetries may give pulsations corresponding to the rev- olutions, and not the poles. For instance, if the armature periphery and the field bore are both eccentric to the shaft, then magnetic conditions are presented which vary directly Avith the revolutions. However, there have been cases where no dissymmetry could be found, and yet which produced enough variations to wink the Hghts. Usually in such cases, the number of armature slots per pole was comparatively small, and the trouble was overcome by materially increasing the number of slots per pole. A second source of winking has been encountered in some three- wire machines in which the neutral tap is not a true central point. In such case, the neutral travels in a circle around the central point and impresses upon the d-c. voltage a pulsation •corresponding to the diameter of the circle. Its frequency how- ever, is that of the machine itself and is therefore more notice- able on low frequency machines, such as engine type generators. Non-Periodic Pulsations or Voltage " Dips." In all d-c. generators, there is a momentary drop or " dip" in voltage with sudden applications of load, the degree of drop depending upon the character and amount of load, etc. The effects of this have been noted most frequently in connection with electric elevator operation, in which the action is liable to be repeated with sufficient frequency to cause complaint. Various claims have been made that certain types of machines did not have such voltage dips, and that others were subject to it. In con- sequence, the writer and his associates made various tests in •order to verify an analysis of this action which is given below. The explanation of this dip in voltage is as follows. Assume, for instance, a 100-volt generator supplying a load of 100 am- peres — that is, with one ohm resistance in circuit. The drop across the resistance is, of course, 100 volts. Now, assume that a resistance of one ohm is thrown in parallel across the circuit. The resultant resistance in circuit is then one-half ohm. However, at the first instant of closing the circuit through the second resistance, the total current in the circuit is only 100 amperes, and therefore the line voltage at the first instant momentarily must drop to 50 volts. However, the e.m.f. PHYSICAL LIMITATIONS IN D.C. MACHINES 289 generated in the machine is 100 volts, and the discrepancy of 50 volts between the generated and the line volts results in a very rapid rise in the generator current to 200 amperes. If the current rise could be instantaneous, the voltage dip would be represented diagrammatically by a line only; that is, no time element would be involved. However, the current can- not rise instantaneously in any machine, due to its self-induction, and therefore, the voltage dip. is not of zero duration, but has a rnore or less time interval. The current rises according to an exponential law, which could be calculated for any given ma- chine if all the necessary constants were known. However, such a great number of conditions enter into this that is it usually impracticable to predetermine the rate of current rise in de- signing a machine; and it would not change the fundamental conditions if the rate could be predetermined, as will be shown later. A rough check on the above theory could be obtained in the following manner, by means of oscillograph tests. For example, it was assumed in the above illustration that with one ohm resistance in circuit, an equal resistance was thrown in parallel, which dropped the voltage to one-half. In practise, the actual drop which can be measured might not be as low as one-half voltage, as the first increase in current might be so rapid as to prevent the full theoretical dip from being obtained. However, an oscillograph would show a certain amount of voltage drop. If HOW, after the current has risen to 200 amperes and the con- ditions become stable, the second resistance of one ohm is thrown in parallel with the other two resistances of one ohm each, then in this latter case, the resultant resistance is re- duced to two-thirds the preceding value, instead of one-half, as was the case in the former instance. Therefore, the dip would be less than in the former case. Again, if 'one ohm re- sistance is thrown in parallel with three resistances of one ohm each, the resuitant resistance becomes three-fourths of the preceding value, — that is, the voltage dip is still less. There- fore, according to the above Analysis, if a given load is thrown on a machine, the dips will be relatively less the higher the load the machine is carrying. Also, if the same percentage of load is thrown on each time, then the dips should be practically the same, regardless of the load the machine is already carrying. For example, if the machine is carrying 100 amperes, and 100 amperes additional is thrown on, the dip should be the same as 290 ELECTRICAL ENGINEERING PAPERS if the machine were carrying 300 amperes and 300 amperes additional were thrown on. Also, according to the above theory, a fully- compensated field machine, (that is, one with & distributed winding in the pole faces proportioned to correctly neutralize the armature magnetomotive force) should also show voltage dips with load thrown on. To determine if this is so, several series of tests were made on a carefully proportioned compensated field machine; Two series of tests were made primarily. In the first, equal in- crements of currents were thrown 'on, (1) at half load, (2) at full load, and (3) at \\ load on the armature. In the second series of tests, a constant percentage of load was thrown on; that is, at half load the same current was thrown on as in the first test, while at full load, twice this current, and at IJ load, three times this current was thrown on. According to the above theory, all these should show voltage dips, although the machine was very completely compensated. Also, in the first series of tests, the dips should be smaller with the heavier loads on the machine, while in the second series they should be the same in all tests. This is what the tests indicated. In the first series, the dips in voltage varied, while in the second series, they were practically constant. The re- sults of these tests are shown in the following table. (The oscillograph prints were so faint that it' was not considered practicable to produce them in this paper.) NORMAL E.M.F.— 1200 VOLTS. Load on generator. Increase in load. Dip in voltage. Test. (Approx). A Amps. 417 Amps. 700 Volts. B 208 " 80 ' 300 " C 417 « 80 200 " D 625 " 80 150 E 417 « 160 300 " F 625 " 240 300 « Tests, B C and D in the table show the dips for the first series of tests, while B, E and F show results for second series. The time for recovery to practically normal voltage was very short in all cases, varying from 0.002 to 0.004 seconds accord- ing to the oscillograph curves, but even with this extremely PHYSICAL LIMITATIONS IN D.C. MACHINES 291 short time, there was very noticeable winking of tungsten lamps, in practically all tests. The oscillograph curves showed practically no change in field current, except in test A. The machine used in these tests was a special one in some ways. It was a 500-kw., 1200-volt, railway generator with compensating windings and commutating poles. In order to keep the peripheral speed of the commutator within approved practise, it was necessary in the design to reduce the number of commutator bars per pole, and consequently the number of armature ampere turns, to the lowest practical limit. This resulted in an armature of very low self induction, which was very quick in building up the armature current with increase in load. This machine therefore did not show quite as severe variations as would be expected from a normal- low-voltage machine of this same construction. However, these two series of tests did show pronounced voltage dips which were sufficient to produce noticeable winking of incandescent lamps. Presum- ably, therefore, all normal types of generators will wink the lights under similar conditions. Data obtained on non-compensated machines of 125 and 250 volts indicate the same character of voltage dips as were found in the above tests. This should be the case, for, by the fore- going explanation, the compensating winding has no direct re- lation to the cause of the dip. It will be noted in these curves that the voltage recovers to normal value very quickly. However, incandescent lamps will wink, even with this quick recovery, if the dip is great enough. There is some critical condition of voltage- dip in each machine which would produce visible winking of lights. Any increments of load up to this critical point will apparently allow satisfactory operation. If larger loads are to be thrown on, then these should be made up of smaller increrhents, each below the critical value, which may follow each other in fairly rapid succession. In other words, the rate of application of the load is of great importance, if winking of lights is to be avoided. Therefore, the type of control for motor loads, for instance, should be given careful consideration in those cases ■ where steadiness of the light is of first importance, and where motors and lights are on the same circuit. An extended series of tests has shown that, in most cases, 10 per cent to 15 per cent of the rated capacity of the generator can be thrown on in a single step without materially affecting 292 ELECTRICAL ENGINEERING PAPERS the lighting on the same circuit, and provided the prime mover holds sufficiently constant speed. However, judging from the quickness of the voltage recovery, the prime mover, if equipped with any reasonable flywheel capacity, cannot drop off materially during the period of the voltage dip as shown in the curves. The dip in voltage due to the flywheel is thus apparently some- thing distinct frdm the voltage dip due to the load. However, if the load is thrown on in successive increments at a very rapid rate, the result will be a' dip in voltage due to the prime mover regulation, although the voltage dips due to the load itself may not be noticeable. The above gives a rough outline of this interesting but little understood subject of voltage variations. Going a step farther, a similar, explanation could be given for voltage rises when the load is suddenly interrupted, in whole or in part. This is usually known as the inductive kick of the armature when the circuit is opened. This may give rise to momentarily increased voltages which tend to produce flashing, as has already been referred to under the subject of flashing when the circuit breaker is opened. Peripheral Speed of Commutator This presents two separate limitations in d-c. design, one being largely mechanical and the other being related to voltage conditions. As regards operation, the higher the commutator speed, as a rule, the more difficult it is to maintain good contact between brushes and commutator face. This is not merely a function of speed, but rather of commutator diameter and speed together. Apparently it is easier to maintain good brush con- tact at 5000 ft. per minute with a commutator 50 in. in di- ameter than with one of 10 in. in diameter. Very slight un- evenness of the commutator surface will make the brushes " jump " at high peripheral speeds, and the larger the dia- meter of the commutator with a given peripheral speed, the less this is. The peripheral speed of the commutators is also limited by constructive conditions. With the usual V-supported com- mutators, the longer the commutator, the more difficult it is to keep true, especially at very high speeds and the higher temperatures which are liable to accompany such speeds. Therefore, the allowable peripheral speeds are, to some extent, dependent upon the current capacity per brush arm, for the length of the commutator is dependent upon this. The per- PHYSICAL LIMITATIONS IN D.C. MACHINES 293 missible speed limits, as fixed by mechanical constructions, have been rising gradually as such constructions are improved. At the present time, peripheral speeds of about 4500 ft. per minute are not uncommon with commutators carrying 800 to 1000 amperes per brush arm. In the case of 60-cycle, 600- volt synchron/Dus converters, 5200- to 5500-ft. speeds are usual with currents sometimes as high as 500 to 600 amperes per arm. In the case of certain special 750-volt, 60-cycle converters, oper- ated two in series, commutator speeds of about 6400 ft. have proved satisfactory. These latter, however, had comparatively short commutators. For the small diameter commutators used in d-c. turbo- generator work, peripheral speeds of 5500 to 6000 ft. have been common. However, such machines usually have very long com- mutators and of the so-called " shrink-ring " construction. The brushes may not maintain good contact with the commutator at all times, and in a number of machines in actual service, the writer, in looking at the brush operation, could distinctly see objects beyond the brush contacts; that is, one could see " through " the contact, and curiously, in some of these cases, the machines seemed to have operated fairly well. One ex- planation of this is that the gaps between brushes and com- mutator were intermittent,, and, with one or more brush arms in parallel, one arm would be making good contact, while another; showed a gap between brushes and commutator. Appar- ently, the commutators were not rough or irregular, but were simply eccentric when running at full speed and the brushes could not rise and fall rapidly enough to follow the commutator face all the time. Incidentally, it may be men- tioned at this point, that with the higher commutator speeds now in use, there has come the practise pf " truing " commutators at full speed. This is one of the improvements which has al- lowed higher commutator speeds. The other limitation fixed by peripheral speed, namely, that of the voltage, is a more or less indirect one. It is dependent upon the number of commutator bars that are practicable be- tween two adjacent neutral points; or, in other words, it is dependent upon the distance between neutral points. The product of the distance between adjacent neutral points and the frequency, in Alternations, gives the peripheral speed of the commutator, (distance between neutral points in feet times alternations per minute equals peripheral speed in feet per 294 ELECTRICAL ENGINEERING PAPERS minute). With a given number of poles and revolutions per minute, the alternations are fixed. Then, with an assumed limiting speed of commutator, the distance between neutral points is thus fixed. This then limits the maximum number of commutator bars, and therefore the maximum voltage which is possible, assuming a safe limiting voltage per bar. From this it may be seen that the higher the peripheral speed, the higher the permissible voltage with a given frequency. In the same way, if the frequency can be lowered (either the speed or ■ the number of poles be reduced) the permissible voltage can be increased with a given peripheral speed. Where the speed and the number of poles are definitely fixed and the diameter of commutator is limited by peripheral speed and other con- ditions, the maximum practicable d-c. voltage is thus very defi- nitely fixed. This is a point which apparently has been mis- understood frequently. It explains why, in railway motors, for high voltages, it is usual practise to connect two armatures per- manently in series; also, why two 60-cycle synchronous conver- ters are connected in series for 1200- or 1500-volt service. In synchronous converter work, the frequency being fixed once for all, the maximum d-c. voltage is directly dependent upon the peripheral speed of the commutator. Conclusion The principal intent, in this paper has been to show that cer- tain limitations encountered in d-c. practise are just what should be expected from the known properties of materials and electric circuits. The writer has endeavored to explain, in a simple, non-mathematical manner, how 'some of the apparently com- plicated actions which take place in commutating machinery are really very similar to better understood actions found in various other apparatus. An endeavor has also been made to show that a number of the present limitations in direct current design and operation are not based merely upon lack of ex- perience, but are really dependent upon pretty definite condi- tions, such as the characteristics of carbon brushes and brush contacts, etc. Possibly a better understanding of the cliaracter- istics and functions of carbon brushes will result from this paper. The writer makes no claims to priority for many of the ideas and suggestions brought out in this paper. However, much of the material is a direct result of his own investigations and those of his associates during many years of experience with direct- current apparatus. PHYSICAL LIMITATIONS IN B.C. MACHINES 295 APPENDIX The following method of determining the maximum capacity which can be obtained with given dimensions and for assumed limitations as fixed by commutation, flashing and other con- ditions, is based upon certain formulas which the writer de- veloped several years ago, and which appeared in a paper before the Institute.* On page 2389 of the 1911 Transactions of the Institute, the following general equation is given : 108 r' ^ '' (0.25/. +0.5) (D+P,) + Ci ~ (0.9 + 0.035 N) + C3 — (1.33 (f, + 0.52 + 2.16 5 Vnf\ p s J C42 per cent increase for . 6o flux distortion, and increased voltages at times, gives 26.3 at full load. r-_. = 1.25 for average constructions. Tj = Varies with the number of coils per slot and the aver- age number of bars covered by the brush, but as- suming 2 bars covered, then C^ = 0.4 approx. with 1 slot chording, and with either 2 or 3 coils per slot. 5, = 1,50,000 lines per sq. in. on the basis of actual iron and all flux confined to the iron. fj, = 0.75. This allows for 90 per cent solid iron and I of the total width taken up by air ducts (about I" duct for each 2" of laminations). 300 N ELECTRICAL ENGINEERING PAPERS = 14 { Two cases have been assumed, one with 3 coils N = 21 [ per slot and 14 slots per "pole, and the other with 2 coils per slot and 21 slots per pole. 14 Slots per Pole. — Substituting the above values in equations (4) and (5),, then for 14 slots per pole equation (4) becomes, Kw, = 3767 £. [ 7pir+-9.i8 (2.5^f + 1) + 19 P, ] ^®^ and equation (5) becomes /P,3 = 18.36 (2.5 designs for the highest speed machines use the through-shaft type of construction. This latter, however, has been retained in some of the more moderate speed large capacity units. 314 ELECTRICAL ENGINEERING PAPERS On account of the high rotative and peripheral speeds, the general design of large capacity turbo-generators turns upon the type and construction of the rotor, father than the stator. Various designs and types of rbtors have been, developed but, with I'are exceptions, only two general types are now built in this country. These may be designated as the radial-slot and the parallel-slot types. Each has a number of advantages over its rival and each has given good results in practice. Radial Slot Type of Rotor In the radial slot, type, as usually constructed for high-speed machines, the core and shaft are forged in one pieee in the smaller and more moderate- sizes, but may be built up of a number of separate plates or disks bolted rigidly together in the larger sizes. In this type, the core is cylindrical in alLcases, and in the outside surfaces are radial slots, usually arranged in groups, in whicLthe Fig. 2 exciting windings are placed. While all radial slot types. of rotors bear a general resemblance to each other, yet there are marked differences in the method of forming the slots and teeth which constitute the outer surface. In some types the solid rotor core has radial slots milled or slotted in the main body of the core. In other cases the slots are formed outside the main core by inserted teeth, usually with overhanging tips, between which the exciting coils lie. These two general constructions are illustrated in Fig. 3. Examples of the inserted-tooth con- struction are found in the large moderate speed rotors of one American company, and in somewhat higher speed machines of a German company However, with the advent of fhe high- speed, high capacity machines, the milled-in construction of the radial slots appears to be taking the lead, due to certain mechanical limitations in the inserted-tooth types. On account of the radial slots and the usiTal concentric arrange- TURBO-GENERA TORS 3] mcnt of the exciting coils, the field or exciting turns cannot 1 assembled and insulated before placing on the core, except the inserted-tooth type of construction. With the milled-in-sli type, the field conductors, usually of flat strap, are dropped in the slot one. at a time, with insulation between individual turn For ease of winding, the ends are usually allowed to overhai the core, and require a very ample outside support in the vei high speed machines This is illustrated in Fig. 4. The cor pleted coils are usually held in place by strong non-magnet wedges in the tops of the slots. These wedges are usually carri« by overhanging pole tips, in the inserted-tooth type, or by groov m the sides of the slots in the milled-slot type The design i the supports for the overhanging end windings has furnished oi of the difficult problems in this type of construction. ExampL Fig. 3 Fig. 4 -of radial slot end windings, and df the rotor complete, are show in Figs. 5 and 6. This general construction- of the radial slot type of rotor -obviously applicable to machines of any number of poles. Wit a two-pole machine there will be only two groups of coil slots an two 'groups of concentric coils, while with four poles or six pol( there will be four or six groups respectively. It is evident tha with this construction, a cylindrical rotor is obtained, regardle: -of the number of poles. It js also evident that the problem i supporting the end windings becomes an increasingly difficu one, as the number of poles is decreased and the span of tl -end windings is correspondingly increased. The support over the end windings usually consists of a hea'v ring which, in very high-spe'ed machines, must consist of materi ■sh\ ELECTRICAL EXCIXEERLXG PAPERS Fi(.. 5 ua^i..MBifl7~-^^ ™ •iiiiiiiiinuiKMr I ^ ' M,. * Fio. (j TURBO-GENERATORS 317 having extra j^ood physical characteristics, for this ring must not only be able to carry itself, but must also carry the. weight of the underlying end windings which it supports. In the German itiser ted-too th rotor, the end windings are suppbrted by steel bands of many layers, instead of the solid steel ring. In some of the lower speed radial slot machines, such as one American type with inserted teeth, the end supports are of ring |erm usually made in a number of sections, which are bolted to an inner shelf by numerous bolts extending form the duter ring between the coils of the end windings to the inner shelf. While this construc- tioh is satisfactory for the more moderate peripheral speeds, yet with the much higher speeds in some of the later practise, this construction has been superseded by a solid ring type of support. Parallel-Slot Type of Rotok In the parallel-slot type of rotor, the slots for the exciting coils , for any number of field poles, lie in planes parallel to one another and to the rotor axis. The arrangement is illustrated by -Fig. 7. As usually constructed, the slots are cut across the ends of the poles, as well as in the sides, so that the exciting coils .are em- bedded in metal throughout their length. The object of this general arrangement of parallel slots is to facilitate the winding of the exciting coils. The rotor can be placed upon a turn-table, or similar device, and rotated, to wind the coils in place under tension. Two or rnore coils can be wound at the same time, as is actually done in practice. As the coils can be wound under tension, and as the conductors .usually consist of thin flat stra.p, which can be wound in very tightly, the resultant winding is a very substantial piece of work. The finished winding is- sup- ported by metal wedges over the coils. It is obvious that, with this construction, no external support is required for the end windings, as the field core proper furnishes the necessary support.. It is largely on account of this feature of well supported end windings that the parallel-slot type took a leading position, during the growth of the larger two-pole, 60-cycle alternators. With the radial-slot type, the sjipport of the end windings presented a more difficult problem, in the large capacity, high-speed, two-pole machines,' which, however, is being gradually solved. In the two-pole, parallelrslot construction, in order to utilize the available winding space to advantage, it is necessary for the windings; to cover the central tiortion of the core end where the 518 l-'-I.ECTR[CAL EXGIXE/UilXG PAPERS Fic. Fig. S Fig. 9 TURBO-GENERATORS 319 shaft is usually attached, as shown before in Figs, 7 and 8. There- fore, with this construction, a separate " head " or driving flange must be bolted to the core at each end, this head carrying the shaft, ,as showti in Fig 8. To avoid magnetic shunting of the 'field flux, this driving head must be made of non-magnetic material, usually of some high," grade bronze, to which the shaft is attached in such a way as to keep the magnetic leakage as low as possible. , This makes a good strong construction, but is necessarily rather expensive, due tp the bronze driving heads. As these cost but little more for a long rotor than for a short one, the construction therefore tends toward relatively long, small diameter cores in order to lessen the relative dimensions of the bronze heads. In two-pole, single phase machines of this construction, the copper cage damper for suppressing the armature pulsating re- action' on the field, is comprised partly of these bronze heads, which form the " end rings " for the copper bar& embedded in the slots in the rotor face. In the four-pole, parallel-slot machine, no bolted-on driving heads are necessary, for the core proper and the shaft may be cast, or forged, in one piece; or in two or more pieces, which are bolted or " linked " together to form a solid core. The principal difference between the. two-pole and the foui--pole parallel slot constructions, is that the latter must have salient or projecting poles, in order to utilize the parallel construction for the slots, while the two-pole machine is preferably made cylindrical. Fig. 9 illustrates this feature. It is evident that there is considerable available space lost by the openings between the projecting poles, while the sections of the poles themselves are cut down very materially by the slots for the exciting winding. The limitations therefore in such a rotor are in the magnetic section of the field poles and in the available copper space, and in these features the four-pole parallel slot rotor is inferior to the radial slot type. In the two-pole machine, however, the difference between the radial slot and the parallel slot is not nearly so pronounced,' as is indicated in Fig. 10' where the two arrangements are shown on one core for compari- son. It may be seen from this that, in the two-pole form, the' tvvo constructions approach each other, to a certain' extent, some of the slots in the parallel construction beings radial, while others depart but little from the radial. One disadvantage in the two-pole, parallel-slot type, however, lies in the smaller amount of 320 ELECTRICAL 'ENGINEERING PAPERS copper space which is obtained, for the slot space must necessarily cover a less proportion of the total circumference than is permis- sible with the radial slot type. This winding space is limited by the physical requirements as regards bending and breaking strains in the overhanging tip a in Fig. 10. In the radial slot type, the slot space has no such limitation. Also, on account of the grouping of the field copper into a narrower zone in the parallel slot type, the heat con- duction from the copper presents a more difficult problem than in the radial type. At first glance, it would appear that the effective length of the field core in the parallel slot type is very considerably diminished by the slots across the ends of the core. However, this is only an apparent effect, for the true length of the core should be taken as that inside .of • the winding slots, and it should be considered that the additional Fig. 10 Fig. 11 length of the core at the pole face is in the nature of a coil support which takes the place of the separate support in the radial slot type. Therefore, if over-all lengths, including- rotor coil .supports, are compared in the two types, there -1C V»lit llfflA Aifff^rf^nnf^ qc inHir»QfAr1 ht\: Pi 11 H r\Tsroirof T URBO-GENERA TORS 321 if the armature core is made of the same width as the pole face, in both types of rotors, then in the parallel-slot type it will be materially greater than in the radial, for the over-hanging pole tips of the parallel-slot machine are also effective magnetically in furnishing flux to the armature. Therefore, as regards the stator, this tends toward a wider core in the axial direction, and a shallower depth of iron back of the armature slots, as indicatfed in ;Fig. 12. Also, on account of the relatively larger polar sur- face, in the parallel slot type of rotor, the magnetic flux density in the air gap is usually relatively smaller than in the radial slot, type, which conduces towards a larger depth of air gap. Also, on account of the larger polar surface, the available space for armature slots and teeth is correspondingly increased. There- fore, this type of construction is better adapted for the straight air gap method of ventilation, as will be described later. The greater section available for slots and teeth at the stator pole face permits a large number of ventilating ducts. The relatively large depth of gap allows a large amount of air to be fed through the air gap to the ducts. Therefore, the " radial " type of stator core ventilation has been used very largely with this type of rotor construction. In the parallel-slot type of rotor, it is obvious that, due to the large polar surface compared with the minimum section of the field core, a limit in design is found in the magnetic saturation in the field core itself. In the four-pole parallel-slot rotor, the field section is more limited than in the two-pole jn-Etchine, due to the fact that con- siderable magnetic space is lost by the notches between the pro- jecting poles. However, in this type of construction, the air gap method of ventilation is relatively easy, due to the fact that these interpolar spaces furnish easy access of the ventilating air to the stator ventilating ducts. In consequence, the problem of ventilation is usually not a serious one in this type of rotor. Due to the polar projections, however, the tendency to noise is obviously greater than in either the radial-slot type or the two- pole parallel type, which are always cylindrical. Nothing has yet been said as to the peripheral speeds obtained in some of the actual designs of th? higher speed generators. These, inisthemselves, indicate some of the limitations which now confront the designer. In the 5000 kv-a., two-pole, 3600-rev. per min., 60-cycle generator already referred to, which is of the parallel-slot rotor construction, the rotor diameter is 26 in. (66 cm.) This gives a 322 ELECTRICAL ENGINEERING PAPERS Fig. 12 Fig. 21 TURBO-GENERA TORS 323 peripheral speed of 408 ft. (124.3 m.) per second, or approximately 24,500 ft. (7468 m.) per minute The core is designed for .a very considerable margin of safety, and is actually tested at overspecds which give practically 30,000 ft. (9144 m.) peripheral speed at the svirface of the core. In certain 19,000 kv-a., 62i-cycle, four-pole, 1875-rev per min. machines now being built, which are of the radial-slot rotor construction, the rotor diameter is 49 in. (124.4 cm.) This gives a peripheral speed of 24,000 ft. (7315 m.) per minute. This compares with a speed of 21,600 ft. (6583 m.) in a 21,000 kv-a. two-pole, 1500-rev per min., 25-cycle, radial-slot machine also being built, the rotor core of which is shown in Fig. 12. Obviously the mechanical limitations are being more closely approached in the 60-cycle machines, up to the present capacities. If> a comparison is made between the above 5000 and 19,- 000 kv-a. rotors, with their parallel and radial type construc- tions, it is found that their limitations lie in quite different features. In the radial-slot type, the core stresses are much lower than in the other, but the supporting end ring is an im- portant problem, requiring for its solution, a very high grade steel for the material of the ring. In the parallel-slot rotor, the maximum stresses are in the core itself, principally in the parts which overhang the slots at the sides and ends of the core. In the radial slot core, there are no such overhanging masses. In both constriictions, the core material is purposely made of re- latively soft steel, having a high percentage elongation, the ob- ject being to obtain a material which can yield sufficiently to transfer the strains from local higher points, to adjacent lower parts, and thus equalize them, to a great extent. The smaller diameter rotor cores are made of steel forgings, in one piece. The larger cores are made up of thick steel plates assembled and bolted together to form a solid mass comprising the core and shaft extensions. By this disk construction, com- mercial material is used which is of uniform quality clear to the center of the disks. The fiber of the material is in a direction best suited to the directions of stress. With corresponding size disks made in one piece, the otitside, to a certain depth, can be given fair physical characteristics, but the center is liable to be glass-hard, as found by experience. However, this may not be a prohibitive condition in machines of more moderate pier- ipheral speeds. Herein lies one great difference between American and European limitations. In American practice, 60-cycles, 324 ELECTRICAL ENGINEERING PAPERS calling for 3600 and 1800-rev. per min. machines, is the standard frequency, while in Europe, 50-cycles is standard, giving 3000 and 1500-rev. per min. machines. These lower speeds make an enormous difference in the possibilities of design and construc- tion. Present Limitations in Design On account of the very great capacities, at high speeds, now being obtained in turbo-generator practice, a number of problems are being encountered, the solutions of which are producing more or less radical changes, both in design atid in practise. Softie of the limitations now encountered are in the relatively high temperatures in' certain parts, high losses in a relatively small space, the difficulty of ventilation, due to the requirement of endrmous volumes of cooling air through limited openings or' passages, the type of insulation, fire risks, regulation and short circuit conditions, etc. A number of these limiting conditions, such as the temperature, ventilation, losses, and insulation, are so closely related to each other, that it is difficult to describe, any one of them in detail, withoyt including the others to a considerable extent, The Problem of VEivrriLATioN In the general problem of ventilation, four conditions must be considered, namely, the total loss, or beat, developed, the surface exposed for dissipating this heat to the kir, the quantity of air required to carry away the heat, and the temperature of the cooling air. In the conduction of heat from the surface of a body into the air, the quantity of heat per unit 9,rea which can be dissipated depends upon the difference in temperature, maintained between the surface of the body and the body of air to which the heat is conducted. The heat dissipated raises the temperature of the adjacent air a certain amount, tod thus tends to reduce the temperature difference, u'nless the air is renewed with suffi- cient rapidity. On the other hand, if the quantity of air is so great, in proportion to the heat dissipated, that there is but little rise in the air temperature, then any increased amount of air over the surface will represent practically no gain in ventilation. In other words, when the amount of air passed over a surface is sufficient to take up the heat dissipated from the surface without an undue rise, then a -further quantity of air- is wasteful, and it may even be considered as indirectly TURBO-GENERATORS 325 harmful, in those cases where the total quantity of air is limited. This has a direct bearing on the size of ventilating ducts or passages in a machine. If the air path through a duct is relatively long, then a considerable width of duct may be required in order to get the necessary quantity of air through it. On the other hand, if the air path is very short, then a very narrow duct may be most effective, for a wider duct may allow more air to pass through than can be utilized in taking up the heat. No matter how thoroughly the ventilating air is distributed through the heat-generating body, or however effective the heat-dissipating surfaces may be, the total air suppHed must be ample in quantity, or its temperature will be raised an undue aniount. As the surfaces to be cooled must always have a higher temperature than ttie cooling air, any considerable rise in the latter will have a direct influence on the ultimate tempera- ture which may" be attained by the body to be cooled. Con- versely, if an ample quantity of cooling air is juipplied, but the heat-dissipating surfaces are insufficient', the ultimate tempera- ture of the body will also be affected. In large capacity, high-speed turbo-generators, the problem of ventilation is one of the most difficult ones encountered. The trouble lies principally in the large total loss expended in a very limited space. The difficulties of the problem may be illustrated by the following example : Assume, in a 1500-rev. per min., 25-cycle, 15,000-kv-a machine, a total efficiency of 96 .5 per cent, including air friction loss inside of the machine. This means a total loss in the machine of 565 kw., which is not excessive for /this capacity, bvit is very large for the limited space in which it is developed. A very large volume of cooling air is required for carrying away the heat due to this loss. A simple approximate rule for determining the quantity of air req^iired is that an expenditure of one kw. in one minute will raise the temperature of 100 cu. f1;. (2.8 cu, m.) of air 18 deg. cent. Therefore, 565 kw. loss would require a supply of ventilating air of approximately 50,000 cu. :ft. (1416 cu. m.) per minute for a rise of the out-going air of 20 deg. above that of the incoming air. Assuming a velocity of 3000 ft-. (914 m.) per minute, this would mean, with a cylindrical ventilating channel, a diameter of 56 m. (142.2 cm.), which is greater than the rotor diameter itself. However, as the cooling air ordinarily would be. supplied to both sides of the machine, the ventilating passage need only be half the above section for each side. 326 ELECTRICAL ENGINEERING PAPERS , Obviously, such passages arc prohibitively large, and much greater air velocities through the machine proper are necessary Velocities as high as 5000 to 6000 ft. (1524 to 1828 m.) per minute are common, while, in some cases, more than 10,000 ft. (3048 m.) per minute has been required in certain constricted sections of the air path inside the machines. Therefore, no matter how the problem is considered, it may be seen that the above condition of the enormous volume of air required, makes the problem of ventilation a difficult one. There are several methods of ventilating large turbo generators, depending upon the systeiti of applying the air. There is, first, the radial system, in which practically all the cooling air passes out radially through ventilating ducts in the stator core. This radial system of ventilating can be subdivided into two alterna- tive methods, depending upon whether the air is partly or wholly supphed through passages in the rotor, or through the air gap alone. These two methods are illustrated in Fig. 13. The straight air gap arrangement may require a relatively large air gap, combined with very high velocity of the air along the gap, while the other method permits a considerably shorter gap. The straight air gap method of ventilation is used, to a considerable extent, in all 60-oycle machines of two-pole con- struction, while it is practically the only one that has been used with the parallel-slot type of machine with either two or four poles. In this parallel-slot type of rotor, however, the air gap can be relatively larger than the radial-slot type of rotor, as explained before, which compensates, to some extent, for the necessity of depending upon this method entirely. In the four-pole parallel- slot rotors, the interpolar spaces are also effective. Moreover, with parallel-slot rotors in general, the openings from the air gap into the stator ventilating ducts can usually be somewhat larger in total section than with the radial type of rotor, as also described before. However, the relatively greater axial length of the core of the parallel slot type of rotor increases the length of the constricted air passages along the air gap in the two-pole machines, which is a material disadvantage. The straight air gap type of ventilation has proven astonish- ingly effective in cooling the rotor in both the radial and parallel- slot types of rotors, and with either type there is usually no great difficulty in forcing through enough air to cool the rotor core; in a fairly effective manner. It must be considered, hovyrever,. that the total rotor loss in large turbo-generators is possibly TURBO-GENERA TORS 327 Only.lO per cetit of the total loss which must be takeii care of, and a r«Ja)tlvely small proportion of the total ventilating air may suffice to cool it; Aeeording to actual measurements, corrobor- ated by general experience, the cylindrical surface. of the rotor core caa give off four or five watts per square inch (6.45 sq, cm.) to the cooling air, with a temperature rise of the rotor surface of about 35 to 40 deg. cent, above the cooling air. To ^hose who have had experience with dissipating heat from electric apparatus, this result will appear to be extremely good. The real difficulty with the air gap method of ventilation, is not so much in getting enough air through for cooling the — * zrTJ^K\^>C\: .^ ^ ^ j^ innnn _ ^ ^^ u^ Fig. 13 Fig. 14 rotor itself, but it is in the much larger quantity required for the stator. For instance,- a one-inch (2.54 cm.) depth of gap (from iron to iron) with a 50-in (127-cm.) diameter of rotor, means a total section of air path into the gap (counting both ends of rotor) of 314 sq.in. or 2.18 sq. ft. (0.19 sq. m.). At a velocity of 10,000 ft. (3048 m.) per minute, this allows a flow of only 21,800 cu. ft. (617 cu.i m.) per minute, which will not take care of a large machine, from the present standpoint of possible (Opacities with the above diameter of rotor. By additional openings in the rotor core, this might be increased to 30,000 cu. ft. (849 cu. m.) per minute, but even this is still 328 ELECTRICAL ENGINEERING PAPERS much less than a machine, with a 50-in. (127-cm.) diameter of rotor, would require if built for capacities . otherwise possible. Therefore, on account of this limitation in the amount of cooling air, other means of ventilation have received much considera- tion. Two other general systems of ventilation, in addition to tlic ga]) method, have been used, namely, the circumferential mcLliod, and the axial. The former has been developed and applied more extensively in the past, but the latter contains possibilities which are bringing it rapidly to the front. In the circumferential method of ventilation, air is supplied to-one or more points on the outside circumference of the stator, and is forced circumferentially around through the air ducts to suitable outlets, also on the outside surface. Air gap ventilation is usually combined with this circumferential method, partly to cool the rotor. The general arrangement is indicated diagram- matically in Fig. 14, in its simpkst form, namely, with one inlet and one outlet diametrically -opposite. A serious objection to this method of ventilation is fovnd in the limited section of the ventilating path. Assuming, for example, a depth of stator core of 20 in. (50.8 cm.) outside the armature slots and a total of 40 |-in. (9.5 mm.) ventilating ducts, or a total. effective duct space of 15 in. (37 . 1 cm.) width then this gives a total section of ventilating path of 20 X 15 X 2 = 600 sq. in., or 4.16 sq. ft. (0.386 sq. m.). On account of the relatively great length of the ventilating path, air velocities of more than 6000 to 7000 ft. (1828 to 2133 m.) are not desirable or economical, but even with 10,000 ft. (3048 m.) velocity, the total quantity of air would be only 41,600 cu. ft. (1166 cu. m.) per minute. Further- more, this method is handicapped in machines with very^ high- speed rotors, by interference between the radial and the cir- cumferential systems of ventilation, so that the full benefit of either is not obtained. Below a certain rotor velocity, apparently the circumferential action can predominate^ and the method is fairly effective up to the permissible air capacity of , the stator ducts; but at very high speeds the radial ventilation may very seriously interfere with the other, so much so, that the radial ventilation alone, even with its very restricted gap section, may give as good results as the two methods acting together. To avoid this interference, various methods have been devised, such as closing part, or all, of the radial ventilating ducts at the air gap to keep the radial effect from interfering with the other. One arrangement which has been used in Europe to a considerable TURBO-GENERA TORS 329 extent is indicated in Fig. 15. In this, the alternate radial air ducts are closed at the outside surface, while all are closed at the air gap. The air enters by the ducts open at the back of the machine, flows both circumferentially and toward the' gap, and crosses' over to the immediate ducts by axial opeiiings back of the armature teeth, and then along these ducts,. to the outlet. This scheme is effective in principle, but is uneconomical in the sense that less than the total section of stator ducts is useful, as regards the quantity of air which can be canned. There is usually one large central duct to allow an outlet for the rotor ventilating air. This particular arrangement of the stator also uses axial ventilation in crossing over from one .set of ducts to the other, which is an effective arrangement. A modification of the simple circumferential method of ntn^ntntntntn Fig. 15 ventilation is to admit air to the back of the stator at two oppo- site sides of the machine, and deliver it at two outlets at inter- mediate points on thej surface, as shown diagrammatically in Fig. 16. By this means, thfe cross section of the ventilating path is doubled and the length is halved. Also, the interference of the radial vefitilation with the circumferential will be less harmful. A serious disadvantage in the Circumferential venti- lation in general is that the ventilating path is relatively long, especially where there is but one inlet and outlet, and therefore the cooling air at the outlet of the channel may be considerably hotter than at the inlet, with consequent less effective cooling action. This means points of local higher temperature in the core, due to the method .of ventilation. In the radial type of ventilation, the coolest air is applied near the seat of the highest losses, namely, at the armature teeth, and immediately back of 330 ELECTRICAL ENGINEERING PAPERS them, and the air, as it becomes heated, passes over the outer part of the iron which has a diminished loss, and therefore normally less heat to dissipate. Therefore, the effect of the in- creased temperature of the cooling medium is offset by the lower loss, and .consequent less necessity for ventilation, in the part where the air is hottest. The radial -system of cooling is therefore theoretically the most effective, but practically, the difficulty is in applying it, due to the limited air passages available. BoUA.the circumferential and the radial methods of cooling are subject to one serious defect, namely, most of the generated heat in the stator iron must be conducted across the lamina- tions to the air ducts. The rate of conduction across the lamina- FiG. 16 tions is only from 1 per cent to 10 per cent as great as along the laminations themselves, according to various authorities. Therefore, if the heat could all be conducted along the lamina- tions to the ventilating surfaces, apparently much more effective heat dissipation could be obtained, provided sufficient surface is exposed to the air, and an ample quantity of air supplied. This has led to the development, of the axial system of ventila- tion, as distinguished from the radial and circumferential. In this method, a large number of axial holes are provided in the stator core which may extend uninterruptedly from one side of the core to the other, or they may extend from each side to one or more large central radial channels which form the outlet. The usual numerous radial ducts are omitted, or may be con- T URBO-GENERA TORS ■331 sidered as combined in one central channel. This general arrangement is' illustrated in Fig. 17. The rotor cooling is accomplished by air along the air gap, and through the rotor core to the large central duct. In this method of ventilation therefore, there is a combination of two types, namely, the axial and the air gap, but there is not the interference between the two, that is sometimes found where the circumferential method is used. From the preceding, it may be seen that the problem of putting a sufficient quantity of air through the machine is an extremely difficult one. In addition, in very large machines, the problem of supplying the reqtiired qufintity of air from a smtable blower forms another serious problem. In smaller capacities, and in slower speed machines, it has been the usual practise to attach blowing fans to the rotor shaft or core, as <^ i 1 _t_ Fig. 17 part of the outfit. There is no particular difficulty in this arrangement, except, possibly, in the high-speed construction of the fans required for 60-cycle, two-pole machines. Such fans can supply an amount of air which is limited by the diameter and other dimensions of the fan itself. Assume, for example, that by lengthening the rotor core, or by other modifications in the construction, the capacity of the machine can be doubled, and therefore double the quantity of air is required for cooling. If the limit of the fan design or operation was reached before, then obviously some radical change is required with the new capacity of the machine. This condition apparently has been reached in some of the later practise in large, high-speed turbo-alternators. One obvious solution of this difficulty lies in the use of separate slower speed, large diameter, fans or blowers. This may appear to be a. step backward, but when the above conditions and limitations are 332 ELECTRICAL ENGINEERING PAPERS taken into account, it is not so. The " tail " must not be allowed to "wag the dog;" the blower, which is an adjunct, must not be^llowed to dominate the construction of the machine itself. Moreover, there are a number of meritoriqus features in the use of a separate blower. In the first place, it can be made somewhat more efficient than the high-speed, rotor-driven fans. Again, with a suitable means to drive. Variable speeds, and thei-efore different air pressures, can be obtained. TJiis feature may prove to be very desirable or advantageous under peak, ox overload, or emergency conditions. One further condition keeps cropping out in the general problem of ventilation, "namely, that of filtering or washing, or otherwise cleaning the ventila:ting air. With 50,000 to 75,000 cu. ft. (14r5 to 2122 cu. m.) of air per minute passing through a large machine, obviously in a year's time, an enormous quan- tity of foreign matter is carried through the machine with the ventilating air. A deposit of a very small per cent of this in the machine will probably be disastrous. In fact, however, the high velocity of the air through the machine serves to keep the air passages clear if no oil or moisture is allowed to enter. That a large amount of foreign matter does go through the machine is very soon shown in case a little oil is allowed to get into the ventilating passages. This oil catches the dirt and in a short time the ventilating passages may be very materially obstructed. On account of the deposit of dust, etc. in the ventilating passages, it is necessary to clean certain types of machines at more or less frequent intervals., and it is advisable to clean all types occasionally. With some systems of ventilation, where such cleaning is difficult, or almost impossible, such as that shown in Fig. 16, provision must be made for cleaning the air before it enters the machine. With the particular construction shown in Fig. 16, air filters are almost always supplied. In the American types of construction, however, such filters have not yet been used, except in a- more or less experimental manner, due probably to the greater accessibility of these machines as regards cleaning. But such filtering processes possess consider- able merit in general. One modification which is being agitated at present is that of washing, instead of filtering, the air. This serves the double purpose of cleaning and cooling the air, and in very hot weather, when the available capacity of the machine is at its minimum, this cooling effect may mean a reduction of 6 to 10 deg. in the temperature of the machine. T URBO-GENERA TORS 333 The Temperature Problem In the general problem of temperatures in electrical apparatus, it is not the rises, but rather the ultimate or limiting temperatures which are' of first importance. Furthermore, the real limitation in ultimate temperature does not lie in the copper and iron, but in insulating materials used; and only insofar as the tem- peratures of the former affect the latter do they concern the general problem. However, as insulating materials in themselves are not usually sources of heat, but as they receive most of their heat from adjacent media, such as iron or copper which may be generating loss, the real temperature problem, as regards insula- tion, resolves itself into the consideration of that of the adjacent materials. Therefore, it is one which, for its full analysis, requires a knowledge of the sources and amounts of heat gener- ated, and its conduction and distribution to other parts. Broadly speaking, there is always a flow of heat from points of higher to those of lower heat potential and the amount of flow is also a function of the quantity of heat generated, the section and length of the paths through which it can flow, and the specific heat resistance of the various materials which con- duct the heat. In an electric generator, for example, heat is generated in large quantities in the armature teeth and in the armature core. It is also generated in the armature coils when the machine is carrying load. Part of the armature copper is buried in the armature slots where it is almost surrounded by iron, which, in itself, develops a loss, while another part, such as the end windings, may be surrounded by, and thoroughly exposed to, the ventilating or cooling air. In such end portions, the flow of heat will usually be from the inside copper, directly through the insulation to the cooling air. The amount of heat which will flow from the copper through the insulation, depends upon the temperature differences between the copper and the outside surface of the insulation, upon the cross section of the path of flow, upon the thickness and " make-up " of the material, and upon the heat-conducting properties of the insulation itself. There is also a considerable temperature gradient from the outside surface to the air. If the surrounding air is not renewed with sufficient rapidity, the flow of heat from the insulation to the air may raise the temperature of the adjacent air, so that the total temperature drop is decreased, and the amount of heat dissipated is correspondingly reduced. In the armature core, the problem is much more complex. 334 ELECTRICAL ENGINEERING PAPERS In the copper buried in the armature slots, there are usually three paths along which the heat can flow. First, it may flow from' the copper directly through the insulation to the iron, T^ovidcd the adjacent iron temperature is lower than that of the copper. Second, it may flow lengthwise of the copper to the end windings to be dissipated directly into the air from that portion of the winding, as described above. Third, in the case of open-slot machines, one edge of the coil may be exposed to the air in the air gap, and there may thus be a direct conduction of the heat through the insulation to the air in the air gap. This latter case, however, only holds for the upper coil, or that next to the gap, in the case of two coils per slot, which is the most common construction. In th6 bottom coil, the only means of conduction in the buried portion of the coil, are to the adjacent iron or lengthwise to the end windings, or to the adjacent upper coil, which, however, would normally have at least as high temperature as the low6r coil. Therefore, the two efl'ective paths should be considered as through to the iron and thence to the air, and lengthwise of the copper to the end windings and to the air. It is the relation of the various factors of these two paths that control the actual temperatures. It has usually been considered that, in the buried copper, the greater portion of the heat is conducted directly into the sur- rounding iron. This, however, is only partially true, depending upon many features in the construction and type of apparatus. The heat conductivity of copper is, roughly, about six times that of laminated iron lengthwise of the sheet, which is possibly ten to twenty times as great across the laminations. In an armature which is comparatively narrow and which has very open, well ventilated end windings, a relatively small difference in tempera- ture between the copper at the center of the core and that in the end windings, may cause a relatively large flow of heat from the buried copper to the end copper. Therefore, in certain designs, a great part of the armature copper heat may be dissipated through the end windings, and not through the armature core, especially in those cases where the armature core in itself has a considerable temperature rise. There even might be no con- duction of heat from the copper to the iron, or there may be conduction from the iron to the copper; lor it the copper is at the same temperature as the iron at the center of the core, for instance, then at each side of the center, or as the edges of the core are approached, the copper temperatures will be relatively TURBO-GENERA TORS 335 lower than at the center, and therefore lower than the adjacent iron, on the assumption that the iron temperatures would be practically constant over the full width of the core. The con- ditions would therefore be as represented in Fig. 18. The solid line a in this figure represents the iron temperature at uniformly 40 deg. cent, rise, and the dotted line & represerits the copper temperatures from the center of the core to the ed^s. Th° tem- peratures at the center being assumed the same for copper and iron, obviously there will be a flow for heat from the iron to the copper near the edges or the core. The effect of this additional heat carried out by the copper would be such as to tend to increase the temperature of the copper at the center of the core by " bank- ing up " the copper heat . Again, if the temperature of the copper at the center is materi- FiG. 18 Fig. 19 ally higher than that of the surrounding core, the conditions may be as represented in Fig. 19. In this case, assuming the core at constant temperature, there will be heat flow from the copper to the iron at the center of the core, and from the iron to the copper at the edges. This study of the problem leads to certain very curious con- ditions which are sometimes found in large machines. At no- load, for instance, with practically no copper loss present, and with high iron loss, there may be a very considerable flow of heat from the armature teeth through the insulation into the copper, and thence to the end windings and to the air. In this way the temperature -of the armature teeth at no-load, and with normal voltage generated, may be considerably reduced by con- duction of the iron heat into the copper, while the copper itself 336 KLEUTKIUAI. lii\uii\Ji±!,Kii\ij rArja.R^ may show a very considerable temperature rise. When load is placed upon such a machine, sufficient to raise the temperature of the copper up to that of the iron in the armature teeth, the lattfer is actually increased in temperature, due to the prevention of the heat conduction into the copper. In this way, therefore, the copper may apparently heat the iron, although there is no direct ^ow of heat from the copper to the iron, but the reverse flow is prevented. Iri high-voltage windings requiring thick insulation, the temp- erature drop from the copper to the outside may be relatively large; that is, with a given difference of temperature between the copper and the surrounding air, a relatively small amount of heat may be conducted through the insulation. Experience shows th^t the amount which can be conducted is a function of the quality of the material, the way it is built up, its thickness, and also the pressure upon it. It is almost impossible, in a machine in service, to calculate exactly the flow of heat, even if all the temperature conditions are known, for the insulating material itselt is one of the variables in the problem. The ability of the insulation to conduct heat will change with operating conditions, to some extent, as, for instance, it may tend to ex- pand somewhat under heat, and thus change its heat conducting qualities. In the armature iron, the problem of heat conduction is just as complicated as in the armature conductor. The principal sources of heat lie in the armature teeth and in the armature core back of the teeth. As a rule, the loss in the portion of the core immediately back of the teeth is relatively greater than at a greater depth, for the magnetic fluxes which cause the tempera- ture rise, generally crowd close to the teeth, so that the density is higher at such parts The heat from the armature teeth can be dissipated' along Several paths. It can. flow lengthwise of the laminations to the end of the tooth and into the air gap, where the ventilatioin is usually fairly good, but the tooth surface exposed is relatively small. In the second place, it can flow back along tlje lamina- tions to the armature core where it can spread out through, a path of much greater cross section and be conducted partly to the back, part of the laminations, and partly transversely to the ventilating ducts. A third path from the armature teeth is across the laminations of the teeth, to the neighboring ventilating ducts. This latter path, however, must necessarily be relatively poor TURBO-GENERATORS 337 in conductivity per unit section of path, compared with the others, but offsetting this, it is frequently of much greater cross section and of relatively small length. In. passing from plate to plate, the heat must pass through the insulating varnish, or other material used, which is of relatively high heat resistance compared with the iron itself. Nevertheless, in machines with radial ventilation, a very considerable portion of the heal due to the tooth loss is carried transversely through the plates to the air in the ventilating ducts, simply because that is the path of lowest total heat resistance, everything considered. In maity cases, the temperature in the core back of the teeth may be as high as that of the teeth themselves, so that the only flow possible is across the laminations to the air ducts, or lengthwise to the tip of the teeth in the air gap. Therefore, the question whether the armature teeth may be hotter than the armature core, or whether the flow of heat is from the teeth to the core, or from the core to the teeth, is a very involved one; and yet upon this question depends, to a great extent, the temperature rise in the buried armature copper. If the armature core is normally hotter, than the teeth, and a considerable amount of heat in the teeth is carried away by the buried copper at no load, then it may -happen that when carrying heavy load, the heat in the teeth will rise very considerably above the no-load condition, and it may actually so " bank-up " that there is still more or less flow from the iron to the copper, eVen with load. With such a condition, therefore, the outside of the insiUation may reach a higher temperature than the inside, while in those cases where the temperature of the copper rises above that 9f the iron of the armature teeth, the inside of the insulation will be hotter. Therefore, the temperature to which the insula- tion is liable to be subjected appears to be largely a problem for the designer to determine from his calculations, based upon accumulated data and experience. This is especially the case with very wide armature cores and large, heavily insulated armature coils, such as found in large capacity, high speed turbo generators'. In such machines, experience has shown that various temperature conditions may be found, depending upon the location and relative values of the losses in the different parts and the means for conducting away the heat. Tests have shown that, in some cases, the armature iron at the center of the core is considerably warmer than the armature copper, while in other cases the opposite is found to be true. 338 ELECTRICAL ENGINEERING PAPERS In. such apparatus, the temperatures actually obtained are liable to be materially higher than the iisual methods of measure- ment will indicate. These temperatures are inherent to the conditions of design and cannot be avoided economically, in certain types of apparatus, such as turbo-generators. In such machines, the limitations in speed, strength of material, etc. force the designer to certain proportions which pre9lude larger dimensions, or lower inductions in the iron, or lower densities in the copper, or increased ventilation- In such apparatus therefore, the development apparently lies in the direction of insulations which will stand the higher temperatures which may be' obtained. These conditions of higher temperatures in some parts of the machine, than indicated by the usual tests, have been recognized for years by designers and manufacturers o£, large electric machinery. A rough indication of these temperatures can be obtained by exploring coils or thermo-couples suitably located. However, it is evident that such coils, if located next to the copper, will not give the correct temperature measurement if the flow of heat is from the iron to the copper, while a coil next to the iron will not give the correct result with the flow from the copper to the iron. Experience has shown that the temperatures, in corresponding positions around the core, may not be uniform, due to local conditions. In consequence, it is not practicable to actually determine the true temperatures of all parts of the insi^lation on commercial machines, exqept by measurements of a laboratory nature, which wt)uld involve such a number of separate readings as to be commercially prohibitive.' On account of the higher temperatures which may be found in such apparatus, and the difficulty of making exact measurc- jnents, except by laboratory methods, manufacturers very generally have adopted the use of mica as an insulating material on the buried part of the coils. Experience has shown that such material, when properly applied, can safely stand temperatures of at least 125 deg. cent. How much more has riot yet been determined. Of such machines it may be said that the manufacturer, with his 'guarantee of 40 deg. cent, by thermometer, actually builds for possible temperatures of 70 to 90 deg. cent, in some parts of the machine, for he expects to find fairly high temperatures in some cases with exploring devices. The usual guarantee Oj 40 deg. cent, therefore should be considered as only a relative indi- cation of a safe temperature in such apparatus. TURBO-GENERA TORS 339 If, for instance, the exploring coils should show 70 deg. ceffi. maximum rise under running conditions, and the ijemiissible ultimate temperature of fibrous or tape insulation is assumed as 90 deg. cent, for continuous operation, then obviously, with air at 40 deg. cent, the insulation would be considered as insuffi- cient from point of durability, except for intermittent service, such as overloads, and such limited^ conditions. Plainly, the insulation, for such temperatures, should be of mica, or equiva- lent material, for which 125 deg. cent, has been found to be safe. Furthermore, it may be stated that with such mica insulation, a turbo generator which shows 75 deg. cent, rise by exploring coils, or thermo-couples, has, in fact, more margin of safety than the ordinary vamished-tape-insulated low-voltage fnachines of any type, which show 40 deg. cent, rise by thermometer or 50 deg. cent, rise by resistance. The foregoing aims to bring out clearly that the temperature problem is a most complex one, in all electrical apparatus, and especially so in turbo-generators. It indicates that no simply temperature test can show all the facts, and that all commercial methods must be considered as approximations. It also shows the absurdity pf classifying a piece of apparatus as good or bad, respectively, according to whether it tests possibly one or two degrees below or above a specified thermometer, guarantee^ Also, following out the above principles on heat flow, various fallacies in temperature measurements might be noted. For example, it is usually assumed that, after shutdown, if a grad- ually rising temperature is shown, this is a more accurate indica- tion of the true temperature. But this may be entirely wrong as Regards windings. If, for instance, the core back of the armature slots is materially hotter than the armature teeth while carrying load, then, upon shut-down, with the air circiolation stopped, the teeth will rise to approximately the same temperature as the core back of the teeth, and there may be a flow of heat into the coils, which condition may not have existed while carrying load. A thermo couple on the coil or in the teeth would thus indicate a false temperature rise after shut-down. This is cited simply as one of many instances, to show tbe possibilities of entirely Wrong conclusions which may be reached in the problem, of temperature. The iKsutATioN Problem The one fundamental condition which must be conadered in the insulation problem, is the durability of the material itself; said 340 ELECTRICAL ENGINEERING PAPERS this must be viewed from two standpoints, — the mechanical, and the electrical. From the mechanical standpoint, the material may have its 'insulating qualities impaired by the action of mechanical forces which tend to crack, or crush, or disrupt the material itself, or it may be affected by being permeated by foreign materials or substances, or it may be injured by such overheating as will partially or wholly carbonize it, or render it brittle or otherwise unsuitable for the desired purpose. From the electrical standpoint, it may be weakened bydeter- ioration of the quality of the insulating material itself or some of its component parts, which may be due to heating", or oxida- tion, or many other causes. The effect of mechanical injury, such as cracking crushing or overheating, on the insulating qualities, will depend upon many conditions. In some cases, with relatively low voltage, any effective mechanical separation of the parts is sufficient for electrical purposes. For higher voltages, continuity of the separ- ating insulating medium is necessary. Experience has shown that, for moderate voltages, tempera- tures which may injure, or even ruin, the insulating material, from a mechanical standpoint, may not seriously affect its insulating qualities. Many insulating materials of a cellulose nature will still retain good insulating qualities if maintained at temperatures as high as 150 deg. cent, for such long periods that the material itself semi-carbonizes. Under such high temperature conditions, however, it becomes structurally bad, — that is, it may become so brittle that it tends to crumble, or powder, or flake off, and thus its value as an insulation is im- paired by displacement of the material itself. In low voltages, therefore, it is not a deterioration in the insulating qualities, but rather a mechanical breakdown of the material itself, which is liable to cause trouble. With high voltages, however, the conditions may be quite different. With some insulating mater- ials, the dielectric strength may be so affected by long continued high temperatures thatthe insulating quality becomes insufficient. This has a direct bearing on large capacity, high-voltage turbo- generators. In the problem of insulation, certain difficulties have been encountered in large turbo-generators, which, while they would have developed eventually in other large machines, yet became apparent more quickly and prominently in the turbo type, due to the abnormal conditions in its design. The two most promi- TURBO-GENERA TORS 341 nent difficulties were, first, that of relatively high temperature in the buried copper, already described, and second, the destruc- tion of the insulation by reason of static discharges between the coils and the armature iron. Dtie to the fact that the ultimate temperature reached in such machines not infrequently exceeds the safe limits for insulation of the fibrous or cellulose type, such insulations will show deterioration eventually in their insulating qualities and their durability. In consequence, with the advent of the larger machines, it became necessary to return to the use of mica for insulating purposes on the buried part of the coil. This type of insulation in th^ form of mica wrappers, had been used extensively on some of the earlier large capacity, slow-speed generators, but it had not been adopted on large turbo-generators, due principally to the difficulty in applying the vefy long wrappers for the straight part ot the coil. However, when the gradual deterioration of the fibrous type of insulatiomiras noted in large turbo-generatorS, the mica wrapper type of insulation was again, taken up and, after considerable experiment, was applied successfully for the outside insulation on the straight parts of the coils. This tfse of mica overcame the deterioration in .the insulating qualities of the outside insulation; but for a while it was considered that a fibrous type of insulation was still effective, between turns in those coils where there were two or more turns in series per coil. As stated before, the instdating qualities of many fibrous materials will stand up astonishingly well under low voltages, when the material is apparently so greatly heated that it ig practically carbonized. Therefore, temperatures which did not carbonize, but simply browned, or darkened, the material, had not been considered dangerous, and undoubtedly many thousands of electrical machines of all kinds are today in operation, in which the insulation is in this condition, and in which no trouble need be expected. For this reason, little or no trouble was expected between turns on the turbo-generators. However, a new con- dition was encountered in large capacity machines, namely, the iasulation between turiis, when it became dry and brittle at the higher temperatures; was liable to be injured by the terrific shocks to which the coils were subjected in such machines, in case of a short circuit across the terminals. The insulation would be cracked, or so disturbed that short circuits, would occur later, without apparent cause. These short circuits on large machines. 342 ELECTRICAL ENGINEERING PAPERS most often appeared as breakdowns to ground, even wdth the mica wrapper insulation on the outside of the coil. Incidentally, several cases were discovered where arcs had occurred inside the coils between adjacent turns, and where they had not yet broken through the outer insulation to gronnd. For many months the writer, with his associates, foHowed up this matter, examining all available coils and windings. Eventually the conclusion was reached that many of the breakdowns to ground had actually started between turns on the inside of the coil. Moreover, as a corroboration, it was noted that in machines with one conductor per coil, the breakdowns were practically negligible. This investigation led to the practise of insulating the individual turns, in each coU, from end to end, with mica iape. After the adoption of this practice, it is noteworthy that the breakdowns to ground practically disappeared, although the outside insulation to ground had not been changed in type or thickness. Many improvements have been made in recent times in the application of this mica insulation. One of these is the Haefely process, developed in Europe, but now being used extensively in this country. By this process, the mica wrappers are ^o tightly rolled on the coil that practically a solid mass of insula- tion, of minimum thickness and greatest heat conductivity is obtained. By means of the mica insulation, the temperature difficulties in general have been entirely overcome, and a durable and non- deteriorating construction, from an insulation standpoint,, has been obtained with the temperatures which appear to be more or less inherent in the large, high-speed turbo-generators. The secoiid trouble, namely, that due to static discharges between the arrnature copper and the iron, was also encountered to a certain extent, on some of the earlier machines. It was found that these discharges were apparently " eating " holes, or even grooves, through the outside insulation of the armature coils. This effect was most pronounced at the edges of the air ducts and at the ends of the armature core, where edges were presented by the iron. After a long period, the holes or grooves would become so deep that the insulation was weakened or ruined. This was a very disturbing condition, when it was once fully recognized and appreciated. Again, a comprehensive investi- gation was made to discover a- cure for this difficulty. Various types of machines and windings were examined. It was noted T URBO-GENERA TORS 343 that the action was a function of the voltage of the machine, but was noticeable, in some cases, at relatively low voltages. During the course of the investigations, it was noted that where mica, wrappers were used with an outside layer of tape, the " eat- ing away " extended only through the outside wrapping in as far as the mica, and that no apparent effect at the mica was visible. Even when examined, with a very powerful microscope, no evidence of any puncture of the mica was found, in any case. Thede investigations naturally led to the' conclusion that the jnost suitable remedy for the trouble was the use of mica insula- tion, which was also a remedy for ^he temperature conditions. This is one of the rare cases in large turbo-generators where two desirable conditions do not conflict With feach other. The mica insulations on the buried part of the coil has now been very generally adopted in this country on high-voltage rnachines, whether of- the turbine-driven, or aiiy other type. This, static trouble was considered so serious at one time that low voltage practice with step-up transformers was adooted by some manufacturers as the safest course, until something positive in the way of a Remedy was proved out. This trouble promised to be one of the most, serious encountered in high- voltage generator work, and even threatened to revolutioViize practise in winding generators for the higher voltages. However, as consistently advocated by the writer, the usfe of mica, suit- ably applied, appears to have entirely overcome this trouble, as evidenced by several year's experience, and all indications now are that there need be no fear from static discharges on windings of 11,000 and 13,000 volts. EVen in the 11,000-volt New HaVejci generators with one terminal grounded, which gives the equiva- lent of a r9,000-volt, three-phase winding with the peutral grounded, the mica insulation appears to be successful and dur- able. Rotor Insulation In most of the early turbo-generators, the rotor winding in the slots was insulated with fibrous material! " fish paper " and " horn " fiber having the j^reference. 'One of the difficulties in the rotor is, that the insulation between the winding and the slot is liable to be crushed or cracked by the hig];i centrifugal forces. In the earlier insulatibns, before fish paper was used, it was ifound that even at very moderate temperatures, the insula- tion got dry and brittle, and cracked readily. Fish paper, or horn fiber, was then adopted pretty generally. Such material 44 ELECRTICAL ENGINEERING PAPERS ipparently stood much higher temperatures than the ordinary ibrous insulations. However, experience also showed that iventually this also became brittle, arid was liable to be cracked, md then displaced, due to the centrifugal forces. There is ilways the possibihty of a small amount of movement in the ield coils when a machine is being brought up to speed, and this novement, in itself, may eventually damage the insulation if t is at all brittle. As the capacities and speeds of turbo-generators were increased ind the space limitations for the rotor windings became more jronounced, the resulting higher normal temperatures led to he adoption of mica for the insulating material in the slots with ither mica or asbestos for the insulation between turns. As he voltage between adjacent turns is always ebctremely low, irhat is needed is really a durable separating medium, rather hail an insulation, this medium being one which will not become risp or brittle at fairly lugh temperatures. Asbestos has erved for this purpose very effectively, and even has some idvantages over mica, as the latter must be applied in relatively mall pieces in the form of strap or tape, and the individual )ieces are more readily displaced or shifted than is the case rith asbestos. Some very severe tests have been made in irder to determine the possibilities of such rotor instdation. n one case, a turbo rotor thus insulated was run at full speed for iver 40 hours, with such a current that the rise by resistance ti the rotor copper was about 250 deg. cent It was the in- ention to continue this test very much longer, but the conduc- ion of heat from the winding to the core, and thence through he shaft to the bearings, was such that finally the bearings >ecame overheated and gave out. After this test, the winding was arefully dismantled, and no evidence of any injury to the nsulation could be discovered. Of course, such temperatures ire not recommended in turbo rotor practice, but this was imply an attempt to find' a temperature limitation. If a Lesigner wants to find the facts in any apparatus, he will obtain he most valuable information if he operates the apparatus ip to the point of destruction. He thus fixes a limit which le must keep below. The use of mica, or mica and asbestos, on turbo rotors has »een very generally adopted in this country at the present time, nd it may be said that, within the writer^s experience, no case if .destruction ot one of these windings through heating, has T'URBO-GENERATORS 345 come to his notice, although a great number of them have been in service for a relatively long time. In many of the older machines with fish paper insulation in the rotors, the conditions of ventilation and the normal ratings of the machines were such that the maximum tempetatures in the rotor windings were relatively much less than in present practise. It may there- fore be said that the use of mica in the rotor has been largely due to the introduction of the larger capacities and higher speeds. Losses in Turbo Alternators The total iron and copper losses in a large, high-speed turbo- alternator are in general no higher than in a corresponding capacity low-speed machine. As far as the iron losses are concerned, no further comment need be made than that the magnetic flux deiisitics in general are somewhat lower than in low^r speed machines of same frequency, and therefore the losses per unit volume of material are no larger. The total armature copper losses in turba-alternators, as a rule, are considerably smaller than £n corresponding .capacity machines of the moderate or low-speed types. This is due partly to the use of a smaller total number of conductors, and partly t6 a lower current density in the armature conductors. As brought out before, in a narrow cpr6 machine, a considerable portion of the buried copper heat may be conducted lengthwise of the conductor into the end winding, and there dissipated into the air. In the turbo-geherator, with its much wider C-4 '' 2 \ \ [ ^ 50 r5 100 135 DEGREES C. Fig. 1 tures as high as 160 deg. cent, on such insulations for a con- siderable period may not entirely destroy their insulating qual- ities, although, mechanically, such temperatures appear to be impracticable, except for very short periods. In order to illustrate the relation between the possible life and temperature of class A insulation. Fig. 1 is shown. This must not be taken as representing actual re^sults, but is simply in- tended to illustrate, in a very approximate manner, the very great shortening of the life of insulation by increase in temperature. It may be assumed that at very high temperatures, the insu- lation will have practically the same life, in actual hours of high temperature operation, whether the temperature is applied con- tinuously or intermittently. For example, if an insulation has 10,000 hours life with a certain high temperature continuously TEMPERATURE AND INSULATION 355 applied, it is assumed that it will also stand the same tempera.- ttire for 10,000 hours in short periods, provided the intermediate temperatures are low enough to represent an indefinitely long life. It is probable that under the intermittent condition, the life will really be slightly greater, due to the fact that depre- ciation will be largely mechanical, and the insulation maj' " re- cover," in some of its mechanical characteristics after each period of high heating. If, therefore, higji temperatures are reached intermittently, with intermediate periods of lower value but still high enough to shorten the life of the insulation, it may be assumed that the total Hfe of the insulation is the resultant of the life under the two temperature conditions. 10 8 -- — 1 — — — - 1 i — u ^4 J _, 2 \ 1 n H^ 50 lOO 150 200 DEGREES C. 230 Fig. 2 In heat-resisting materials, such as those of class B tempera- tures of -125 deg cent are comparable with 85 deg cent or 90 deg cent in.classA,andl50deg cent in the former is comparable wnh 100 deg. cent in the latter. Fig 2 illustrates very approxi- mately the life-temperature curve of such insulations As in Fig 1 , this should not be taken as an exact representation of the actual life Due to the greater heat-resisting qualities of such materials. It appears that relatively higher temperatures-are not as quickly harmful as m the first class In class C materials, it is difficult to give any reasonable indi- cation as to the limits of temperature, except that very high temperatures, (practically up to the point of incandescence') are found in some heating appliances. '■irtCi ELECTRICAL ENGINEERING PAPERS Temperatures and Flow of Heat As the insulation, in itself, is not usually the seat of generation of loss or heat, it is the temperature of adjacent materials which must be considered in defining the conditions in the insulation. The temperatures of the adjacent materials should therefore be considered only m so far as they affect the insulation itself, and where such temperatures do not affect the insulation, or the hfe of the apparatus, or its normal perfomance, they are immaterial Considering the influence of the temperatures of the adjacent media, the direction and amount of heat flow must be taken into account, as the maximum temperature in the insulation is de- pendent upon these. In the case of armature windings, for instance, the heat flow may be from the buried portion of. the d 1 j 1 i,a 1 ^J)^"'^ — "V 1 c a M 1 B Fig. 3 coils toward the end windings. It also may be from the buried copper through the insulation to the armature teeth, or there may be a reverse heat flow from the iron to the copper, depending upon the various factors of construction, heat conductivity of the materials,, amount of heat generated in the various parts, ventilation, heat dissipation etc. Depending upon conditions of heat flow and distribution, various methods of temperature determination may be used. No method is accurate, unless all the conditions of heat flow are accurately known, which is never the case in commercial ma- chines. The difficulties in the problem of commercial temperature determination are illustrated by Fig. 3. TEMPERATURE AND INSULATION 357 In the figure, a represents the temperature inside an armature coil, h the temperature between the insulation and the iron of an armature tooth, c that in the body of the tooth, and d that in the body of the core at some point back of the coils and teeth. Lei the temperatures at no load be represented on the ordinate A Then, at some load, represented by ordinate B, the relations of the various temperatures have changed. At C, D and E, there are still greater changes, depending upon the heat genera- tion and distribution. If the rated capacity of the machine is at E, for instance, then the armature copper is hotter than the iron, while if rated at B, the reverse would be true. Obviously, no rule can be formulated to cover these various conditions in different machines, nor even in a given machine, unless all the heat generation, distribution, and dissipation characteristics are known. Obviously, as far as the insulation is concerned, the temperatures of a and b are the only ones which need be consid- ered. All temperature determinations of a commercial nature, are necessarily approximations, or. relative indications, upon which proper margins must be allowed for the ultimate temperature possibly attained. Therefore, in apparatus where there are liable to be discrepancies of 10 deg. between the measurable and the actual ultimate temperatures, a limit of 90 de<; cent, should be allowed by conventional temperature measurement on insu lations in which 100 deg. is set as the maximum tcm])craturc with a reasonable length of life. The conventional methods of .temperature measurement, a^ bv resistance, and by thermometer, do not usually give the maxi- mum temperature, but give either the average, or the outside sur- face, values, and, when measuring the temperature by these methods, which are the only ones generally applicable, an allow- ance, must be made in windings for ]:)0ssible local higher temperatures. These methods apply espedally to tliose ma- chines of moderate or low voltages in which the insulation is relatively thin, so that the heat gradient from the inside copper to the outside surface is small. Also, they apply particularly to those machines in which the conditions of ventilation are not nor- mally difficult, and in which a fairly thorough distribution and dissipation of heat occurs among the various parts, such as in ordinary direct-current armatures, induction motors primaries, stators and rotors of moderate speed alternators m which the width is relativelv small compared with the diameter, etc 358 ELECTRICAL ENGINEERING PAPERS As the ultimate temperatures obtained by the apparatus de- pend upon its rise above the room temperature, or that of the cooUng medium, and as such temperatures may vary over a wide range, it is not practicable to specify or guarantee ultimate tem- perature of apparatus without also specifying the. elements upon which it depends This, therefore, results in specifying the temperature rise in relation to that of the cooling medium. While most apparatus operates at materially lower cooHn;^ temperature than 35 deg. cent, to 40 deg cent., yet such tem- peratures are sometimes reached for considerable periods of time m steam stations, and it appears therefore as justifiable to choose the permissible temperature rise, such that, at room temperature of 35 deg. cent, to 40 deg. cent., an ultimate temperature of 85 deg. cent to 90 de^. cent, by conventional methods of measure- ment, IS not exceeded This means, therefore, a temperature rise of 50 deg. cent, with conventional methods of testing, such as by increase of resistance, or by thermometer, in those insula- tions which can stand a continuous ultimate temperature of 100 deg cent with a comparatively long life. This allows an excess of 10 deg. cent, to 15 deg. cent, for local spots, or for the temperature gradient through the insulation. A less allowance should be made for this difference when methods of temperature measurement other than the conventional are used, and which approach more closely to the highest temperature actuallv at- tained When the above temperatures are liable to be materially exceeded for long periods, heat-resisting insulation of class B is recommended With such materials, a temperature of 125 deg. cent IS comparable with 85 deg cent to 90 deg. cent, m the materials of class A Therefore, on this basis of a room tem- perature at 40 deg cent or 45 deg cent., rises of 85 deg. cent or 80 deg ceiit should not be considered harmful However, m those specia] cases .where the conventional methods may not sufficientl\' approximate local high temperatures, as mav be the' case m large turbo-generators, or in wide core alterna- tors of large capacity, the rises of 80 deg cent, or 85 deg cent should not be specified by resistance or thermometer, but preferably some lower temperature such as 50 deg. cent, thus allowing a very considerable margin for local higher tem- peratures. In such apparatus with the higher temperature.-^, which require class B insulation, there is liable to be less unifonn- itv of heat distribution TEMPERATURE AND INSULATION 359 If special-methods of temperature measurement, such as ex- ploring coils or thermo-couples are used m such apparatus, the temperature hmit of 125 deg cent, should be considered, and not the conventional '50 deg cent rise. In those machines of this class which have relatively thick insulation, and consequently' may ha\-e a high heat gradient between the copper and the iron, (depending upon how much heat is flowing from the copper to the iron) an ultimate temperature of the inside insulation of 150 deg. cent is considered as the limit, this being comparable with 100 deg. cent with insulations of class A. In certain classes of apparatus which are artificially cooled by air from outside the room, the cooling is accomplished partly by dissipating heat to the artificial air supply, and partly by dissi- pation into the surrounding room. If the temperatures of the cooling air and of the room are widely different, the resultant of the two temperatures should really be taken as that of the cool- ing medium. The variation of the temperature rise has heretofore been considered as having a definite relation to the temperature of the cooling medium. However, it appears that it does not follow any definite simple law, but it is sometimes positive and some- times negative, so that no satisfactory correction for room tem- perature is possible at present. It is therefore desirable to make the temperature tests at a room temperature as nearly as pos- sible to some specified reference temperature, so, as to make any temperature correction negligible The reference temperature in the guarantees should therefore be such as can easily be secured ; that is, it should be the average temperature of the places at which the apparatus may be operated This is from 20 deg. cent to 25 deg. cent , and as it is easier to raise than to lower the room temperature, the upper figure is advisable as a reference value. This reference temperature therefore should be chosen as 25 deg cent., which is m accordance with the previous A I E.E standard. Me.asurement of Temperature In the conventional methods of temperature measurement, by thermometer, and by resistance, many conditions should be taken into account, and good judgment is required, in all cases, or fallacious conclusions may be obtained. There are many conditions which affect both the accuracy of the resistance and the thermometer methods of measuring tem- perature The resistance method measures only the average 360 ELECTRICAL ENGINEERING PAPERS temperature rise, and not that of local hot spots. However, it measures the internal temperature of windings, and therefore no correction is required for the temperature gradient through the outside insulation The proposed margin between the result by the conventional method, and the actual temperature can therefore be allowed, in the resistance measurement, as the dif- ference between the warmer and the average temperatures in the windings. In the resistance method of measurements, the rate of transfer of heat from one part of the winding to another will not greatly affect the result, as the measurement indicates an average temperature, which is that obtained if the heat were equalized throughout the winding. However, the rate of flow of hekt from the windings through the outer insulation to other parts, will affect the temperature measurement by resistance, and preferably the measurement by this method should be taken during operation in those parts where this is practicable, as in field coils, and some other instances. In those parts where the resistance cannot be measured during operation, this should be done as quickly as possible after shut-down, and the time taken to. shut down the apparatus should not be unduly long. Prefer- abl)', during shut-down of rotating apparatus the normal current should be maintained on the apparatus until at least a relatively low speed is obtained. This would represent onlj^ an average condition, as the ventilation at lower speed is very greatly de- creased, while the losses in the windings will remain normal, thus tending to give an increased temperature in the windings. It would be difficult to fix any definite rule which would give. the exact temperature conditions during shut-down. In the measurement of temperature by thermometer, con- siderable judgment is required Wherever possible, the tem- perature should be taken during operation, but the thermometer with its pad should be so placed that it does not interfere with the normal air circulation. In thermometer readings, as usually obtained on windings, the heat gradient through the insulation must usually be allowed for, this being 10 deg. to 15 deg as previously defined However, depending upon the method of taking the temperatures, this allowance should vary over a con- siderable range, depending upon whether or not the method of measurement approximates the actual internal temperature For instance, the total heat gradient frdm the inside copper to the outside air will be that through the coil insulation, plus the thick covering pad over the temperature bulb If the gradient TEMPERATURE AND INSULATION 361 through the covering pad is very large compared with that through the insulation, the thermometer may indicate almost exactly the internal temperature of the copper; that is, the heat gradient through the insulation to the thermometer, maybe rela- tively small compared with the total gradient to the air. This is particularly true where the thermometer rests on a metallic seat which covers a considerable portion of the coil surface. In this case, the heat which affects the thermometer- bulb will pass through a relatively large section of surface, with a correspond- ingly small drop in temperature, so that the bulb more closely approximates the tempera:ture of the inside copper. Where there is local heating in the windings, and a consequent liability of rapid transference of heat to other parts, the results obtained by the thermometer method will vary to some extent with the rapidity with which the actual meastirement is made; that is, the more quickly the thermometer can be brought up to the full temperature, the more accurately the temperature of the hottest part is determined. With a very rapid method of measurement, it may be possible to measure practically the in- ternal temperature of the copper of the winding before any great heat transference or dissipation has occurred. In such cases, obviously, the full allowance for the usual temperature margin shovild not hold. It should be fully understood that it is the ultimate temperature, and not the temperature rise, which should be considered as the limiting condition, and that the measured rise, plus the allowances for temperature gradient, plus the measured room temperature, is simply an indication of the possible ultimate temperature. By whatever method the temperature measurement is made, in all cases the results may be considered as more or less approximate, and in the end, it is the' manufacturer who must supply the necessary margin over the approximate measurement, in order to make the machine safe. A blind adherence to some particular rule or method of taking temperatures, may lead to fallacious results in some instances. In armature windings, in particular, incorrect readings may be obtained after shut-down. For example, if the armature iron back of the armature teeth were hotter than the armature teeth and coils during operation, then the temperature to which the insulation 13 subject during operation may be considerably lower than that in the hottest part of the machine, due to the ventila- tion conditions when running. However, upon shut-down, the 362 ELECTRICAL ENGINEERING PAPERS temperature at the insulation may rise to that of the hottest part of the machine, and therefore a false temperature, by any method of measurement, might be indicated. Recommendations That with class A insulation, 90 deg. cent, be taken as the ultimate temperature limit, as indicated by conventional methods of measurement, or those which give similar results, and that 100 deg. cent, be considered as the maximum ultimate tempera- ture permissible in the insulation, where a cornparatively long life is a requirement. That 40 deg. cent, be taken as the limiting temperature of the cooling medium, or room, and that, therefore, 50 deg. cent, be the permissible rise by conventional methods of measurement, with class A insulation. That 25 deg. cent, be taken as the reference air temperature. With the permissible 50 deg. cent, rise, this gives 75 deg. cent, as the average operating condition, by conventional methods of measurement, or 85 deg cent, actual temperature, when the usual margin represented by the temperature gradient is added. An exception to the rise of 50 deg. cent, can be made in those cases where space or weight limitations are such that higher temperatures, with consequent reduced life, are commercially economical, such as in railway motors. In such cases, with class A insulation, a rise of 65 deg. cent, with reference air at 25 deg cent, is at present accepted as good practice. With class B insulations, 125 deg cent be taken as the ultimat temperature limit, as indicated by conventional methods of measurement, or by equivalent methods, and 150 'deg. cent, be considered as the maximum ultimate temperature permissible m the insulation- It follows therefore that 80 deg cent, to 85 deg. cent, rise is allowable, with such insulations, by the usual methods of measurement No temperature correction should be made for variation of the cooling temperatures from the refere4-ice temperature of 25 deg. cent When the method of temperature measurement shows the highest temperature actually obtained in the insulation, the maxi- mum temperatures specified for the given type of insulation should hold . In the final decision on questions of temperature rise, the ulti- mate temperature should be the basis, rather than the rise. TEMPERA TURE DISTRJB UTION 363 TEMPERATURE DISTRIBUTION IN ELECTRICAL MACHINERY FOREWORD — This paper was presented at the Chicago Section meeting of the American Institute of Electrical Engineers, November 27, 1916. A number of papers by the author dealing with the temperature problem had appeared before, but the purpose of this paper was to put the subject in more definite shape and bring it more nearly up to date. During the discussion of the paper, considerable new data was presented by the author, and it has, therefore, been included in this reprint. — (Ed.) HPHE laws ^governing heat flow and temperature distribution ■•■ are so similar, in many respects, to those governing electric current flow and electric potentials, that it is rather surprising that the former have received so little attention in comparison with the latter. Some of the laws of heat flow are so well recog- nized that their apphcation to the problem of temperature dis- tribution in electric apparatus should have been a leading feature in the early developments in such apparatus; whereas, on the contrary, it is only recently that very careful study has been made of such application. One object of this paper is to indicate, in a comparatively simple manner, some of the conditions which fix the tempera- tures in different parts of electric apparatus. Before going into the general problem, certain simple conditions may be stated, such as: 1. The heat flow between two points is proportional to their temperature difference and to the heat resistance of the path of paths between them. Note the resemblance to Ohm's law As a corollary to the above, it should be eviderit that between 364 ELECTRICAL ENG'INEERING PAPERS two points at the same temperature, there should be no flow of •heat. 2. The total temperature drop between any two points or media of different temperatures will be the same through all paths of heat flow. 3. There are no true non-conductors of heat, and, conversely, no perfect conductors. 4. Heat conduction and electric conduction bear some quan- titative relation to each other, in the broad sense that all electric insulators are relati^"ely poor heat conductors, while good electric conductors are correspondingly good heat conductors. There is apparently no rigid relation between the heat resistance and electric resistance of the various materials used in electric ma- chinery, but the general relation holds and there are apparently no radical exceptions. 5. The rise in temperature at any point, due to generation of heat, is dependent (a) upon the total heat generated, and (b) upon the amount of heat which can be carried away along all available paths per degree of temperature difference. The tem- perature, will rise until the heat dissipation equals the heat generation. 6. There are two ways to lessen the heat flow along any path, (a) By interposing higher heat resisting materials, (b) By lessening the temf)erature difference, as by raising the tempera- ture of the part through which the heat is to be con-ducted. Conversely, the heat flow can be increased along any path by the use of better heat conducting materials, or by paths of lower heat resistance, and by lessening the temperature of any part to which the heat is to flow. What makes the problem unduly complicated, in electrical machinery, is the fact that there are several different sources of heat generation, which may be, and often are, all active at the same time. Moreover, the heat losses may be distributed through the various heat conducting paths in such a way as to render any calculation very difficult and more oj less inexact, except in a general way. For example, there is heat generated by losses in the copper conductors, obeying one law; while there is heat generated in the iron parts under a quite different law; and there may be heat generated by windage and friction, according to a third law. As tb.ese different losses may act in different parts of the heat conducting circuit, it should be evident that the problem of determining tlie exact heat distributions, TEMPERATURE DISTRIBUTION 365 and the temperature, is a very complex one. Such a determina- tion is in the province of the expert analj'tical designer of such apparatus, but certain general conditions are of interest to all users of electric apparatus. Consider first the general conditions of heat dissipation from an armature coil. In Fig. 1 is represented an armature slot with the surrounding iron, and with two separate "coils" per slot, as is now the most common practise. Let it be assumed that the point a represents the "hot spot", or part at highest temperature in the apparatus. The heat from this part can flow along two general paths, namely, longitudinally through the copper 'con- ductor itself to the end windings, and thence to the air, and ^^ Fig. 1 laterally through the insulation to the surrounding iron, or to the ventilating ducts. From the iron the heat flow is then through various paths to the external cooling air. Longitudinal Heat Flow Considering first the longitudinal conduction of heat in the coil, then starting at the point a, the first unit of length con- ductor will have a certain loss. If the heat generated by this first unit loss were all that need be considered, then the drop in temperature, from the point a to the end windings, would be simply a function of the heat-conducting properties of the con- ductor itself. But the next unit length is also generating its 366 ELECTRICAL ENGINEERING PAPERS own unit loss, so that the heat flow from the second to the third unit length is due to two units loss; in the same way, the flow to the fourth unit length will be due to three units loss, etc. Therefore, the temperature drop, or temperature difference per unit length of conductor, increases more rapidly as the point a is departed from, and if it is at a considerable distance from the end winding, and the losses per unit length are compara- tively high, a very high temperature may be required at a to conduct all the heat longitudinally to the end windings. In very wide core machines the longitudinal drop may be so great that the temperature at a in practise will be so far above that of the surrounding iron, that a very large percentage of the actual heat is conducted laterally through the insulation to the iron, even if the iron is at a comparatively high temperature. However, in narrow cores, the drop to the end windings may' be, in some cases; so very low, possibly 5 to 10 degrees, that with good heat dissipation from the end windings themselves, the point a may have, for instance, an actual temperature of 40 deg. cent. If the iron next to a dho has a temperature of 40 deg. cent, then there would be no flow of heat from a to the iron. Fur- thermore, in such a case, as the iron temperature over the whole width of the core may be fairly uniform, and as the copper temperature decreases from a to the end windings, obviously as we depart from the point a, there would be heat flow from the iron to the copper, and thus the windings would tend to cool the core. This is frequently; the case with light loads on a machine, for in such conditions the coil loss is low, while the iron loss remains fairly constant for all loads. In such case there may be heat flow from the iron to the copper along the whole length of the buried portion of the coil. At some higlier load, the copper, loss varying as the square of the load, the in- creased longitudin:il drop will bring the copper temperature above that of the iron so that the heat flow is from copper to- iron. This condition is illustrated by Fig. 2. It must be recognized that the lateral flow of heat, from the coil to the iron, reduces the longitudinal drop, such reduction depending upon the relative percentages of heat flow al.ong the two paths. It must also be borne in mind that in order to have such longitudinal heat flow, the end windings must be able to dissipate their own heat at lower temperature than would ba attained at a, of in the core. If the end windings have little or no ventilation, or heat dissipating capacity, then their own. TEMPERATURE DISTRIBUTION 367 generated heat may bring their temperatures higher than those of the armature iron so that the heat flow actually may be from the end windings toward a, and then laterally through the in- sulation to the core In such case, the hottest spot will be in the end winding rather than in the buried part of the coil Obvi- ously when such condition occurs there is no possibility of either the end windings or the buried part, of the coil being cooler than the iron, for the heat flow throughout is toward the iron. Lateral Heat Flow Considering next the lateral flow of heat through the insulation to the iron, the amount of heat conducted is a function of the temperature difference and the resistance of the conducting path. Or, in other words, if a given amount of heat is to be conducted through a path of given resistance, the tempera- ture in the heat generating part rron Temp's. wiU rise Until the required heat Ligtii Load Iron Temp's Copper Temp's. ^ Copper Temp's, jg couductcd away, M Copper Temp's. A Iron Temp's. _&b_ /30°C. Drop >— 70°C-4O°C. Rise, in Iron with Air at 30'C Width ol Core JM» _ Fig. 2 Pig. 3 To illustrate this problem more concretely, let Fig. 3 represent the temperature conditions in a section of an armature. Assum- ing, for example, the temperature of the copper inside the coil insulation as 100 deg. cent., the iron temperature as 70 deg. cent., and the air temperature as 30 deg cent , then the following conclusions may be drawn. (a) From the outer coil (the one next to the air gap) through the wedge to the air gap, the tempefature drop will be 100 — 30 = 70 deg. cent. Obviously, any tertiperature measurement made outside the wedge, next to the air, will approximate the 368 ELECTRICAL ENGINEERING PAPERS temperature of the air and not of the copper Any temperature measurement made beneath the supporting wedge will measure some intermediate temperature between the copper and the air If the temperature drop through the wedge should be eqiial to that through the insulation, then a measurement underneath the wedge should show half the temperature drop through in- sulation and wedge, and obviously, the measured temperature would be far below that of the copper. (b) If the temperature is measured at the outside of the coil, between the iron and the insulation, it would approximate the average of the temperatures of the iron and of the outside .insulation, or practically the temperature of the iron. If the iron should be at different temperatures at the sides of the slot and at the bottom, then obviously different readings would be obtained, depending upon the location of the measuring device. It is evident that such temperature measurements give no in- dication whatever as to the true internal temperatures of the coil, for the heat flow and the resistance of the insulation are nowise involved in the measurement. (c) At a point a, between the two coils, there should be but little heat flow through the insulation, unless the copper is comparatively narrow. If there is but little heat flow through the insulation at this point, then eventually the temperature at the point a must rise to approximately that of the copper in the two coils. Therefore, a measuring device located at a will approximate the temperature of the copper itself, and is, in general, a good indication of the hSt spot at that part of the winding Therefore, as a practical method of temperature determination, a thermo-couple located at a is about the most satisfactory volts, which would not be anything like as bad as before. If, now, the load is removed from the machine, the brushes still retaining their lead, the three volts due to the main field will still be generated in the short- circuit armature coils, and there will be a no-load short-circuit e. m. f. of three volts, which would set up a local short-circuit current. However, as no work current is present under this con- dition, the short-circuit current could obviously be practically as great as the maximum value of the resiiltant current at the full load conditions. Therefore, if three volts short-circmt e. m. f. is permissible at full load, then four or five volts would be permissible at no load with practically the same commutating conditions as at full load. Therefore, the brush could be shifted forward into a field representing four volts, for instance, and thus at no load the short-circuit voltage will be four volts, while at full load it would be 6 — 4 = 2 volts. Therefore, by this means an impossible com- mutating condition, represented by no lead at the brushes, be- comes a possible and practicable condition by giving a certain amount of lead. On non-commutating pole machines where a slight amount of lead is almost always required, a resultant short- circuit e. m. f. of three volts across the brush may be permissible, in some cases, at full load, but this cannot be assumed to be true in all cases, for there are other conditions, besides commutation, which are dependent upon the amount and distribution of currents in the brush. Of these other effects, the principal ones may be classified as, burning of the commutator and brush faces, high mica, and picking up of copper. "wear" or "eating away" of commutator and brushes An elaborate and long extended series of tests has shown that when a relatively large current passes from a brush to a commutator or collector ring, or vice-versa, there is a tendency for undue "wear," as it might be called, of either the commutator or brush face, depending upon the direction of current. If the current is from the commutator to the brush, then the commutator face "wears" or is "eaten" away, while with the current from the brush to the commutator, the brush shows increased wear. This SYNCHRONOUS BOOSTER CONVERTERS 445 is not a true mechanical wearing away of the commutator or brush, but is more Hke an electrolytic action, except that usually the particles taken from one surface do not deposit on the other. This rate of wear, as shown by test, is a function of the current density, the area of siurface through which the current passes, and the contact drop. It is not directly proportional to the contact drop, or the current, but increases in .a much greater proportion than either, or possibly even more rapidly than the product of the two. However, this is difficult to determine defininitely, for with the wear once started, the trouble tends to accentuate itself. In other words, this wear will increase the contact drop and in turn the increase in contact drop wiU exaggerate the wear, so that the action is ctimulative. This wearing action is apparently very slight in amount at true brush densities of 50 to 60 amperes per square inch, with carbon brushes, and if the commutating characteristics are very good, even much greater true densities are practicable, possibly up to 100 amperes per square inch. If the apparent density could be brought up to the true density; that is, if no current but the work current were present, then this high current density in the brush might be utilized in well designed machines, but this implies the absence of all local currents, also, perfect division of the current between the various brushes and brush arms, as will be referred to later. These two conditions are rarely attained in practice, and it would probably be dangerous to attempt apparent densities of 100 amperes per square inch in the ordinary carbon brush; but with commutating pole machines, where an opposing e. m. f. is generated in the short-circuited armature coils, the condition of relatively small local currents can be obtained by very careful proportioning of the commutating pole field. This means therefore that higher crurent densities in the brushes are feasible in commutating pole machines in general than in the non-commutating pole type. This has a direct bearing on the synchronous converter problem, as will be shown later when considering high speeds and maximum outputs with a given number of poles. However, the condition of perfect division of current between the different brushes has not been obtained in any simple, practical manner, and therefore some margin in brush ctirrent density must be allowed, even in commutating pole ma- chines. 446 ELECTRICAL ENGINEERING PAPERS HIGH MICA When the maximum current density in a brush contact is comparatively high, due to local currents or other causes, the commutator and brush "wear" may be relatively rapid compared with the mechanical wear due to friction of the brushes on the commutator. Under this apparent wear the conmiutator copper will be slowly eaten away by the current, but the commutator mica will not be materially affected. The mica must wear down by the mechanical friction of the brushes. If the "eating away" of the copper exceeds the mechanical wear of the mica, then a condition is reached which tends to increase the defect . As soon as the copper face is burned even an infinitesimal amount below the mica, the brush face tends to "ride" on the mica and thus has a reduced contact on the copper surface, or even none at all. This condition increases the burning action and eventually results in the so-called "high mica" where there is an actual gap between the binsh and the commutator face. Such a condition, once started, does not tend to cure itself, except under certain special conditions of operation. This high mica is frequently charged to the use of "hard" mica, which tends to produce a similar condition. In some cases, this trouble from high mica may not be due to either excessive local currents or hard mica, but may be due to a relatively high proportion of mica to copper surface. Where comparatively thin commutator bars are used on a machine, the thickness of mica between the bars is not reduced in proportion, so that the percentage of mica may be relatively high. In con- sequence of this high percentage, the mica itself does not wear as rapidly as where a less total amount is used, while the copper may eat away at the same rate. This may therefore tend toward high mica, even where the local currents are relatively small. This condition of high percentage of mica is foimd particularly in high voltage machines where the number of bars is necessarily great and the thickness of each bar correspondingly small. On the other hand, with low voltage machines, the percentage of mica is relatively less, but other conditions may enter which partly neutralize this advantage. With lower voltages, for a given capacity, the current is correspondingly greater and, with a given contact drop, the losses are correspondingly increased and the tendency to produce noise by the brushes is also greater. To overcome these objectionable features, a soft, low resistance brush is frequently used. This, however, increases the tendency for SYNCHRONOUS BOOSTER CONVERTERS 447 local currents and thus increases the copper wear, while at the same time a softer brush has less grinding action on the mica. There- fore, the low voltage machine may also tend toward high mica. A common, and very effective, remedy for this tendency toward high mica is to "undercut" the mica so that, everywhere on the -brush wearing surface, it lies slightly below the copper surface. This does not remove the initial cause of the trouble, namely, the tendency to eat away the copper surface. But it must be considered that this initial tendency is usually very slight, and that the major part of the wear is due to the lessening of the contact between the brushes and the copper, thus increasing the burning tendency. In consequence of undercutting the mica, the brush can always maintain good contact with the commutator face, and thus the actual burning may be so slow as to be practically negligible. The true gain from undercutting the mica thus lies in the maintenance of more intimate contact between the copper and the brush. This eating away of the commutator face may occur in service and yet the conmiutator may polish beautifully. This is found in some cases where the burning action is pronounced, and yet the conditions of operation are such that the mica can be worn down mechanically as rapidly as the copper bums away. This is not infrequently the case with machines where there are heavy peak loads of relatively short duration, followed by very much longer periods of operation with but little load. Under such conditions the biiming action during the peak loads, with a consequent tendency to high mica, is hidden by the grinding action of the brushes on the mica during the long periods of operation at light load, so that the mica is kept practically flush with the copper and the copper surface is polished. That real burning is present is often indicated, in such machines, by relatively rapid wear on the commutator in grooves when the brushes are not well staggered. "picking up copper" Another condition which sometimes accompanies high current density in the brushes, is the so-called "picking up of copper." Apparently, under some conditions, particles of copper, eaten away from the commutator face, will collect on the brush face. This may result in glowing at the brush contact, eventual burning away or "honey-combing" of the brush sxxrface and general trouble at the commutator. This difficulty is possibly largely cumulative in its action. A sHght coating of copper, or copper 448 ELECTRICAL ENGINEERING PAPERS "spots," may form on a brush. This gives a more intimate, or lower resistance, contact with the commutator face. With many brushes in parallel, an undue percentage of the total current may then pass through this one point, or brush, or low resistance con- tact, and the cvirrent density at this point may even become so great that the burning will be excessive. The resistance of the carbon brush, in itself, does not help this condition, for, un- fortunately for this case, carbon has a negative coefficient of resistance so that heating lowers its resistance and thus accentuates the unequal di^dsion of current. One remedy for this condition is a more uniform contact resistance between the brush and the commutator. Experience has shown that undercutting the mica will frequently overcome this difficulty of picking up copper, particularly so if the machine can be "nursed" until the com- mutator face acquires a glaze. In some cases, a different grade of brush will be an improvement, but it is generally difficult to predict the most suitable brush, unless the inherent commutating characteristics of the machine are well known. This picking up of copper appears to be, to a great extent, a function of the cur- rent density, and is apparently somewhat of an electrolytic action, the copper eating away from the commutator and depositing upon the brush. Whatever tends to materially reduce the tendency for the commutator face to eat away, also tends to reduce the picking-up effect. The foregoing features, while apparently minor in nature, are all of fundamental importance in commutating machinery in general, and particularly so in the case of commutating pole rotary converters, especially in those commutating pole rotaries which have what might be called self-contained or "auto" regula- tion of voltage, such as those with synchronous boosters, or with regulating poles. COMMUTATING POLES In the direct-current generator of large capacity and high speed the commutating pole has proved to be a real necessity. In such machines, due to the reduced number of poles and high armature ampere turns per pole, and consequent large fields or fluxes set up by the armature, together with the high speed, the inherent short-circuit voltages across the brush have reached excessive values, such as 12 to 14 volts at normal load. Such e. m. f.'sr unless largely neutralized, would obviously set up exces- SYNCHRONOUS BOOSTER CONVERTERS 449 sive short-circuit currents under the brush. As a resultant short- circuit voltage under the brush of about 2 volts or less at full load is desirable, it is obvious that some such device as the commut- ating pole, which introduces an opposing e. m. f. in the short- circuited armature coils, is practically a necessity; and, further- more, this opposing e. m. f . must vary practically in proportion to the load, in order to keep within the permisssible short-circuit limits across the brush at all loads. Shifting the brushes forward into an active field to neutraUze 12 volts, for instance, is obviously impracticable, for if a sufficient opposing e.m.f., such as 10 volts, is thus introduced into the short-circuited coils at full load, then it is so large that it will give prohibitive currents at no load if the same brush lead is maintained. Therefore with such a machine of the non-commutating pole type, the brushes must be shifted with the load, which, in many cases, is not a practicable condi- tion. Consequently, the commutating pole, with its neutralizing e. m. f. varying in proportion to the load, is a necessary device with such machines. In the rotary converter, however, the conditions are not so severe. On account of the alternating and direct currents in the armature winding opposing each other, the resultant armature magnetizing effect is very small compared with that of a corres- ponding D. C. generator. Therefore the magnetic fields set up by the armature winding are relatively much smaller, and the inherent short-circuit e. m. f.'s are also lessened. Therefore, the speed, current, nimiber of poles, etc., being equal, the rotary converter would naturally have a materially lower inherent brush short- circuit .e. m. f. than the D. C. generator. In many cases this e. m. f. may be within the permissible limits of the 6 to 8 volts, when the brushes are to be given a fixed lead, while the corres- ponding D. C. generator might have 10 to 12 volts, which cannot be sufficiently corrected by a fixed lead. Therefore, the addition of the commutating pole to the rotary converter usually will not represent the same gain or improvement as in the D. C. generator, and its use, in some cases, is more in the nature of a refinement of operation than an absolute necessity. It may be suggested that, by the use of commutating poles, the inherent short-circuit e. m. f . naight be made higher, or given the same values as in D. C. gen- erators, with a consequent gain in cost of the machine, due to the use of higher speeds or a reduced amount of material. There might be some saving, with such a procedure, but, on the other 450 ELECTRICAL ENGINEERING PAPERS hand, there are certain operating conditions in commutating pole rotaries, not encountered in D. C. generators, which make it inadvisable, in many cases, to work at as high commutating limits as on commutating pole D. C. machines. In D. C. gener- ators the armature has a definite magnetizing action, depending upon the current carried, and this magnetizing action is always of the same value for the same armature current, regardless of speed, voltage, or any other condition. The function of the commut- ating pole winding is to overcome or neutralize this armature magnetizing effect at the point where the armature coils are short-circuited, and in addition, to set up a magnetic field in the opposite direction to that which the armature winding will tend to establish. A positive relation is thus estabHshed which is practically unaffected by conditions of operation. In the rotary converter, however, the conditions are somewhat different. As the resultant armature ampere turns are normally very small, the commutating pole ampere turns required are cor- respondingly reduced, and. have a much smaller value than on a corresponding D. C. machine. If the resultant armature ampere turns always held a definite value, for a given direct current delivered, under all conditions of operation, then the commutating pole winding could readily be given the necessary proportions for setting up the desired commutating - field. But the resultant armature ampere turns in the rotary can vary over a considerable range, while delivering a direct current of practically constant value, and consequently with a constant commutating pole strength. Obviously, with a constant commutating pole strength and a resultant armature magnetizing effect which can vary over a considerable range, the resultant short-circuit e. m. f. can also vary up or down, while commutating a given current, and, if the variation is excessive, bad commutating conditions will result. As the average value of the resultant ampere turns of the rotary converter armature is only about 15 percent of that of the same armature as a D. C. machine, it is obvious that a relatively small unbalancing of the opposing alternating and direct currents may give a great increase in the resultant ampere turns, which may greatly disturb the commutating pole conditions and set up relatively large resultant brush short-circuit e. m. f.'s. As such disturbances can actually occur in rotary converters from several causes, it is usually advisable to make the inherent short-circuit e. m. f as small as possible, without undue sacrifice in SYNCHRONOUS BOOSTER CONVERTERS 451 the design of the machine. One condition which can produce the above unbalancing between the alternating and direct currents is "hunting." When a rotary hunts it alternately stores energy in the rotating parts and returns it to the system, during which the speed of the rotary oscillates with respect to the frequency of the supply system. While storing energy in the moving parts the alternating-current in-put is higher in value and, in restoring power to the line, is lower in value than is required for the average D. C. output. In consequence, where hunting occurs, the result- ant armature ampere turns periodically vary in value and there is a corresponding periodic short-circuit voltage across the brush which may reach excessive values and cause vicious sparking, or even flashing. Another cause of variation in armature reaction is found in sudden changes of load on a rotary converter. When a sudden load is thrown on, the rotary may momentarily carry part of its load as a D. C. generator. This means disturbance of the com- mutating field, in the wrong direction, at the very moment that this field should be at its best. But by avoiding too high normal short-circuit voltages in the armature winding, the above condi- tions of undue voltages across the brush can be relatively lessened. In rotaries with "self-contained" regulation, another dis- turbance is introduced, which will be described later. RELATION OF SPEED TO CURRENT CAPACITY, ETC. In the design of all rotating machines for transformation pur- poses, as high speeds should be chosen as conditions of economical design will allow. In D. G. generators, the speeds and the nimiber of poles have no rigid relation to each other. Thus, a 1000 kw, 500 r. p. m. generator could have from 4 to 12 poles, as desired. For 600 volts, and corresponding currents, it could have 6 poles, for instance. For half this voltage, with twice current, it could have 12 poles, with the same speed. There is therefore a certain freedom in the design of such a machine. In the rotary converter, however, the above condition is absent. The frequency is fixed, which at once fixes the relation of the number of poles to the revolutions per minute, for the frequency is the product of the two. Therefore, if a 600 volt, 1000 kw 25 cycle rotary converter would require 6 poles at 500 revolu- tions, then a machine with half this voltage and twice the current and with 12 poles, must operate at 250 revolutions, and not 500. 452 ELECTRICAL ENGINEERING PAPERS In rotaries where the current per brush arm, and per pole, is at the highest permissible hmit, the ntimber of poles must vary directly and the speed inversely, as the total current to be handled. Thus, for example a 270 volt rotary of large capacity will inherently have more poles, and will run at a lower speed, than a 600 volt rotary of equal capacity, which is not necessarily the case with D. C. generators. The minimtim number of poles in either a rotary converter or a D. C. generator is practically fixed by the direct current to be handled. There is a practical limit to the current per brush arm, as fixed by the permissible current density in the brushes and the permissible breadth of the commutator face. There are physical conditions which limit the breadth of the commutator face, de- pending upon the speed, expansion conditions under temperature, etc. The maximum breadth being determined for any given case, the circumferential thickness of the brushes being fixed by limits of inherent short-circuit e. m. f., and. the current density in the brushes being fixed by limits of brush and commutator wear, as before described, it follows that the maximum current per brush arm is pretty definitely fixed, with present constructions. For a given total output in ourent, the limiting current per brush arm thus fixes the total number of brush arms and poles, and thus fixes the speed for a given frequency. These Umiting conditions are pretty closely approached in recent 25 cycle rotaries of the com- mutating pole type. LIMITING CURRENT PER BRUSH ARM As indicated above, this is a function of the length of the commutator, which depends, to some extent, upon the peripheral speed of the commutator face. With 25 cycle rotaries, consider- ably lower peripheral speeds are obtainable than with 60 cycle rotaries, without imdiily decreasing the distance between adjacent brush arms or neutral points. The peripheral speed, in feet per minute, of any commutator is equal to the distance in feet between two adjacent neutral points, multiplied by the frequency in alter- nations per minute; thus, with 25 cycles per second (or 3000 alternations per minute) with one foot, or 12", between adjacent neutral points, the commutator peripheral speed wUl be 3000 ft. per minute. With 60 cycles per second (7200 alternations per minute) with 8", or 2-3 ft. between adjacent neutral points, the peripheral speed of the commutator wUl be two-thirds of 7200 = SYNCHRONOUS BOOSTER CONVERTERS 453 4800 ft. per minute. Or, in other words, with equal peripheral speeds, the 25 cycle rotary can have 2.4 times as great distance between neutral points as a 60 cycle machine. The above relation of commutator speed to frequency holds true regardless of the number of poles. It therefore follows that, as the 25 cycle ma- chine can have much lower peripheral speed at the commutator, the difficulties of building the commutators should be very much less. It shotdd therefore be practicable to build much wider commutators for 25 cycle rotaries than for 60 cycle, and experience bears this out. With the wider commutators, at 25 cycles, the brush bearing surface is increased, and thus with a given width of brush, the number of brushes per arm can be correspondingly greater than for 60 cycles. In the second place, even with considerably lower peripheral speeds at the commutator, the thickness of the commutator bars will be considerably greater, in most cases, than can be used on 60 cycle machines of the same rated voltage. In consequence, with a given thickness of brush, fewer bars will be short-circuited on the 25 cycle machine, than on the 60 cycle, and therefore, in general, somewhat thicker brushes are permissible for given inherent brush short-circmt limits. This, again, allows more current per brush, so that the 25 cycle machine has an advantage in total current per arm, due to the thickness of brushes, and to the number of brushes which can be used per arm. On the basis of a brush ^" thick, and a current density of 50 amperes per square inch, experience shows that a normal rated current of about 1000 amperes per brush arm is possible on large 25 cycle rotaries which are designed to carry heavy overloads for moderate, periods, such as two hours. With such brush thickness, these rotaries can be designed for moderately low inherent short-circuit voltages and abnormal refineinent in proportioning of the commutating pole dimensions is not required, as extremely close adjustment of the resultant short-circuit voltage is unnecessary. With thicker brushes, such as 1" instead of ^", it is possible to operate at somewhat higher current per arm, possibly up to 1200 amperes, but this is at a certain expense in higher inherent short-circuit e. m. f.'s and less all-around margin in general. With the thicker brush there is necessarily a greater tendency for local currents, and therefore closer proportioning of the commutating poles is required. However, with equally careful proportioning, with the ^" thickness of brush, the resvdts would be relatively better also. 454 ■ ELECTRICAL ENGINEERING PAPERS One of the possible troubles with very heavy currents per brush arm, lies in the difficulty of obtaining equal division of current among all the various brushes per arm. The possibility of trouble is apparently considerably increased, the greater the current per arm, and if this greater current per brush arm is ob- tained by the use of thicker brushes rather than by greater length of commutator, then the result is practically equivalent to working the machine harder, or nearer the limit. If the operation of two commutators be compared, one with a ^^" thickness of brush and the other with a 1" brush, both having such brush capacity that they are worked at equal apparent current densities, then, other conditions being equal, the commutator with the ^" brush will be found in general to give superior results. And ex- perience has shown that in many cases the 1" brush can have its width cut down to %" width, mth apparent improvement in operation. However, if both the %" and 1" brush actually show the same true current density; that is, including all local currents and unbalancing of current between brushes, then with equally well proportioned commutating poles, there should be but little difference in the operation with the two thicknesses of brushes. Assuming 1000 amperes as representing the limiting current per arm with %" brushes on 25 cycle machines, then on 60 cycle rotaries, which usually have brushes of less than ^" thickness, and considerably narrower commutators on account of higher peripheral speeds, the maximum rated current per arm will be in the neighborhood of 600 amperes. This smaller current per arm should apparently handicap the 60 cycle machine compared with the 25 cycle, but, in compensation, on the basis of equal revolu- tions per minute, a 60 cycle rotary will have 2.4 times as many brush arms, which more than makes up for the lower current per arm. Therefore, from this standpoint it should be feasible to operate the 60 cycle rotary at considerably higher speed than the 25 cycle. This, however, has not been carried to the limit, in present practice, as the speeds which would be obtained would be so high, in some cases, that present commercial conditions will not allow them. This means, therefore, that we have probably not yet reached the possible maximum speeds which are practicable with 60 cycles. SYNCHRONOUS BOOSTER CONVERTERS 455 E. M. F. REGULATION OF ROTARY CONVERTERS There are three well-known methods for varying the D. C. e. m. f. of rotary converters, with a fixed A. C. supply voltage. These three are known as the induction regulator, the syn- chronous booster, and the regulating-pole methods of control. In the induction regulator method, an induction regulator varies the A. C. voltage up or down over the range necessary to give the desired D. C. voltage change. In the synchronous booster method, an A. C. generator of a capacity corresponding to half the range of control is operated synchronously with the rotary converter and, by means of direct-current field control of this booster, the A. C. e. m. f. supplied to the rotary is varied up or down. In the third method each main pole of the rotary proper is made up of two or more smaller poles, one or more of which may have the excitation varied and by this means the ratio of the D. C. to the A. C. e. m. f ., in the rotary converter armature itself, may be changed. Each of these three methods has certain possibilities, advan- tages, and disadvantages, depending upon the conditions of oper- ation. The induction regulator method has been used very con- siderably in the past, but is but little advocated, in more recent work, due probably to the fact that it is more complicated and expensive than other methods. Both the synchronous booster and the regulating pole methods of voltage reguladon have been used more or less extensively, however, principally with- out commutating poles. With the introduction of the latter, a new problem enters, which has a very considerable bearing on the design of such apparatus, especially in machines of very large current capacity where the maximum permissible current per brush arm is approximated. This problem lies in the variable armature magnetizing force of the rotary, with change in D. C. e. m. f., while delivering a given current. Obviously, if the resultant armature ampere turns vary, the commutating pole ampere turns should vary a corresponding amount. But if the commutating pole winding is in series with the direct-current armature current, which may not be varied with change in voltage, the desired conditions are not met by such an arrangement. In this lies the real problem. In the rotary converter with synchronous booster, but with- . out commutating poles, the difficulty of variable armature reaction 456 ELECTRICAL ENGINEERING PAPERS such as indicated above, exists also, but is usually not serious, as indicated by the following : In a rotary converter without synchronous booster or regulat- ing poles, the ratio of the alternating current to the direct current is in normal operation pretty definitely fixed. The two currents oppose each other in the armature winding to such an extent that the resultant ampere ttims vary between about 7 percent and 22 percent of the value in a D. C. machine, or with a mean of about 17 percent, when a ftill pitch armature winding is used. When a "fractional pitch" or "chorded" winding is used, this value is reduced, depending upon the amount of chording. This small resultant acts in the same direction as on a D. C. machine, and sets up a small field which affects the commutation slightly. Any- thing which will increase the ratio of the alternating-current in-put to the direct ctirrent will tend to reduce the resultant armature ampere turns, for normally the D. C. effect is slightly in excess. Therefore, if the rotary should act, to a certain extent, as a motor, thus receiving some A. C. in-put which is not trans- formed to direct current, the restiltant armature ampere ttuns will be reduced, and may even pass the zero value and be in the op- posite direction. Again, if the rotary converter armature transforms some mechanical power received at its shaft, into direct current, so that the direct-current output is correspondingly greater than the A. C. input, then the resultant armature ampere turns will be increased. In the synchronous booster method of regulation, the above is just what happens. The normal A. C. e. m. f . corresponds to the midway point on the D. C. e. m. f. range. When the booster neither "boosts" nor "bucks," the alternating current suppHed corresponds properly to the direct current delivered, and the resultant armature ampere turns have a mean value of 17 percent approximately, assimiing a fvill pitch winding. If the D. C. e. m. f . is boosted 15 percent, for example, the A. C. supply e. m. f. re- maining constant, then obviously the current supplied to the alternating end is increased with respect to the current delivered by the D. C. end, in the ratio of the percentage boost. Therefore, the normal restiltant armature ampere turns are reduced to 17 — 15 = 2 percent. Again, when the D. C. e. m. f. is reduced 15 percent, the direct current is increased 15 percent relatively to the A. C. and the resultant armature amperes are increased 15 percent, and SYNCHRONOUS BOOSTER CONVERTERS 457 beccjme 17 -|- 15 = 32 percent. Therefore, with a boost and buck of 15 percent voltage, while carrying the same direct-current load, the resultant armature reaction would be varied from 2 percent to 32 percent of that of a D. C. armature. This, however, is not serious in a rotary converter without conunutating poles, as even with 32 percent armature reaction, the conditions are much better than in a D. C. machine where the armature reaction is 100 percent. But when commutating poles are introduced the conditions are quite different. The commutating pole winding normally should be equal to the effective or resultant armature ampere turns, plus the magnetizing ampere turns for setting up the required magnetic field under the commutating poles. This latter com- ponent usually is small. Counting the effective armature ampere turns as 17 percent of that of a D. C. armature, and assuming the magnetizing component as 25 percent, then normally the total commutating pole turns would be 42 percent. If this 42 percent is fvimished by series excitation from the D." C. end of the rotary, then it wiU be constant in value, with a constant value of the direct current, regardless of the variations in the D. C. e. m. f. Now, suppose the D. C. voltage is boosted 15 percent by means of a synchronous booster, then the resultant armature ampere turns faU to 2 percent, as shown before, and, the commut- ating pole ampere turns remaining at 42 percent, the difference, which is 40 percent, will all become magnetizing. Therefore, with a boost of 15 percent, the magnetizing component of the com- mutating pole winding is increased from 25 percent to 40 percent, although the current to be commutated is unchanged. In the same way, if the D. C. voltage is bucked 15 percent, then the armature ampere ttuns become 32 percent and the magnetizing component of the commutating pole field winding becomes 42 — 32 = 10 percent, when it shotild be 25 percent. Therefore, the commutating field strength actually varies up or down 60 percent from the reqtured value, due to the synchronous booster action, when, in reality, it should remain constant. If a resultant short-circuit e. m. f . of 3 volts across the brushes were allowed, then, this 60 percent variation in the commutating pole strength, would mean that the inherent short-circuit e. m. f. is only 5 volts, which is normally neutralized by the commutating field.. However, an inherent short-circuit e. m. f. of 5 volts is so low that it would require a rather difficult and expensive design, and therefore seven to 8 volts inherent short-circuit e. m. f . shoidd 458 ELECTRICAL ENGINEERING PAPERS be considered in most cases. Obviously, with the above conditions of variable armature reaction, this would lead to vicious sparking' conditions, especially at heavy overload, or at no-load conditions. Therefore, series excitation of the commutating pole by the. direct current delivered, should not give satisfactory results. What is needed is a variation in the commutating pole excitation in accord- ance with any changes in the armature reaction of the rotary; that is, a reduced excitation at boost and increased excitation at buck. Looking at the variable elements, it may be seen that the field cun-ent of the synchronous booster had its current in one direction at boost and the reverse direction at buck. Herein would appear to be a solution of the problem, by putting the booster field current through an auxiliary winding on the commut- ating pole, so that it opposes the series commutating pole coil at boost and adds to it at buck. At first thought, this seems to fit the conditions perfectly, and, in fact, it does, at one definite direct current delivered, but does not do it perfectly at other loads. This is shown by the following figures. Assume the preceding value of 42 percent series ampere turns on the commutating pole, with an additional auxiliary winding having the same percent ampere turns at full load as the percentage boost or buck. For example, with 15 percent boost, then at full load the auxiliary winding has 15 percent ampere turns, which are in opposition to the 42 percent series turns, while at IS percent buck, at full load, the 15 percent auxiliary winding acts with the 42 percent series. With 15 percent boost at full load, the armature reaction is lessened by 15 percent, and the total commutating field excitation is also reduced 15 percent by means of the auxiliary winding. Thus the resultant magnetizing component of the field winding remains at the required 25 percent. At no boost or buck, where there is no current in the booster field and auxiliary commutat- ing pole circuit, the resultant magnetizing component of the com- mutating field winding remains at 25 percent, as explained before. When the booster field is reversed, in order to buck the A. C. voltage, the auxiliary field ampere turns on the commutating pole also are reversed, and at 15 percent buck they add 15 percent to the series commutating pole winding, and thus give an effective mag- netizing value of 25 percent instead of 10 percent, as given before. Hence, with this arrangement, the resultant commutating field strength is correct for all the voltages, at the assumed full load current. SYNCHRONOUS BOOSTER CONVERTERS 459 Considering, next, the half -load condition, then the armature ampere turns, both A. C. and D. C. are halved and the resultant armature reaction is also halved. However, for the same percent- age boost or buck in D. C. voltage, the synchronous booster must operate over the same voltage range as at full load, and therefore, if the booster field current is the same for the same voltage range, regardless of load, then the auxiliary winding on the commutating pole adds or subtracts 15 percent, when, for correct commutating field conditions, it should add or subtract only 7}/^ percent. Therefore, the excess field strength at the two extremes of voltage is 7 3/^ percent, or 30 percent of the normal full load magnetizing component of the commutating pole winding of 25 percent, which was assumed as that required to neutralize the assumed inherent brush short-circuit e. m. f. of 8 volts. A 30 percent component of this would mean 2.4 resultant volts across the brush. Prac- tically the same condition would also be found at 50 percent over- load. This apparently would not be a prohibitive condition if it represented the full range of operation. At no-load, however, the excess effect of the auxiliary winding would be 15 percent instead of 7}/2 percent, giving a magnetizing component equal to 60 percent of the normal magnetizing effect of the commutating pole winding, or 4.8 resultant volts across the brush, which is higher than desirable. The above arrangement therefore fails for extreme changes in load, if the synchronous booster excitation is constant for a given percentage boost or buck, independent of the load on the rotary. What is required with this scheme is an excitation of the synchronous booster, which, for the same range of voltage variation, increases and decreases with the load on the rotary. If, for instance, the 15 percent boost or buck could be obtained at no-load on the rotary, with one-half the field excitation that would be required for full load, then the excess ampere turns in^the auxiliary winding on the commutating pole would be only 7}/^ percent total, at no load, instead of the 15 percent indicated above, and the resultant short-circuit e. m. f. across the brush at no-load would be 2.4 volts, which is entirely practicable. From the above analysis, the solution of this problem is in- dicated. It lies in giving the synchronous booster such characteristics that its field current varies greatly with change in the load on the machine. This can be done in various ways, but most readily by designing the synchronous booster with relatively high ampere turns on its armature compared with its field ampere 460 ELECTRICAL E.YGIXEERIXG PAPERS turns, which is the ver\- cunslruclion needed for making the most efficient and least expcnsi\-c b(joster. In sucli a booster, with very high armature reaction, the field current can be made to \'ary over a relatively wide range, with a given percentage boost or buck, with an}- consideralilc changes in the armature current. With this construction therefore, it is practicable to build a synchronous booster type of rotan* conA'crter with commutating poles which will automatically adjust its commutating pole exciting conditions to suit changes in both load and voltage, and thus there is no occasion to revert to the induction regulator, or other outside means of control. As a proof of the correctness of the aV)0\-e principles, may be cited the largest capacity synchronous bocistcr, commutating pole rotarv converters vet built, nameh- those reccnth' furnished to the FIG. 4. XEW YORK EDISON 3500 Kw. SYNXHRON'OUS BOOSTER COMMU- TATING POLE ROT.\RIES. New York Edison Company, one of which is shown in Fig. 4. These machines have a nomial continuous rating of 3500 kw at 270 volts, and 13,000 amperes D. C. They must also carrj' 50 SYNCHRONOUS BOOSTER CONVERTERS 461 percent higher current for two hours, or 19,000 amperes at 270 volts. In addition, by means of their synchronous boosters, they can vary the voltage from 270 up to 310 of down to 230, while still carrying the rated ctirrent. As these are the most remarkable machines of this type yet constructed, a more complete decrip- tion of them will be in order. The contract included five machines of 3500 lew, of the horizontal shaft type, and two machines of 3000 kw of the vertical shaft type, these latter to fit existing foundation plans. Each 3500 kw machine has a normal rating of 270 volts D. C. at 13,000 amperes, and is arranged to boost and buck approximately 15 percent. The synchronous booster therefore has a normal capacity of about 525 kw. The A. C. end of the rotary is arranged for 6-phase double-delta connection, requiring about 165 volts normal. The alternating current handled by each of the six collector rings is enormous, being approximately 6300 amperes. As the rotary has 28 poles, the current from each collector ring is carried by 14 leads, through the armature windings of the booster, to the rotary converter armattire, where it divides into 28 paths, or one per pole, in the usual manner. The normal alternating ciorrent per armature circuit in the booster thus becomes 450 amperes, and in the rotary it is 225 amperes. As the machine has 6 collector rings, with 14 leads per ring, there are 84 windings on the synchronous booster. Each winding, however, simply consists of a single group of coils. As the booster has 28 poles, the same as the rotary itself, it has therefore three groups of coils per pole, the same as an ordinary three-phase generator. The booster armature is therefore simply an ordinary type of three-phase generator, except that the various groups of coils in each phase of the armature are not connected in series, but are in reality con- nected in parallel at the collector rings and at the main armature winding. This arrangement of the booster armature between the collector rings and the rotary converter armature thus presents a relatively simple arrangement, and- tends toward compactmess and symmetry in the complete armature unit, as shown in Figi 5. The brushes on the collector rings are of a metal-carbon type, arranged in box-holder's somewhat like ordinary carbon brushes. The type of metal-carbon brushes used has a very low contact drop under normal operation, being approximately 1-10 that of ordinary carbon brushes. The total number of brushes per ring is 20, and each brush has a section of 2.15 square inches, 462 ELECTRICAL ENGINEERING PAPERS thus giving a normal current density of 147 amperes per square inch. On the direct-current end there are 28 brush arms, giving a normal rated current per brush arm of 930 amperes, approximately, and, for the two hours overload, of 1400 amperes approximately. There are 15 brushes per arm, each of %" x IM" section, thus giving an apparent current density of 473^ amperes per square inch. The armature winding of this rotary converter is thoroughly cross connected in order to equalize the circuits, — a point of very considerable importance in commutating pole machines. The field -poles are also equipped with heavy, well distributed copper dampers in order to destroy any tendency to hunt, which is a very important condition in commutating pole rotaries, as previously explained. V The air gap tmder each main field pole is one-half inch. The use of this large gap naturally lessens any tendency for magnetic noises. As the brushes are of a lubricating type, and as the brush holders have special devices for adjusting the brush tension very accurately, the machines run very quietly. The commutator mica is undercut about 1-32 inch. As these rotaries are equipped with both synchronous boosters and commutating poles, very careful designing as regards commut- ation characteristics, had to be done. The variable armature reaction, for all the various conditions of load and boost and buck of the D. C. e. m. f ., were carefully calculated, and the commutat- ing field proportions for correcting these reactions were deter- mined. In the analysis and example previously given, showing what conditions of inherent short-circuit e. m. f., etc., could be allowed, and stiU obtain permissible results, and armature reaction of 1 7 percent under normal conditions and a magnetizing compo- nent of commutating pole strength of 25 percent were assumed, giving a total of 42 percent. It was shown that, with a suitable auxiliary winding on the commutating pole, satisfactory conditions could be obtained from no-load to 50 percent overload, with 15 per cent boost or buck, with an inherent brush short-circuit e. m. f . as high as 8 volts. But in these 3500 kw New York Edison rna- chines, by very careful analysis of the conditions of commutation, the inherent short-circuit e. m. f . at full load was gotten down to 6.3 volts instead of 8, while the average armature reaction was naade as low as 11 3/2 percent, instead of 17 percent, both of which SYNCIlROyOl'S BOOSTER CONVERTERS 463 conditions arc very favorable, compared with the' tVmncr assumed permissible limits. The normal or series conimutatin<^' field ampere turns are 39 percent, instead of 42 percent, sc) that the ma.t;netizing component is 27^ o percent, the other 11,^ 2 percent simi.)ly opposing the nomial armature reaction. This large magnetizing component is obtained by the use of a •''4" air gap under each commutating FIG. ,s. pole. Such a large ga].), in itself, is of direct assistance in obtaining the desired distribution of the cnmmutatmg field flux, and thus makes the design problem some\\-hat easier. A brief description of some of the imusual features of these machines may be of interest. The rotary converter main frame or field is (.)f cast steel, in order to reduce somewhat the OA'crall dimensions, and keep inside the customer's requirements. The synchronous booster field frame is of cast iron. Both the main field and the booster have bolted-in laminated poles. 464 ELECTRICAL ENGINEERING PAPERS The commutator, main armattire, booster armature, and col- lector rings are each assembled on separate spiders. The com- niutator spider, however, is pressed on the hub of the main ar- mature spider, so that the shaft can be removed for shipment, without disturbing the connection of the armature winding to the commutator. The commutator is of the through-bolt construction with heavy steel "V-ringg." These rings are of large section in order to avoid distortion under the heavy clamping strains to which they are normally subjected. The commutator bars are designed to give the same deflection at all points. The commutator diameter is 120", and the width of exposed face is 30". The two 3000 kw vertical units for the same company are of practically the same general design as the 3500 kw except that they are somewhat smaller, and operate at higher speed. They have 22 poles, and the normal current per brush arm is 1010 amperes^ compared with 930 on the 3500 kw. An extensive series of tests were made on both the 3500 kw and the 3000 kw, some of the results of which are as foUows : Armature iron loss at 270 volts, 3500 kw, 16.3 kw. 3000 kw, 13.8 kw. That is, the normal armature iron loss in both sets is less than 0.47 of one percent,' — a remarkably low figure. The booster aramttue of the 3500 kw unit showed an iron loss at 15 percent boost or buck, of 4.6 kw, or less than 0.9 of 1 percent of its own rating, which is only 15 percent of that of the unit. At 310 volts D. C, the 3500 kw unit showed 20.1 kw iron loss, and at 230 volts, 8.8 kw. The total iron losses, including the booster armature, thus varies between 13.4 kw and 24.7 kw over the entire range of voltage operation, or between 0.4 and 0.7 of 1 percent of the rated capacity of the machine. Under all conditions the efficiency of the -unit showed apprec- iably higher than the guarantees, due partly to the relatively low iron loss, as given above. The following temperature test resiilts were obtained on the 3500 kw unit: Hrs. Run. Arm.Rise Comm.Rise: 10 31.5°C. 37°C. 5 21.5 29.5 6 31.5 30 Amp. V. 13000 270 12950 231 12950 317 SYNCHRONOUS BOOSTER CONVERTERS 465 On account of lack of certain facilities, a 50 percent overload temperature test was not made on this vinit, but this was carried out on the 3000 kw unit, as shown in the following resiilts of tests : Amp. V. Hrs. Run. Arm. Rise Comm.Ri *11100 270 13 17.5°C. 28.5°C *16650 270 2 42 45.4 11100 232 8 16.5 36 11000 312.5 8 27.5 30.5 In these temperattire tests, the first run was made, in each case, for a period long enough to reach constant temperature. The other tests followed, while the machines were hot, so that steady temperatture conditions were reached in a shorter time. In the commutation tests, the results were equally satisfac- tory. At 270 volts the 3500 kw machine was tested from no-load up to 19,300 amperes; also, at 310 volts from no-load up to 14,000 amperes; and at 230 volts from no-load to full load; and at 257 volts, up to 15,550 amperes. Under all these conditions the com- mutation was remarkably good, and this may therefore be taken as evidence of the correctness of the principles given in the earlier part of this paper. Furthermore, as an illustration of the accuracy that is possible in the design of such apparatus, when the fvmda- mental principles are sufficiently well known, it may be stated that, in the case of this 3500 kw imit, all drawings were made up, and aU the above shop tests made on the completed machine, without any changes whatever, in the electrical or magnetic design, from the original engineering design specification. Also, on shop test, absolutely no re-adjustments were necessary in any of those parts where provision is usually made for such adjustment by reason of possible sHght variations in material or workmanship, or inabiHty of the designer to predetermine certain characteristics with sufficient accuracy. ♦These runs were duplicated. The results given are the highest rises obtained from either test. 466 ELECTRICAL ENGINEERING PAPERS SIXTY-CYCLE ROTARY CONVERTER FOREWORD — This paper was prepared for the twenty-ninth annual convention of the Association of Edison Illuminating Companies at Cooperstown, N. Y., September, 1913. At the time it was presented, the subject of 60 cycle rotaries was becoming a very ' 'live" one, as improvements in this type of machine were bringing it very rapidly to the front as a competitor of the 25 cycle rotaries. — (Ed.) ONE of the most significant developments in the past year has been the greatly increased purchase of large 60 cycle rotaries by central station plants. Here is an example of a type of ma- chine which has been more or less discredited in the past, but which, all at once, is coming prominently to the fore. The present machine itself is not radically different from its older forms, 'but it contains many minor improvements which, individually, do not stand out prominently, yet, collectively have served to overcome those little difficulties which formerly were just sufficient to put the machine in the questionable class. However, a number of general conditions were also involved in this improvement. It is the purpose of this paper to show wherein the new machine is superior to the older type, and also to indicate wherein a number of modifications, each in themselves of a small amount, have combined to form a relatively large improvement. Shortly after the 25 cycle rotary began to be prominent in electrical work, that is, about 15 to 18 years ago, the problem of 60 cycle rotaries was also presented, as the relatively numerous 60 cycle plants also had need of economical means for transform- ing to direct 'current. In consequence, there being a field for 60 cycle rotaries, such machines were built and installed in a number of places. These early 'machines were in some cases, fairly suc- cessful, while, in others, they were failures. Apparently ,in some of these cases of failure, the rotary itself was not entirely to blame, as it was operated under conditions which would now be considered impracticable, with our present knowledge and experience. These early 60 cycle rotaries were very greatly handicapped in design by the limitations of commercial and manufacturing practice of those days. Relatively low speeds were considered necessary from the commercial standpoint, and with 60 cycles 60 CYCLE CONVERTERS 467 this meant a large number of poles, even for relatively small out- puts. Also, manufacturing limitations called for relatively low peripheral speed of the commutators. In those days commutator speeds of much in excess of 4,000 feet per minute were considered excessive, and unduly dangerous, both from the manufacturing and operating standpoints. Herein was a handicap of the worst sort upon the design. The peripheral speed of the commutator is equal to the distance between adjacent neutral points multiplied by the number of alternations per minute (revolutions per minute X No. of poles). On this basis, 3600 feet peripheral speed with 60 Com mutating Zone w/t/) no /eacf. Fig. 1. Commutating Zone with No Lead. cycles per second (7200 alternations per minute), gave 6 inches, betw:een adjacent neutral points. Even 4200 feet peripheral speed gave only 7 inches between neutral points. It is obvious therefore that, even with this higher peripheral speed of the commutator, there was undue crowding of the brush holders. 468 ELECTRICAL ENGINEERING PAPERS which in itself was a bad feature. But the worst feature was in the fact that, with only 7 inches between commutator neutral points, the maximum permissible num.ber of commutator bars was unduly limited. Assimung, for example, a thickness of bar-plus-mica of 3-16 inches, which is very thin, the 7 inches between neutral points would aUow about 36 commutator bars between neutral points, or per pole. This ntunber was ample for 250 to 300 volt machines, but for 600 volts, experience indicated that it was on the ragged edge, especially with the field flux distributions obtained with those early machines. In consequence, 60 cycle rotaries for 250 to 300 volts, rendered a better account of themselves than the 600 volt machines, and the latter were very much inclined to flash at times, due to the small number of commutator bars, and a high maximum voltage between bars. The field fiux distribution had something to do with the questionable operating conditions. With these earlier machines, very high peripheral speeds of the armatiure core were considered objectionable, for several reasons. One was, that the construc- tions of that time did not allow very high peripheral speeds of the armature windings, and, a second reason was that, with the relatively low speeds, and consequent large number of poles, the armature dimensions and cost would have been excessive for a given output. In general, a 12 inch pole pitch was considered as large as desirable or practicable, which corresponds to 7200 feet per minute at 60 cycles per second. With this smaU pole pitch, in order to obtain a sufficiently wide commutating zone between the poles, it was necessary to make the poles relatively narrow. The use of narrow poles led into one di2ic\ilty, as regards flashing, as will be explained later, while widening the pole and narrowing the interpolar space led into another difiiculty of flashing which was equally serious. The situation can be illustrated by Figs. 1 and 2. In Fig. 1 the field flux distribution is indicated for an extreme case of a pole face as wide as 8 in. and with only 4 in. interpolar space, the total pole pitch being 12 in., and the poles without polar horns. The "field form," which indicates the flux distribution, in this case has a relatively wide top, and the proportions are such that the maximum e. m. f. between the bars is about 40 percent greater than the average e. m. f. per bar. With 36 commutator bars, for instance, at 600 volts, the average volts per bar would be 16 2-3, and the maximum voltage per bar almost 24, which, in itself, may be a safe figure if eo CYCLE CONVERTERS 469 never exceeded. However, the flux distribution in the interpolar space, as indicated by Fig. 1, is such that there is almost no width to the neutral or comtnutating zone, and therefore the brushes are short-circuiting the armature coUs in an active field, even at no load, and, in some cases, this short-circuiting action may be so great that th^e are excessive local currents in the brushes. Fur- thermore, with the neutral point so narrow, a very slight forward shiftihg of the brushes, to take care of load conditions, would place the brush in such a strong field at no-load, that there is danger of Commutat/ng Zone with no /eaef. Fig. 2. Conmiutating Zqne with No Lead. flashing when the load goes off, or changes suddenly. Therefore, with such proportions, the neutral point woiild be too narrow for reasonably safe operation. The remedy for this particular condi- tion, with these former machineis, was obviously in the use of wder interpolar spaces, and consequently narrower poles, the pole pitch being limited to about 12 inches as previously stated. 470 ELECTRICAL ENGINEERING PAPERS In Fig. 2 is illustrated the conditions with the wider inter polar space , and narrower pole face , these being taken as 5 3/^ in , and 6 3^ in . respectively, instead of 4 in. and 8 in. Obviously the flux conditions in the interpolar space are much better than in Fig. 1, and it should be possible to shift the brushes slightly for full load conditions with- out excessively bad conditions as regards sparking and flashing at no load. But the same figure also shows that the field flux distribu- tion as a whole is considerably narrower at the peak value than in Fig. 1, and therefore the ratio of the maximum value of the e. m. f . per commutator bar to the average e. m. f. is much greater. In this case, the maxium per bar is about 65 percent greater than- the average, and, with 16 2-3 average per bar, the maximum be- com.es almost 28 volts, which is in the danger zone as regards arcing between bars, except in relatively small machines. There- fore, in overcoming the sparking and flashing difficulties incident to the narrow neutral zone of Fig. 1, an equivalent difficulty is encountered, due to the narrow field distribution, or field form. Plainly, in these older machines, whichever way we turned, we were in difficulty. An obvious remedy for the above difficulties was in the use of wider pole pitches, which would allow both the commutating or neutral zone of Fig. 2, and the wider field form of Fig. 1. But in- creasing the pole pitch, with a given number of poles and given speed, means increasing the diameter of the armature, and even though the armature could thereby be narrowed, the cost of the larger diameter machine would necessarily be somewhat in- creased. The remedy for this condition was in reduction in the number of poles as the pitch was increased, thus keeping down the size of the armature for a given output. But reduction in the number of poles necessarily means higher speeds, which were formerly considered commercially objectionable, as no one had yet been educated up to high speeds. Therefore, between com- mercial-limitations, difficulties in design and manufacturing condi- tions, the 60 cycle rotary was in a bad way. Mild attempts were made from time to time to increase the speed by decreasing the number of poles, but this could only be done commercially in relatively small steps. In such increases in speed, and decrease in the number of poles, other difficulties began to be encountered, such as somewhat poorer inherent commutating characteristics, due to the higher speed and greater cvurent per brush arm to be commutated. The higher the speed was made, and therefore the 60 CYCLE CONVERTERS 471 more commercial the machine became as regards size and cost, the greater were the inherent diffictilties in the design. However, with increased experience in commutator constructions, one great advance was made by increasing the commutator speeds of the 60 cycle rotaries. Instead of approximately 7 in. between points, the distance was increased to 83^'or 9 inches for 600 volts, giving 5 1 00 to 5400 feet peripheral speeds at the commutator face. This allowed as many as 45 to 48 commutator bars per pole, which is well within the range of good direct-current 600 volt practice. This increase in the nimiber of bars reduced the average and the maxi- mum volts per bar. In this manner, one of the principal weak- nesses of the former designs was eliminated. Also, by improved mechanical design which allowed higher peripheral speed of the armature windings, the pole pitch could be increased to about 16 in., instead of 12, without an unduly large diameter of armattire for a given output. This also allowed a much better field flux distri- bution, or field form, such as in Fig. 1 , and better interpolar space of Fig. 2. In consequence, the maximum voltage per bar on the 60 cycle rotaries has been brought down well within accepted D. C. practice for 600 volt work. For lower voltage work, these limit- ations have never been so prominent, but the same steps in the development have proved advantageous in lower voltage rotaries also. With increased speed and decreased number of poles, the current per brush arm on the larger 60 cycle rotaries has gradually increased until it is practically double what it was On former ma- chines of same capacity. This higher current per arm, with the increased number of connmutator bars per pole, and the higher speeds, all tend toward making the commutation problem more difi&ctilt. But at this stage in the development, the commutating pole began to loom up as a possibility in rotary converters, and this has furnished the latest important step in the improvement of these machines. By the addition of commutating poles, still higher revolutions are permissible than formerly . While relatively high -speed rotaries of large capacity can be made without com- mutating poles, yet the addition of such poles has rendered the design less difficult and has allowed still further increases in speed, which are very welcome in the way of improving the standing of the 60 cycle rotary. With these higher speeds, and greater out- puts with a given diameter of machine, the losses have not in- creased anything like in proportion, so that the difBculties of the 472 ELECTRICAL ENGINEERING PAPERS 60 cycle rotaxies have been gradually increasing untU now they are treading on the heels of the 25 cycle. In fact, when the greater efficiency of the 60 cycle step-down transformers is taken into account, the difference between the efficiencies of 60 cycle and 25 cycle converting iinits in large capacities, is not enough to attract any particular attention. Thus the use of the commutating poles has been of advantage principally in allowing higher speeds, with consequent better characteristics in general. The modifications above described cover electrical defects principally. However, there were a number of other minor condi- tions in these earlier machines which might be considered as mechanical defects, or mechanical and electrical combined. These were found principally in the brush holder and commutator con- structions, and in the materials in the commutator. On the higher voltage machines, in which a large number of commutator bars per pole was necessary, the thickness of each bar was, and still is, very small, and thus the proportion of thickness of mica between bars to the thickness of the bars themselves is a very considerable percent. On this account, it has been difficult to obtain, in many cases, a wear or abrasion of the mica equal to the so-called copper "wear," which, in reality, is more in the nature of slow burning than actual wear from friction. No matter how perfect the commutation may be in appearance, there is always a sHght tendency to bum the face of the commutator by the cur- rent passing between the commutator and the brushes. This . burning normally may be at an extremely slow rate, but if the mica does not wear down at the same rate, the result will be that, after a time, the mica lifts the brush siuface away from contact with the copper, and thus an almost infinitestimal gap exists between the brush and the copper of the commutator. This gap then exaggerates the burning tendency, and the difficulty thus accentuates itself. The hardness and wearing quality of the mica must be such that it wiU always wear down as fast as the copper bums away, so that normal contact is maintained between the copper and the brush. Where the percentage of mica is high, and where the mica varies in hardness, as is liable to be the case in practice, it is difficult to avoid more or less tendency to high mica and consequent trouble. This trouble is also accentuated by high commutator peripheral speeds, as it is more difficult to maintain uniform contact between the brush face and the commutator. In consequence, in 60 cycle rotary converters in general, and in high 60 CYCLE CONVERTERS 473 voltages in particular, experience has shown that it is advisable to undercut the mica slightly, in order to avoid any tendency toward high mica, and also in order to be able to use brushes which contain some lubricant such as graphite. It is obvious that where any con- siderable grinding action by the brushes is necessary, to keep down the mica, such lubrication is not practicable to the same extent as where no grinding action is necessary. In consequence, on later types of 60 cycle rotaries, the commutator mica is usually tmder- cut, thus allowing good contact to be maintained, and thus reducing any resultant burning action to the minimum. The true causes of the dif&ctilty with high mica were not thoroughly ap- preciated, in the older 60 cycle rotaries, and, in consequence, in many cases, brushes of a hard, grinding character were used, with consequent increased losses and other disadvantages. Also, on some of the older machines, even with the much lower peripheral speeds than at present, the design and construction of conmiutators were not as nearly perfected as at present, and there was always more or less danger from unevenness, and other defects, which, while not showing in themselves any particularly harmful restdts, woiild very often show indirect harm by causing high mica, sparking, brush troubles, etc. Furthermore, in many of the earlier machines, the brush holders were not as rigid or as well suited for operation on high speed commutators as in present practice. In some cases, the operating characteristics of the rotary were greatly modified by simply changing the angle of inclination of the brush to the com- mutator, or the direction of inclination, or the brush pressure, etc. Brush chattering was not uncommon, and if there is any- thing which will surely cause bad commutators and commutation, it is severe chattering at the brushes, as this prevents good contact between the face of the carbon and the commutator face. On many of the earlier machines the brush holders were not arranged with due regard to harmful results from incipient arcs between bars, or in the neighborhood of the brush holders and brushes. On sudden changes in load, or partial short-circuits, or even in normal operation in those cases where the maximimi voltage between bars is unduly high, the not uncommon "ring-fire" around the commutator, due to burning of the carbon or graphite de- zosited on the mica or between bars, may develop into small arcs, with consequent vaporization of copper, the resultant vapor being a good conductor. If the brush holder or other parts are in close 474 ELECTRICAL ENGINEERING PAPERS proximity to the point where such small arcs may form, the con- • ducting vapor may bridge across from the commutator face to the adjacent parts, where there is any considerable difference of potential between them, and may develop real arcs or flashes which are of a destructive nature, possibly necessitaing the shut- down of the machine until the commutator can be smoothed up. In many of the older machines, with their very small distances between brush holders, and their generally crowded conditions, and their voltages per bar, such arcs were much more liable to occur than in the modern machines. In the development of the 60 cycle rotary converter, there were other conditions beside commutation, flashing, etc., which had to be taken into account. The rotary converter is a synchronous machine, and must follow rigidly in step with its source of e. m. f. supply, or there will be difficulties in the operation. The early rotaries, in many cases, were operated from generators driven by slow speed reciprocating engines, which did not run at uniform rotative speed, there being pronounced periodic speed fluctuations during each revolution. In some cases this condition was so bad that the generators in the power house would not operate decently in parallel. As the engines and generators varied in speed period- ically, obviously the frequency of the electric circuit varied to the same extent, and any synchronous apparatus operated on such system would also have to vary in speed to the same extent, if the conditions were such that the machine should follow the supply system, as is the case in rotary converters. If the rotary did not follow rigidly, it would periodically either ' ' under-run " or " over- run." This action is called hunting, and it was very serious at some of the early plants. Not infrequently the generators at the power house would not hold a rigid relation to each other, and hunted badly. There are causes of hunting, other than variations in speed of the prime mover on generating unit, but usually these have been of secondary importance, and will not be considered further. The action of hunting of the rotary, with variations in speed of the generator, may be explained briefly, as follows: The rotary gen- erates an alternating e. m. f . wave similar to that of the generating or supply system, but in opposition, or as a counter e. m. f. If the generator momentarily runs faster, then its e. m. f. will be ahead of that of the rotary. A motor current flows, tending to raise the speed of the rotary to that of the generator. If the 60 CYCLE CONVERTERS 475 generator now drops back in speed, its e. m. f . wave drops back, and the rotary tends to deliver current to the generating system, thus tending to slow the rotary converter speed down to that of the generator. The action in the rotary is therefore one which tends to speed it up or slow it down to follow the generator. This action of the rotary, acting alternately as a motor or as a generator, is what constitutes hunting. Usually this action of following the generator speed is not a serious one, as a relatively small current may produce the necessary accelerating or retarding action. The difficulty is that the rotary may over-run ; that is, it may speed up too much or drop back too much, and thus have an increased motor or generator action. In other words, this accelerating or retarding action may exaggerate the swinging effect, just as in the case of a swinging pendulum, where a very slight force, if timed just right, may gradually increase the swing of the pendulum. In those cases in the early rotaries where hunting was most severe, the periodic speed changes in the generating system were usually timed just right to cause the rotary to over-run, and thus exag- gerate the hunting action. The direct result of this hunting was visible in bad operation at the commutator. In the normal rotary converter, when run- ning properly in synchronism, there is practically no armature reaction in the armature winding, such as is found in direct-cur- rent machines, for the alternating current supplied to the armature winding is in opposition to, and practically neutralizes, the magnet- izing effect due to the direct current delivered. Therefore, as far as reactions on the field are concerned, the rotary is quite different from a direct current machine, and, at full-load, the armature has very little more effect on the field than at no-load. However, when the rotary is hunting, the current due to the hunting action above described is not balanced by the direct current delivered, so that this current acts like that in a straight A. C. or D. C. machine, and sets up magnetic fluxes in the interpolar space, and under the edges of the poles, which are harmful in character. These fluxes create bad commutating conditions by reason of the armature coils under the brushes being short circuited in a periodically varying magnetic field, which is not the case when the rotary is not hunting. Therefore, as a rotary hunts, there is usually periodic sparking at the brashes, which is in time with the periodic "beat" which usually can be heard in a machine when it hunts. This sparking will get more and more severe as the rotary hunts more, 476 ELECTRICAL ENGINEERING PAPERS until it may become so bad that the machine flashes over. This hunting in some of the early machines was a very puzzling phen- omena, and it was not until its nature and cause were determined that an effective remedy was applied. The corrective now uni- versally applied consists in the use of copper dampers, or "cage windings," in the field pole faces of the rotaries. It is not within the province of this paper to explain the action of these dampers, but it may simply be said that they exert, to a certain extent, a. braking action on the over-running action of the rotary, and also they damp out the field distortions due to hunting, such distortions materially exaggerating the hiinting action. The dampers thus reduce one sottrce of accentuation of the hunting, and exert a braking action to overcome the effects of the others. Such damp- ers were used early on 60 cycle rotaries, but in comparatively crude forms. Moreover, the angular variations in speed with 60 cycle generating units, were usually greater, in degrees per electrical cycle, than in 25 cycle machines, due to the much larger number of poles, and this made the hunting tendencies of the rotaries much greater, and the damping problem correspondingly more difficult than in 25 cycle rotaries. In consequence, 60 cycle rotaries should have had more damping action than 25 cycle machines, while, on the contrary, they actually had much less. The 60 cycle rotary was therefore considered a much more delicate machine as regards hunting, than its 25 cycle brother, and yet the fatilt was really in the generating plant in many cases. The advent of the later 60 cycle ttorbo generating plants have been a large item in the successful development of the later type of 60 cycle rotaries. The problem of angular variation in speed of the prime mover has disappeared, and therefore the dampers on modem 60 cycle rotaries have to take care of only those secondary causes of himting, which were present in the old conditions, but were masked by the much greater cause in the generating condi- tions. Also, with the newer high speed rotaries, with their relatively wider poles, it is practicable to add many more damper bars per pole than in the older machines, and, in fact, with the later machines, the problem of hunting is rarely encountered. However, a new problem in connection with hunting has come up in connection with the advent of the commutating pole, both in 25 and 60 cycles. In the commutating pole generator, the ampere turns in each commutating pole coil is sufficient to not only neutralize the entire magnetizing force of the armature winding 60 CYCLE CONVERTERS 477 per pole, but also to furnish an excess flux for commutating. In the commutating pole rotary, there is normally but little resultant magnetizing effect in the armature winding, due to the A. C. and D. C. ciirrents being normally in opposition, and therefore the commutating pole winding must only be strong enough to neutralize the very small resultant armature reaction, and give, in addition, a magnetic flux sufflcient for commutation. In consequence, the ampere turns on the commutating pole winding may be only 30 percent to 40 percent of the total armature ampere turns, con- sidered as in an A. C. or D. C. machine, whereas, in a D. C. gen- erator, the commutating pole winding is usually at least 125 percent of the armature ampere turns. Therefore, the rotary converter with its 30 percent to 40 percent commutating pole ampere turns, instead of 125 percent, cannot act as a generator or motor with good commutation, as its commutating pole strength is then much less than required. As a generator or motor, the armature reaction may not only over-power the commutating pole winding, but may set up a strong magnetic flux in the wrong direction. The commutating conditions may thus become much worse than if the commutating pole were absent. Therefore, the commutating pole rotary converter, when acting as a generator or motor, is a much worse machine than if the commutating pole itself were omitted. Herein lies a source of possible trouble with commutating pole rotaries. In case there is hunting, the arma- ture will act alternately as a generator and motor, and, under such conditions, the magnetizing force of the armature may be such that it will demagnetize the commutating pole, or even reverse the fltix under it, so that the machine acts in the same way as if it were operating with the current reversed in the commutating pole winding, which would obviously give very bad commutating conditions. Therefore, when the commutating pole rotary h\mts, it represents a worse condition than when a non-commutating pole machine hunts to an equal extent. In consequence, with commutating pole machines, it is very important to suppress any hunting tendency, and this, in general, requires somewhat better damping conditions than in the non-commutating pole machine. Therefore, although improved conditions of generation, etc., have eased up on the damper requirements, yet the necessities of the commutating poles have made the damper requirements more rigid. In some cases, this has led to a very curious situation. It is well known that the commutating pole, whether on a direct-current 478 ELECTRICAL ENGINEERING PAPERS machine or on a rotary, should not have any closed conducting circuit around it, as such closed circuit acts as a secondary or op- posing circuit in case of sudden change of load, preventing the commutating pole flux from rising or falling in step with changes in load. Therefore, from the standpoint of commutating pole construction, there should be no closed circuit surrounding the commutating pole itself. However, from the standpoint of the best arrangement of the damper to prevent hunting, a complete cage winding, tying all the poles together, as shown in Fig. 3, is, in general, the most economical and effective. But such a closed !o| I O I Icj !o! jL HI FIG. 3. winding forms a rather effective closed secondary circuit around the commutating pole. One would therefore assume that it is in- advisable to close this damper circuit around the commutating pole, and that a break at a or 6 in Fig. 3 for instance, would be an improvement. However, in some instances, experience has shown that the improvement in the damping action as a whole, in pre- venting htinting, by tying together at a and b, more than over- balance the harmful effects of the closed secondary circuit around the commutating pole, caused by the closed damper winding. This is not necessarily always the case, the restdts depending upon individual and local conditions, to some extent. The same damping effect as tying together at a and h might be obtained theoretically by special proportioning of the damper on each pole, but, in some cases, especially on 60 cycle machines, space require- ments do not permit such proportioning of the damper, so that it may prove better, to tie the dampers together at a and b. 60 CYCLE CONVERTERS 479 A new condition also developed in connection with self starting of comrautating pole rotaries. In the older 60 cycle rotaries, start- ing motors were rather commoiily used, due, not to the inability of the rotary to start itself, but to the effect of the large start- ing current upon the relatively small generating plants of those days. Later practice tends strongly toward self -starting, except in special cases. There are some very considerable advantages in this self -starting, and at the same- time, there are some disadvan- tages, especially in the 60 cycle rotaries. The greatest advantage lies in the rapidity with which the rotary can be started from rest and brought up to synchronism, together with the fact that no synchronizing devices are required. With the old starting motor, the machine had to be brought to synchronous speed and then thrown in step. This was more difhciilt with 60 cycles than with 25 cycles, and self -starting eliminates this trouble. On the other hand, while starting and accelerating, the rotary converter is purely an induction motor of a rather crude sort, and will take a relatively large starting current — ^in some cases approximately full load current from the line — and this current is at very low power factor ; that is, at least, 90 percent to 95 percent of it is purely watt- less. When starting a large capacity rotary, this will represent a relatively large inductive load thrown suddenly on the power plant. However, the new condition which developed with the advent of commutating poles, lies in sparking, and not in the starting current. As the rotary converter at start acts like an induction motor, it has a rotating magnetic field flux set up, which travels around the armature. The armature coils short-circuited by the brushes form secondaries to, or are cut by, this field, and therefore have relatively large e. m. f.'s set up in them, which , develop large local currents. The e. m. f 's set up in the short-circuit- ed coils are usually comewhat greater in the 60 cycle rotaries than in 25 cycle, due primarily to the fact that there are usually fewer conductors in series for the normal voltage of the machine, and therefore the normal voltage per conductor is relatively higher than in the 25 cycle rotary. In consequence, at start, assuming that similar voltages are appHed for starting both 60 and 25 cycle machines, the relative voltage per conductor generated by the rotating field set up by the armature winding will also be higher in the 60 cycle rotary. Also, the number of commutator bars covered by the brush will usually be greater on the 60 cycle rotary. 480 ELECTRICAL ENGINEERING PAPERS In consequence of these two conditions, the short-circuiting action of the brushes and the sparking will be worse on the 60 cycle rotaries, but it is Uable to be excessive on all large machines. With the advent of the commutating pole rotary converter, a still naore difficult condition has been encountered in self -stating, namely, that the flux conditions in the zone of commutation of the short-circuited coils are materially higher than in the non-com- mutating pole machine. In the latter type, while the short- circuited coils cut an alternating flux and therefore have local currents set up in them, these coils, in commutating or reversing these currents, Ue midway between the poles, and therefore in the region where the conditions of reversal are easiest. But in placing the commutating pole directly over the short-circuited coils, the conditions of reversal of the short-circuited current are made much more difficult during starting. In consequence, during starting and accelerating, the sparking conditions in the commut- ating pole rotary, both for 60 and 25 cycles, are much worse than in the older non-commutating pole type. In fact, in the larger machines, the conditions are so bad that it has been foimd neces- sary to add brush lifting devices which will lift aU. the brushes but two, during starting and bringing up to speed. This is an added complication, but it is offset, to some extent, by the fact that, with the brushes Ufted, there is no sparking at all, and therefore the commutator does not suffer at all diuing the operation of starting. In the earlier 60 cycle rotaries, the question of variable voltage came up in connection with 250 to 300 volt machines. The general means of voltage variation in these machines was almost entirely by means of induction regulators, or step-by-step trans- formers. It is only in very recent years that the self-contained tmits, such as the synchronous booster rotaries, and the regulating pole type, have been brought forward. For 60 cycle, the syn- chronous booster appears to be the only really practical method, due largely to limitations in design and in space requirements. In commutating poles are to be used, then the regtilating pole type of machine, with main and auxiliary poles, in addition to the com- mutating poles, reqmres a very crowded design of field, unless a larger pole pitch is chosen than in the synchronous booster naa- chine, in which there are only the commutating and the main poles. When synchronous boosters are used with commutating poles, the problem of proper adjustment of the commutating pole 60 CYCLE CONVERTERS 481 Strength, with varying loads and voltages, comes in. This has been treated before rather fiilly in a paper before the association, and nothing further need be said, except that this problem of adjustment is just as pronounced in 60 cycle machines as in 25. Where the range of voltage is relatively small — say, never exceed- ing 10 percent up or down — ^it is practicable to so proportion the commutating pole windings that, without any automatic or hand- adjusting devices, good commutation can be obtained over the whole working range. However, if materially higher voltages are needed, such as 15 percent to 20 percent up or down, practice indicates that some auxiliary device is required for automatically, or by hand, adjusting the field strength at the extreme condition, which appears to be at no-load with maximum boost or buck. For this condition, an automatic device has been developed, which, when the main current falls to a relatively low value — say, one- fourth full load — automatically short-circuits that part of the commutating pole winding which is in series with the booster field. The same operation cuts into the circuit a resistance equivalent to the section of the winding cut out. This latter is a necessity, due to the fact that any variation of the resistance of the booster field circuit will vary the amount of boost or buck, and thus affect the maih voltage of the machine. Any automatic device therefore shoiild hold the resistance of the booster circuit constant. Such devices have been installed on a number of synchronous booster, commutating pole type machines. They can be located at the rotary, and, being piu-ely automatic in their action, require no attention from the switchboard operators or anyone else. As such a device operates only very infrequently, but at fairly regular intervals, such as once or twice a day, it is not liable to wear out due to excessive operation, or to stick due to non-use. Experience has shown that, except for extremely wide ranges in D. C. voltage, only one step is needed in such automatic device. 60 cycle rotary converters are now being manufactured in relatively large capacities, such as 1000, 1500 and 2000 kw for 270 volts with synchronous boosters, and up to 2500 kw for higher voltages. Larger capacities, for either voltage, can be constructed without difficulty, and with as good performance as in the capac- ities mentioned. The modem 60 cycle rotary converter, for either 270 or 600 volts, is approaching very close to the 25 cycle rotary in its general characteristics, such as efficiency etc. In commu- tation, it can be fully equal to the 25 cycle. In general relia- 482 ELECTRICAL ENGINEERING PAPERS bility, the modem machine is far ahead of the older types. This development of the 60 cycle rotary therefore removes one of the most serious handicaps formerly encountered by the large 60 cycle generating systems. IRON LOSSES IN D.C. MACHINES 483 IRON LOSSES IN DIRECT CURRENT-MACHINES FOREWORD — This paper was presented before the Schenectady Section of the American Institute of Electrical Engineers, March, 1916, before an audience composed entirely of engineers of the General Electric Company. It is prin- cipally of interest to designing engineers, in general, and it brings out some of the problems actually involved in an analysis of the loss conditions occurring in direct-current machinery. Certain explanations of eddy current losses, due to saturation of the armature teeth, are brought out here for the first time, the author believes, and some approximate methods of calculating these losses are given. In fact, a careful study of this paper will indicate wherein the calcula- tion of the no-load losses in any direct-current machine is, necessarily, more or less empirical, while the conditions with load are very much worse. — (Ed.) IRON LOSS is a general term to cover a number of losses, o£ various kinds, which, by the nature of the t6sts, are included in one set of measurements and which, in reality, should be known as core loss. The term has been, used so promiscuously, without indicating what it really includes, that many have come to believe that it means the true iron loss and nothing else. In fact, however, the true iron loss, in many cases, may be only a moderate percentage of the core loss. Usually no distinction has been made between losses simply located in the iron, and those due to the magnetic conditions in the material itself! The readily practicable methods of measuring the core losses show only their sum and there is no true indication of the relative values of the various com- 484 ELECTRICAL ENGINEERING PAPERS ponents. To separate the total core loss into its various com- ponents, except by complicated and expensive laboratory meth- ods, appears to be almost impossible. However, it is possible to indicate the various components and their probable causes, and in some cases they can be segregated very crudely by calculation. In most rotating machinery the calculation of the individual elements, which make up the total core loss, is necessarily only approximiate, m comim.ercial apparatus. This is due partly to the fact that there are many possibilities of variation in loss on account of conditions of manufacture and materials, as will be described later. This is evidenced by the fact that two machines, built at different tim.es from the same draw- ings and the sam.e tested grade of materials, will ofttimes show materially different core losses. If two such machines vary twenty -per cent from each other in core loss, it is obviously impracticable to expect any refinement in calculation closer than twenty per cent. Even if we always could come within twenty per cent by direct calculation and could place any great reliance upon the results, it would be a great step ahead, in certain types of apparatus. In the discussion of the various losses and their causes, given throughout the following paper, it will be shown why it is imipracticable to calculate, with any exactness, certain of these losses. In separating the total core loss into its comipcnents, two principal classifications of losses may be rradc. Cne of these is eddy current loss, either in the iron laminations themselves or in other conducting parts wherein e.m.fs. are generated during rotation. Such e.m.fs. will set up local currents where closed paths are possible, and if such paths are in the lamiina- tions themselves, instead of in neighboring solid parts, it is simply incidental. Eddy current loss in the lamiinations is, therefore, not a special kind of loss, and it should rightly be classed with other eddy losses in the machine. The second, class of losses includes those due to changes in the magnetic conditions in the iron itself; these are known as hysteresis losses. These latter are dependent upon the material itself and not its structure. Lamination is primarily for increasing the resistance in the eddy current paths and not for the purpose of afifecting the hysteresis. In fact, lamiina- tion may increase the hysteretic losses, for a given volume of material IRON LOSSES IN D.C. MACHINES 485 The principal object of this paper is to show causes for some of the principal losses. These arc usually related to two sets of frequencies, namely, the normal frequency (revolutions per second times number of pairs of poles), and some very high frequency, dependent upon the number of slots, commutator bars, etc. The hysteretic losses are undoubtedly affected by these higher frequencies but apparently not to the same extent as the eddy losses. These high-frequency losses are liable to be present in most classes of rotating machines, while in some instances they may overshadow all other losses. Certain of them are characteristic of certain types of machines only, while others are liable to be present in any type of rotating machine. In most classes of rotating machines, only the no-load core tosses can be- measured with any accuracy by ordinarily con- venient methods of measurement. However, if the various components of the 'no-load loss can be approximately deter- mined, then it is possible to indicate in what way these same components- will be affected by load. A quantitative deter- mination of the component losses with load is, however, very difficult to determine except in a very few classes of machines. In direct-current machines the principal no-load armature core losses are the hysteresis loss in the iron, eddy losses in the iron and copper, and eddy losses in other adjacent conduct- ing parts, which may be seats of e.m.fs. The relative values of these losses are dependent upon many conditions. In a thoroughly well designed machine the eddy losses in the copper and any other parts than the iron should be relatively small compared with the iron loss proper. Again, the pro- portion of hysteresis to eddy loss in the iron itself depends upon many conditions, such as the various frequencies in the machin-e, the grade of material, the degree of lamination, the perfection of the insulation of the laminae from each other, the distortion of the material in handling and building, the conditions of punching, treatment during , assembly, grinding, filing, etc. Here, at once, so many variables appear that one cannot reasonably expect any great accuracy in any prede- termination of eddy loss in the iron itself. Hysteresis loss is also affected by some of these conditions. It is a fact well known to designers that the iron loss tables used by transformer engineers do not directly apply to ro- tating machinery, but that an increase, in some cases, of one hundred per cent or more is necessary, depending upon the 486 ELECTRICAL ENGINEERING PAPERS type of machine. This increase is due largely to additional causes of loss which do not occur to any appreciable extent in transformers. Some of these additional losses are as follows: (a) Handling of iron. Experience shows that well annealed armature iron will have its losses very materially increased by springing or bending. If a lamination is given a decided "bend, beyond the elastic limit, and then is straightened out, the loss at the part which has been bent may be increased as- much as 100 per cent. This fact must be taken into account in ma- chinery where armatures with many light teeth are used. Here it is almost impossible to prevent some abuse of the iron, especially in the teeth, which are the parts usually worked the hardest. Furthermore, tests have shown that, if iron is bent, even at a small angle, and not beyond the elastic limit, the loss is materially higher with the iron in this strained con- dition, although the loss may return to normal when' the iron, is allowed to spring back to normal position. And if the. iron is annealed in a curved or warped position, then when straight- ened out in building the strain is present, with increased loss. In building up armature cores, undoubtedly part of the iron is put under stress, especially in the teeth. Any dent in the iron, produced by hammering or otherwise, also tends to"^ in-.' crease the loss. (b) A second source of increased loss in the iron is due to the operation of punching. In shearing the iron a small amount adjacent to the sheared part is affected much in the same way as when iron is bent beyond the elastic limit. In transformer, plates this strip next to the sheared edge represents but a very small percentage of the total volume of each plate, or lamina- tion. However, in armatures with many coinparatively long narrow teeth, this sheared part may represent a relatively large percentage of the whole plate and, moreover, this is a part which often has the largest losses. But this may not 'have as great effect on the losses as another result of the shear- 'ing, namely, the sharp burrs which are left on the iron. These • may be very small or almost negligible in appearance and yet 'represent quite a large percentage of the thickness of the plate. .For example, a burr of two mils height, or 1/500 in., seems to be very small indeed, and yet it is about 12 per cent of the thickness of a 17-mil lamination. Dies must be maintained in very good condition to keep the burr below two mils. The effect of this burr is to bring increased thickness and pressure IRON LOSSES IN D.C. MACHINES 487 at the edge of the sheets, particularly at the teeth. If the laminations are all turned one direction in building and the edges match perfectly the sheets might fit • together so accur- ately that the burr would cause no extra thickness. But it is impossible to obtain such accuracy in practise and, there- fore, the burrs of one sheet "ride" upon the surface of the next sheet, thus increasing the total thickness of the built- up iron. In -practise, however, the iron is pressed down to approximately uniform height throughout. This means that the burrs carry considerable of the pressure at the 'armature teeth and there is more or less of a tendency to cut through the insulating film on the plates, thus increasing the eddy current losses. This is 'obviously a variable condition depend- ing upon the accuracy of building, upon the condition of the dies, etc., and no method of calculation can take this loss into account with any accuracy. In small machines with low voltage per unit length of core , this loss usually is not of great importance. However, in high-speed large-capacity machines, it becomes increasingly .important and in some cases special means are used ■ for removing tjie burr before insulating the indi\'idual armature plates. (c) Another source of iron loss, and one which also is be- yond the scope of calculation, is found in the filing of armature slots and cores. In ideal armatures with perfect punchings and assembly, there should be no occasion for filing. However, the practise, in many cases where the armature iron does not build up with perfectly smooth surfaces in the slots, is for a limited amount of filing, to be done. Usually this takes off only isolated high spots, so that the adjacent laminations are not bridged over to any great extent by the burrs due to filing. The tendency of most workmen is to file down to a nicely polished surface, whereas a coarse filing gives better results as it tends to break the laminations away from each other. Filing is most harmful in machines having a relatively high voltage per unit length of core. A milling cutter for cleaning out slots is usually worse than a file, as it produces greater burring of the edges. However, if the milling is followed by filing with a very coarse file the results may be just as satis- factory as with filing alone. Obviously, no method of cal- culation can show accurately the losses due to such burring. (d) The iron losses are affected to a certain extent by pres- sure, that is, by the tightness with which the core is damped: 488 ELECTRICAL ENGINEERINO PAPERS The loss due to this is probably closely related to some of the preceding losses, such as bending and springing of plates, effect of burrs, etc. -In small machines the effect of pressure apparently is of little moment, but in large very long cores it may become very appreciable. It is particularly noticeable in large turbo-generator armatures where the cores are very wide. In such machines, in attempting to draw the core down to a sufficiently solid condition as a whole, the parts next to the end plates are liable to receive abnormal pressure, with consequent increase of loss -in those parts. For this reason, it is the practise in some cases to add an extra separation of paper at frequent intervals near each end of the core. Ex- perience shows that this equalizes the losses and temperatures very materially. That this is due to undue pressure and not to stray field or other conditions, is indicated by the fact, that when high temperatures are found in the iron, at each end of the core, very often the condition can be relieved by s'mply lessening the pressure to a comparatively small extent. The writer has known cases where the temperature in the end sec- tions of the iron has been reduced 30 to 50 per cent by "easing off" the end plates. The total loss in the core may not be re- duced very much, for the reduction in pressure usually affects only the end sections to any great extent. Presumably this loss is due to increased contact between the adjacent plates, possibly from the burr, but not entirely so, for similar results have been found in some cases where the burr had been fairly well removed before enameling the plates. The character of the enamel coating used for insulating purposes also has some- thing to do with this. In connection with pressure, the effect of heating of the core may be considered. Cases have been noted where the effect of high temperature of the core has been to increase the pressure between the laminations, due to expansion. This in turn increased the loss and thus still further increased the tem- perature. This effect has not been uncommon, to a minor extent, but a few cases have occurred where the combined pressure and temperature cumulatively have resulted in ex- cessive core temperatures. In one case which the writer has in mind, a certain large machine operated for about two years without any noticeably high temperature in the core. Then, in a comparatively brief time, it showed evidence of increas- ing temperature until finally an entirely prohibitive ternpeTa- IRON LOSSES IN B.C. MACHINES 489^ ture showed at one place. Examination showed that the core' was very tight and all evidence indicated that increased tem- perature was causing increased pressure and thus further in- creasing the loss. In this machine, fortunately, the construc- tion of the armature core and winding was such that the end plates could be released very easily about \ in. on each end. This was tried as an experiment and the temperatures all returned to the former normal of about 30 deg. cent. rise. As an interesting* side issue, it may be mentioned that on this machine the armature teeth at each end of the core had been breaking off, although stout brass supporting fingers had been used. Apparently under the increased pressure, due to heating, the fingers would be bent away from the core, thus releasing the tooth laminations. Repeated tightening of the brass fingers did not relieve this condition. However, when the end plates were released \ in. at each end of the core, the brass fingers were then sprung in against the teeth and afterwards remained in position so that no breakage of tooth laminations was ever reported afterwards. Obviously, with losses dependent upon pressure, no extreme accuracy in calculation of such losses is possible. However, in moderately small size machines, and especially in those oir^ very moderate frequency and of very low voltage per unit length of core, the effect of pressure is not serious, within a moderate range of practicable pressures. (e) Another. source of iron loss, but which is not in the arm- ature core, is that of the pole face, due to the tufting or bunch- ing of the flux between the field pole and the armature teeth, where slotted armatures are used. Obviously, with all other conditions the same, this pole face loss will depend upon a number of variables in the lamination of the material itself. The effect of burrs from punching, the burring over of the surface due to turning, the effect of pressure, etc., all appear in the pole face loss. Therefore, it is evident that great ac- curacy in the calculation of such loss is impossible, in com- mercial apparatus. There are other conditions that affect this pole face loss which will be considered later under this subject. Armature Ring Loss. The true iron loss in the armature ring is dependent upon the total flux per pole, distribution of flux, rate of change of flux, etc. The problem is much com- plicated by the fact that the flux distribution in the ring usually 490 ELECTRICAL ENGINEERING PAPERS 'is not uniform, that is, certain parts of the core have higher maximum densities than other parts. However, in ordinary practise the core densities used are relatively low, so that the losses can be approximated by averaging the inductions, in certain parts. However, the rate of change of flux in the ring is dependent, to a certain extent, upon the flux distribution in the air gap and armature teeth, and this introduces some' error, always in the direction of increased loss. The distribution of flux in the armature ring is also depend- ent upon the effective length of the various flux paths. These latter will naturally depend upon various conditions, such as the number of poles, diameter of armature,- flux distribution; in the air gap and teeth, etc. Therefore, any method which does not take this distribution into account is necessarily cnly approximate. How-ever, in practise there are so many other, variables, as already described, in connection with manufactur- ing conditions, such as burring, filing, etc., that empirical rules have been developed, based upon numerous tests, which ap- proximate the armature core loss in a standard type of ma- chine about as- accurately a£ any attempt toward exact cal- culation. A rmaiure Tooth Losses at No-Load. Apparently the flux densities in the armature teeth can be calculated with more accuracy than in the various parts of the core, for in the teeth the fluxes are limited to fairly definite paths. Therefore, exclusive of the losses due to manufacturing conditions, as already described, the tooth losses can be fairly accurately calculated, probably with much greater accuracy than many other losses, as will be described. The tooth losses may be considered further as follows: The flux density in each individual armature tooth passes through a cycle, indicated by the shape of the field form. - With the field form of the shape illustrated in Fig. 1, the tooth den- sity will be a maximum at A, and this density will remain practically constant as the tooth moves toward C until the point B is reached. It will then decrease as the ordinate of the field form curve decreases and will reach zero value at C. The cycle of flux change is not sinusoidal, and therefore, the actual tooth iron loss should not agree with that represented by the usual iron loss curves based upon sinusoidal changes in induction. The difference, however, may be relatively small in the ordinary types of machines. The error may be IRON LOSSES IN D.C. MACHINES 491 taken care of by some suitable correcting factor, which, of course, will be only approximate for the average case. The density in the armature teeth is involved in the iron loss. This density is not uniform over the entire depth of the tooth, with the usual parallel-side slots, for the section of the tooth tapers off. This difference of section, in small diameter machines, may be very considerable. However, a higher density at the base of the tooth, tending to give higher iron loss, is compensated for, to some extent, by the reduced volume of material. In consequence, the mean section at, some- point from one-half to two-thirds the way down the tooth may be taken and the mean density and volume of ma- terial, based upon this section, may be used for approximating the iron loss. The accuracy, of this method will be dependent, to some extent, upon the actual density used. For instance, if both the minimum and maximum densities in the tooth are relatively low, then the loss calculated for the mid-point density, at the mid- point section, will be closer to the true loss than if the y maximum density is exces- ^ sively high. PiQ 1 Armature Copper Eddy Cur- rent Loss at No-Load. There may be a number of eddy current losses in the copper, some of which are of a minor nature. However, there may be two relatively large losses, depending upon the design of the ma- chine. One of these is due to the flux from the' field poles entering the armature slots and cutting the conductors. This is, to a certain extent, a function of the saturation of the tops of the armature teeth. It is also dependent upon the width of the slot opening compared with the iron-to-iron clearance. At first thought, one would say that the larger the air gap the more- would the lines from the pole pass into the tooth top. However,-the opposite is the case, for the larger the gap, the nearer do the lengths of paths into the slot approach to the iron-to-iron clearance, in percentage. In moderate size machines with relatively small air gaps and moderate slot widths, the eddy current loss from fringing into the top of. the slot is comparatively small, and, as a rule. 492 ELECTRICAL ENGINEERING PAPERS no special precautions need be taken to minimize it. This particular loss is usually greatest in high-voltage, large-capac- ity turbo-alternators, where relatively wide slots, up to 1.5 in. or more, may be used, and where the air gaps are very large. In such cases lamination of the top conductors to avoid eddies from this cause may be desirable. ■ The second source of eddy current loss in the copper, which is liable to be larger than all others combined, is due to the peculiarities of flux distribution in the armature teeth. Let Fig. 2 .represent the magnetic conditions in a given machine. It is evident from this figure that under the central flat part of the field form, the armature teeth are worked at a uniform induction, assuming that there is no field distortion. How- ever, at ■ the edges of the pole the tooth density decreases slightly. If the saturation of the teeth under the flat part of the field form is very high (materially above 120,000 I .^- I lines per sq. in.), the ampere- I y^. —-^^ i turns required to magnetize J^^y$|4?^'-n n nU the teeth may be very con- J |Jj[J g.g:g U U U L|j L siderable. However, at the \ i '' I edge of the pole a compara- I i tively small decrease in the i r\ y flux density in the teeth (15 J \^ to 20 per cent) will mean a relatively enormous decrease in the ampere-turns for the teeth. For instance, the tooth c in Fig. 2, under the central flat part of the field form, may require 2000 ampere-turns, while the next tooth h, under the pole edge, which is worked at possibly 20 per cent lower den- sity, rnay require only 10 to 20 per cent as many ampere-turns. Assuming such conditions, then the magnetic potential at the top of tooth c will be higher than that at the top of h by 1600 to 1800 ampere-turns. Therefore, under this condition there will be a very considerable flux across the slot between c and b. A little earlier or a little later in the rotation this flux across this slot will not exist to any extent, for the ampere-turns for h and c will then both be comparatively low or very high, while the difference between them will be small. In conse- quence, near each pole edge, there is a very rapid rise and fall of flux across the armature slots. This is illustrated in Fig. 2. Obviously, the armature conductors lying in the path of Fig. 2 IRON LOSSES IN D.C. MACHINES 493 this flux will be the seat of e.m.fs. which will tend to set up local currents, the value of which will be some function of the e.m.f. producing the current, of the dimensions of the conductor, etc. If the flux across the slot is large, this^ e!m.f. may also be considerable, for the rate of this flux change will be high compared with the normal frequency of the machine. As the e.m.f. generated is a function of the maximum difference between the ampere-turns required for two adjacent teeth and as the loss in any given case will vary as the square of the e.m.f., obviously the loss in one slot will vary as the squareof the maximum difference between the anipere-turns of two ; X ^ —- OJ C^ CO r* to CD rH CO O O to "O l> l> O 00 Oi O CO CD OJ 1-1 00 00 M to CO rH N CO CO N to 00 w to O O O O O O O M CI to tP to to CO to 00 CO 00 O b- W.M d M M CO Width of slots; ■s 0.406 0.391 0.312 0.328 0.625 o «-, 2° 10 o ■3 S K o o o O O Tp (N CSI .H Single air gap; •s .a lO (N U5 ^ ^ M to * to t» to O 1-1 jH N to 00 t^ to 00 bo ^ cq th CO CM eq CM O O O o o o o o o o Z o 'o p, (O (D o » o iH r-l <2- .3 S 1150 1200 Tj< Tti ■^ to to to g S o o o o to to o o M to CD b- g g 750 75.0 1000 502 ELECTRICAL ENGINEERING PAPERS ings which are as symmetrical as possible. Also, the arrange- ment of the winding should be such as always to generate bal- anced e.m.fs. in parallel circuits. This condition is not in- frequently overlooked in the design of direct-current machines. A second cause of undue loss in the armature winding may be occasioned by short-circuiting one or more of the armature coils under an active field. The brushes may be shifted from the magnetic neutral point so that some of the armature con- ductors are short-circuited under the main field flux; or the neutral point may be so narrow and the brush so wide that some of the armature turns are short-circuiting in an active field, even when the brush is set for the no-load neutral. An armature winding which is considerably "chorded" in a field with a narrow neutral point may have two sides of a coil short- circuited in fields of the same polarity. The e.m.fs. in the two' sides of the coU should, therefore, balance each other if the brush is set at the true neutral. However, if the brush short- circuits several coUs or turns, obviously only one of them can be at the true neutral and have balanced e.m.fs-. set up in its two halves. The other turns may have more or less local current in them, which may be a source of considerable loss. A third condition may occur when there are considerable pulsations in the reluctance in the air gap under the main poles as the armature teeth move under the poles. This vary- ing reluctance usually gives varying main flux and at a relatively high frequency. The armature coils short-circuited by the brushes will act as secondaries to these .pulsating fluxes and in consequence there may be some loss in the short-circuited cojils due to this cause. Any solid parts of the yoke or poles may also have losses due to this cause. Usually, however, such losses are small. A fourth source of loss may rise from stray fluxes from- 'the main fields to the armature, which do not pass through well laminated parts of ihe armature core. For instance, the ventilating spacers may be so dimensioned and shaped that eddies can be set up in them. Also, the finger plates at each end of the core, the end plates, etc., may carry light fluxes which produce some loss. Bands on the armature core or at the ends may also be the seat of e.m.fs. and will have some loss in them. These losses are difficult to determine, and, in practise, should be eliminated as far as possible. IRON LOSSES IN D.C. MACHINES 503 Full Load Losses It is evident from the foregoing that the no-load core losses are dependent upon so many variable conditions that there can be no great accuracy in predetermining such losses unless all the details of construction, material, treatment, etc., are known for each individual machine. The impossibility of accurate calculation is shown by the fact that the individual machines built on the same stock order will vary considerably i from each other, especially in certain types. While the no-load losses are difficult to predetermine, the full load losses are still much mote difficult to calculate, as will be shown in the following rough analysis. Here, the effects of flux distortion by the armature magnetomotive force tend to exaggerate the pole face losses and those in the arma- ture copper; which are the two relatively large losses which are most difficult to calculate at no-load. Also commuta- tion and brush losses, due to load, now enter into the problem. The individual core losses may be considered briefly as follows: Armature Ring Loss, with Load. This loss should not change greatly with load, provided the total flux at load is practically the same as at no-load. ' Under this condition a variation in the distribu- tion of this flux is about the only factor which should pro- duce any material change -in loss. The • full load field form may be illustrated by Fig. 5. It -is evident from this figure that the flux is now crowded toward one pole edge and, therefore, the major part is concentrated in a narrower space. The average length of the fliix path may, therefore, be somewhat greater than at no-load, but in some cases this may tend to distribute the flux more uniformly through the depth of the ring. However, where the flux enters the core at the base of the teeth there will be slightly more crowding and, therefore, somewhat increased loss. Taking everything into consideration it would appear that, in general, the arm- ,ature ring loss can be considered as practically constant with, constant total flux and speed, independent of the variation in load. / ' A A / / 1/ \ yy — ■ — &^ /" \ A Pig. 5 504 ELECTRICAL ENGINEERING PAPERS In variable-Speed and adjustable-speed d-c. machines, the- armature ring loss may vary over a wide range due to changes^ in total flux and speed. Such cases are difficult to calculate with any degree of accuracy, although no more so than other losses in the same machines. Armature Tooth Loss, with Load. As shown by Fig. 5, the tooth flux density at one edge of the pole is decreased and at the other edge is increased when the field flux''is distorted by the armature magnetomotive force. The increased density in the armature teeth means increased iron loss and, if the dis- tortion is very great, the increase in tooth loss may be very large, being in some cases even doubled or trebled, compared with the no-load tooth loss. No direct rule can be given for the calculation of this ioss, except that it inay be determined approximately by calculating the flux distribution with load and thus determining the flux densities in the teeth. In variable-speed and adjustable-speed machines, particu- larly in the latter, the tooth loss with load will be affected very considerably by changes in both speed and total flux. In variable-speed machines of the series type, reduction in speed usually accompanies increase in total flux, so ttiat, as regards- the losses, one effect partly neutralizes the other, so that the increase in tooth loss with load may be less than in a constant- speed machine. In adjustable-speed machines, however, es- pecially in those of constant horse power and constant voltage, the tooth losses will vary over a very wide range with change in speed. Here, the armature magnetomotive force is con- stant (assuming a constant horse power) and the field flux is varied from a maximum value at lowest speed to one-quarter value at four times speed, assuming a four-to-one range. The total flux, therefore, varies inversely as the speed and the two^ effects should nearly compensate each other, as regards losses, if it were not for the variation in flux distortion. At lowest speed, with considerable saturation in the pole horns and armature teeth, the armature magnetomotive force, even if relatively large compared with the field magnetomotive force, may not produce very large distortion, so that the tooth loss- is not increased excessively over the no-load tooth loss. How- ever, as the field is weakened, the armature magnetomotive force remaining constant, the distortion is relatively increased, so that the peak value of the distorted field may remain -almost constant in height. As the armature tooth losses are dependent- IRON LOSSES IN B.C. MACHINES 505 ■Upon the peak value of this field, then obviously the combined effect of this field and the increase in speed will mean very greatly increased tooth losses. With very low field magneto- •motive force, the distortion may be so great as to give a double peak, as indicated in Fig. 6. This double peak gives, to some extent, the effect of a double frequency and thus further in- creases the loss. Eddy Currents in Copper. When the field form is distorted, with load, the ampere-turns in the teeth at one pole corner are greatly increased, while those at the other corner are de- creased. Therefore, there will be an increased loss in the copper at one pole edge and a decreased loss at the other pole edge. However, as this loss at high inductions will vary al- most as the square of the ampere-turns in the armature teeth, it is evident that the re- duction in the loss at one pole corner may be small compared with the increase in loss in the copper at the other pole corner. The resultant loss can be calcu- lated approximately by using the formula already given for no- load conditions, but with the ampere-turns in the teeth based on the load conditions. This would give a loss corresponding to no-load with the maximum induction in the teeth raised to peak value with load. This would include losses for the two pole corners; therefore, the result should be halved, as the peak density occurs at only one pole edge. If the empirical formula given for the copper loss repre- sents the facts, even to a merely approximate degree, the re- sults are very startling when applied to some of the old-time machines. The calculations show that in some cases the eddy current copper loss at heavy load was sei/eral times greater than at no-load. This should be true, but to a much less extent, in more modern types of machines. The results indicate that in many cases there will be considerable gain by reducing the field distortion through high saturation in the pole face, pole horns, etc. This saturation, however, would have to be Fig. 6 506 ELECTRICAL ENGINEERING PAPERS SO arranged as to give the most beneficial field distribution with load, and haphazard methods of cutting off pole comers, without regard to the field form with load, would have to be avoided. In fact, in the past, the cutting away of pole corner laminations, in many cases, has been largely for the purpose of improving cominutation, and not to obtain the best field form with load. Pole Face Losses, with Load. The pole face losses will obvi- ously be affected locally by change in the flux density in the air gap or at the pole face. Field distortion will tend to increase the loss at one pole corner and decrease it at the other. The increase will usually considerably exceed the decrease, but the resultant will not be increased in anything like the same pro- portions as the copper eddy current losses under the pole corners are increased, with load. A rough approximation for the in- creased iron loss could be obtained by comparing the squares of the densities, at several points along the distorted field form, with the squares of the densities of the no-load field form corresponding to the total induction. As the increase in pole face losses with load will, in some instances, be considerably less than the increase in the eddy current losses, it might be advantageous in such cases to de- crease the field distortion by pole face saturation, even at the expense of increasing the no-load pole face losses. For example, if, in an extreme case, the air gap were decreased .20 per cent and the air gap ampere-turns thus gained were expended in suitably saturating the pole face material, then the full load field 'distortion might be much less than with the larger gap, with the same total field magnetomotive force. The no-load eddy current copper losses would be practically unchanged, while the no-load pole face loss would be increased. However, the full load pole face loss, due to the reduced distortion, might be no greater than with the larger gap, while the eddy current losses in the- copper might be very much less than with the larger gap.- In consequence, while the total no-load losses would be increased somewhat, the full load loss would be smaller than before, and the carrying capacity of thcr machine would be actually increased. This would apply, however, only to those machines where the no-load eddy current and arm.ature tooth losses are relatively high and where the distortion is rather large with load. Stray Losses! When the machine is carrying load, the stray IRON LOSSES IN D.C. MACHINES 507 losses given under the no-load conditions may also exist and at the same time some of these may be greatly exaggerated/ Also, other losses may appear which are not found at no-loadJ Copper loss due to short-circuiting the ariTiature coils in an active field will sometimes be more pronounced than at no-' load, particularly in non-cornmutating pole machines in which ^ the brushes are shifted into an active field to produce com- mutation. This field, as a rule, will only be of proper value to produce proper commutation at some definite load, while at other loads there may be very considerable local currents in the short-circuited coils which may produce loss. As the main field flux is crowded toward one pole corner and the field form becomes more pointed in shape, the effect of variable reluctance in the air gap may become more pro- nounced than at no-load, and, therefore, pulsations of the inainl field flux may cause more loss in the short-circuited armature coils. ■ Stray fluxes from the main poles will be distributed differ- ently from the no-load condition and the densities of these ^ stray fields may be considerably higher at certain points and! thus give increased losses. Additional losses at full load may be due to. fluxes set up} by the magnetomotive force of the armature winding itselfi when carrying load. For instance, the armature winding will set up magnetic fields, through the end windings, which fields are fixed in space, in a rotating armature m.achine. -Bands or supporting parts, or other solid metal, rotating with the endi winding, may cut these stationary fields or fluxes, and thus losses may be set up which are a function of the load. Another source of loss at load may be found in the operation ^ of commutation itself. A magnetic field or flux is set up by the armature winding across the slots from one commutation zone to the next. At the point of commutation this flux is re- versed in direction with respect to the armature conductors, and, therefore, there will be local currents set up in the arm- ature copper itself, due to this action. This, however, should be more properly charged to commutation loss rather than to armature core loss. The above covers the principal core losses in direct-current machines. It was the original intention to analyze the core losses in the various types of rotating machines, but it soon developed that the subject was too .extensive for the scope 508 ELECTRICAL ENGINEERING PAPERS of this paper, therefore it was limited to d-c. machines only. However, many of the conditions which hold for d-c. machines also apply, to a certain extent, to many other types. In ad- dition there are losses in d-c. machines which are relatively large compared with those in other apparatus, due to the fact that the tooth saturation in d-c. machines is frequently carried, much higher than in other apparatus. The foregoing treatment of core losses is qualitative rather than quantitative, and it deals with the simpler phenomena only. It omits some very complex conditions, such as the effect of pulsations in flux superposed on high densities, dis'^ placed minor hysteresis loops, etc., which mean i additional losses. The principal object of the paper is to give a better idea of the possibilities and impossibilities of the problem of core losses. IRON COMMUTATORS 509 IRON COMMUTATORS FOREWORD — During the past two years (1916-1918) frequent inquiries have been made as to why iron is not used instead of copper in the construction of commut- ators. Obviously this question is inspired by rumors and statements that Ger- man manufacturers are using iron in commutators, due to the scarcity of copper since the outbreak of the war. Apparently also it has been assumed, in some instances, that the present use of copper is more or less of a fad and that other metals, such as iron, could be used if desired. This paper appeared in the Electric Journal, July 1918. — (Ed.) TN the cotirse of the development of commutating machinery -'■ various metals have been tried out in commutators, all the way from pure copper, both hard and soft, through various alloys and brasses, cast copper of various purities, aluminum, wrought iron, clear down to cast iron. All such materials have received con- sideration at some time or other and have been given fairly con- clusive tests. Experience has shown that all of them could be used in commutators if one is willing to pay the price, this price being in the first cost of apparatus, in maintenance or in less satisfactory operating characteristics, or a combination of all. Under the stress of war conditions it may be necessary to pay any price, and apparently this is the condition which has confronted German manufacturers. In consequence, materials and constructions are used simply as a matter of necessity which, however, may not con- form to conditions of even reasonably good design. The use of copper in modem commutators is a matter of development and not simply a fad. In fact, most of the early commutating machines used other metals in their commutators, which would now be considered quite unsuitable. Cast copper and various brasses and bronzes were used quite extensively, with more or less bad results. Pure copper was considered too expen- sive for general use and it was only after very considerable de- velopment that the conclusion was reached that its apparent higher first cost was more than neutralized by improved mainten- ance and operation. Even after pure copper had come into general use for commutator construction, it was not known, or understood, why it was so superior to other metals. About twenty-seven years ago the writer made extended tests on the use of iron in street railway commutators. The ma- chines soon developed "high mica" and the commutators grad- ually blackened, the contact surfaces bHstered and sparking 610 ELECTRICAL ENGINEERING PAPERS gradually increased until the commutating conditions became practically impossible from the operating standpoint. These conditions repeated themselves in every test until finally this con- struction was given up as impracticable. The difficulty was blamed largely upon high mica, as it was assumed that, in some way, the metal wore below the mica, thus causing bad brush con- tacts, with resultant burning and blackeriing. It was not recog- nized that the converse was really the case and that the high mica was the result of burning rather than the cause. In all machines of those days there was more or less tendency for the commut- ators to "wear" considerably, and it was not recognized that such was not true mechanical wear, but that it was the result of burn- ing away the contact surfaces. A Uttle later, the writer made quite complete tests on the use of aluminum on street railway motor commutators. This material worked better than iron, in the sense that burning and blackening and high mica did not appear as quickly as with the iron. How- ever, like the iron commutator, there was no tendency to polish, but the commutator soon assttmed a diill appearance which gradually changed to a blackened and burnt condition. Bronzes and brasses were tried on similar railway commut- ators, and while these gave better results than the aluminum or iron, yet they developed high mica much more quickly than the copper commutators. With such evidence at hand, the use of forged or drawn copper for commutator bars was a natural con- clusion. However, even with the best copper obtainable, there was some tendency toward blackening and burning of the com- mutators, generally accompanied by high mica, and the difficulty was blamed primarily on the mica. It was assumed that the cop- per bars did not wear as rapidly under the carbon brush as was the case with other metals. At the same time it was recognized that when the machine was operated without current none of these metals seemed to wear unduly. It was only when considerable current was carried that the wear was excessive. At that time, the real explanation of this difficulty was not ftiUy appreciated. Later investigations on collector rings and commutators, de- veloped the fact that whenever a cturent is carried between a stationary brush contact and a moving surface, there is a tendency to btim away either the brush contact face or the moving siirface, depending upon the direction of the current and upon the current density. It was found that this bvuning action, which is some- IRON COMMUTATORS 511 what similar to that occxirring in an arc, was to some extent a ftmction of the contact loss. This was indicated partly by the fact that the burning was a ftinction of both the brush contact drop and the current density. A given current, for instance, might produce very httle btiming as long as the contact drop was qtiite low; whereas, if for any reason such contact drop increased materially, noticeable burning would begin. If the current was from the brush to the commutator or collector, the brush contact stirface would tend to bum away more than the opposing surface. If, on the contrary, the current was from the collector or commut- ator to the brush, then the collector surfaces would tend to bum and, in some cases, deposit the btimt material on the brush face. When carbon brushes are used, there is usually a very con- siderable contact drop due, apparently, to the natture of the ma- terials in the brush itself. This drop, in many cases, is in the nature of one volt for each contact and it is fairly constant over quite a wide range of current. In consequence, the contact loss varies nearly in proportion to the current and not as the square of the current. Due to this very considerable loss with carbon brushes, there is a tendency to bum away the brush stirface and to bum and blister the commutator or collector surfaces with which the brush is in contact. This tendency to btun is dependent upon the actual current density in the brush (including local or short- circuit ciurents), but the resultant btiming is largely a function of the material in the commutator or collector face. As the brush cannot make perfect contact with the metalHc surface to which it is opposed, there are minute arcs at the contact and these evi- dently bum away the siufaces. However, the real burning action is dependent upon the inability of the surface to conduct away heat rapidly, for if the heat developed in the surface fiilm is not conducted away with sufficient rapidity, then such surface is liable to be bUstered or bumed locally, even though moving with respect to the brush. Such burning or blistering naturally roughens the contact surface and increases the contact drop and thus tends to increase the arcing and btmung action. Thus, if there is any huming action it tends to grow worse, cumulatively. This burn- ing away of the surface leaves the metal surface of the commutator slightly lower than the mica, unless the latter wears mechanically at the same rate that the commutator metal btims away. As this is not usually the case, high mica soon develops, simply by the action of burning away of the metal. Thus high mica is a result 512 ELECTRICAL ENGINEERING PAPERS of the trouble, rather than the cause. However, as even a very- gradual burning away will eventually leave the mica above the sttrface, modern practice has tended toward undercutting of the mica, so that even with a sHght btu-ning tendency the brush still maintains contact with the metal, thus preventing accentuation of the trouble. As mentioned before, this burning action is a ftinction of the contact voltage, the current density, and the non-bviming or non- blistering qualities of the metal constituting the commutator. It is in this latter feature that copper has proven so superior to other metals. Extended experience shows that the heat conducting qualities of pure copper are so good compared with most other metals or alloys that the burning or blistering action of the current under the brush is very small, except for high current densities. Anything which tends to decrease the heat conducting properties of the commutator metal, tends to increase burning action. This has been very clearly demonstrated in elaborate tests of carbon brushes on collector rings, etc., where questions of commutation did not come in to disturb the conclusions. Such tests have been made covering copper; bronzes and alloys of various sorts, wrought iron, cast iron, etc. In practically all cases, with high ciorrent densities, the biu-ning and blistering action appears to be dependent upon the ability to conduct the heat away from the contact stir- face. By such conduction the local heating of the contact film of metal is kept at a low point which results in reduced fusion of the metal, and with very good heat conducting materials the fusion of the metal may be so minute that the polishing action of the brush keeps the surface in a smooth glossy condition. It is an interesting fact that the electrical conductivities of the . metals and their mixtures and alloys, bear a fairly close relation to their heat conductivities. Experience shows that very little im- purity in copper will reduce its electrical conductivity to possibly one-third or one-quarter, and its heat conductivity will be decreased nearly in proportion. Most of the alloys of copper have a very low conductivity compared with copper itself, while wrought iron is worse than most of the copper alloys in this regard. The series of tests above referred to, indicated quite clearly that the burning tendency varied very much as the electrical resistance of the ma- terial, that is, with the heat resistance. Wrought iron, having from eight to ten times the resistance of copper, wotild burn or blister and get rough at very much lower ciirrent densities than .' IRON COMMUTATORS 513 copper commuliators or rings. Even some of the alloys which ap- peared to be good for collector rings, showed blistering effects at very much lower limiting current densities than copper. Conse- quently, it developed that the limiting carrying capacity of dif- ferent metals in commutators and collector rings, varied roughly with the heat conducting qualities, and thus copper proved to be superior to any of its alloys or any other available material. Ac- cording to this line of reasoning, silver should be better than cop- per, but this is not an available metal. The above also explains why alloys of copper in which other elements have been introduced for the purpose of hardening, etc., usually do not have the ultimate carrying capacity found in copper. Aluminum has fairly good heat conductivity, if pure, but it is so easily oxidized and the re- sistance of the oxidized surface rises so rapidly, that presumably this fact neutralizes- any possible gain otherwise. Experience on actual commutators showed that aluminum did not take a polish, even under moderate current densities and, in fact, it acted very much like some of the higher resistance metals used in the tests. It should be evident from the above that, when materials of higher heat resistance, that is, with poorer heat conductivity than copper, are used in commutators, the operating current densities should be reduced accordingly. Thus, it may be possible to use iron or steel for commutator bars, provided the brush current densities are reduced stif&ciently. In very small machines, this might mean only an increase in the dimensions of the commutator and brushholders. In larger machines, however, any material modification in the proportion of the commutator may lead to radical changes in the machine as a whole, so that the total cost would be materially higher than in the copper commutator ma- chine. This depends entirely upon how much sacrifice is to be made in operating conditions and maintenance. If these are to be kept at the same high standard as on present copper commut- ator machines, then it is questionable whether the iron commut- ator would prove to be practicable under any conditions. The same conditions hold true, to a certain extent, with certain alloys instead of copper in the commutator. As such alloys, as a rule, cost nearly as much as copper itself, it should be obvious that any material increase in the dimensions of a commutator wiU soon balance any possible gain. In larger machines one serious condition would be liable to be encountered with other than copper commutators. At present 514 ELECTRICAL ENGINEERING PAPERS these machines are btiilt for qtiite high peripheral speeds of the commutators, and construction difl&ctilties are encountered which would make any increase in their length or diameter very objection- able. Consequently, serious modifications in the general construc- tion of the machine, and possibly in its speed conditions, are liable to be necessitated. In fact, in many cases the whole design of the machine is predicated on the commutator construction In such cases the use of a poorer material in the commutator would un- doubtedly be a backward step in the development. It is thus obvious, that the use of iron in commutators, while possibly practicable under the urge of necessity, is not in the di- rection of an advance in the art. In fact, it is a big step backward. It should be assumed naturally that if, in the past thirty years of development in commutating machinery, iron commutators have not come to the front, it is for very good reasons, and the pre- ceding is simply an attempt to bring out some of the foremost reasons. INDUCTION MOTOR— SINGLE PHASE SECONDARY 515 POLYPHASE INDUCTION MOTOR WITH SINGLE- PHASE SECONDARY FOREWORD — ^This is a non-mathematical explanation of the characteristics of an induction motor with one phase of its wound secondary disconnected so that it has but a single-phase secondary circuit. Repeated requests had been made of the author, from time to time, for an explanation of the half speed character- istica of a polyphase induction motor under the above conditions. This anal- ysis does not require more than a working knowledge of the characteristic principles and curves of the polyphase induction motor. This paper appeared in the Electric Journal, September 1915. — (Ed.) TT is well known that when a polyphase induction motor is -•• operated with only one secondary circuit closed — -that is, with a single-phase group-wound secondary, it will develop at full speed a maximum torque much less than with polyphase secondary, and then with increasing load will drop to approximately haH speed, where it wiU develop comparatively high maximum torque. However, a simple non-mathematical explanation of the causes of this action has not yet been put forward, to the writer's know- ledge. Such an explanation is possible, and the following is one which should be grasped without difiBiciilty by those who are familiar only with the general characteristic curves and actions of induction motors. To begin with, consider the relations between the rotation of the magnetic field and the mechanical rotation of the rotor in the two cases of rotated and stationary primary windings — ^that is, with the primary winding (1) on the rotor, and (2) — on the stator. In the case of the primary winding on the rotor, let the direction of the magnetic field rotation be in the lef thand direction (counter- clockwise), as indicated in Fig. 1. This rotating field tends to pioll the stator around in the same direction (left hand). As the stator core and winding cannot rotate, the rotor, due to the torque between the rotor and stator, turns in the opposite direction (right hand). When starting from rest, the stator or secondary has the same frequency as the primary; then as the rotor speeds up in the right-hand direction, the magnetic field set up by the primary winding rotates in the left-hand direction, and therefore its speed relative to the stator becomes less and less until S3m.chron- ism is reached. Thus the secondary or stator frequency generated by the rotating primary field decreases until synchronism is reached, which is the normal action of the polyphase rotor. 516 ELECTRICAL ENGINEERING PAPERS Considering next a stationary primary winding and a rotating secondary, and asstiming right-hand rotation of the magnetic field, as shown in Fig. 2, it is obvious that the secondary winding and core will be pulled arotond in the same direction — that is, the rotor will turn in the right-hand direction. Thus, left-hand rotation of a primary magnetic field on the rotor, and right-hand rotation of a primary magnetic field on the stator, both tend to give right-hand rotation of the rotor. These two conditions are explained rather fully as they enter into the following explanation of the action of the induction motor with single-phase secondary. FIG. 1. PRIMARY ON ROTOR FIG. 2. PRIMARY ON STATOR The next consideration is the resolution of a single-phase field in an induction motor into two componenets rotating in opposite directions at the same speed, each of half the maximum value of the single-phase field. Such component rotating fields, it may be asserted, do not actually exist, but, nevertheless, the resultant of two such fields of proper value and rotation will actually be a single-phase field corresponding to that set up by a single-phase winding on the motor. This assumption of two oppositely rotating fields as the equivalent of a single-phase field is a great aid in explaining the speed characteristics of the poly- phase motor with single-phase .secondary. Assume next that the induction motor with single-phase secondary has its primary winding on the rotor and its secondary on the stator.* Consider first the standstill condition. The primary field is assimied to be rotating in the left-hand direction, as in Fig. 1. This field, cutting the secondary winding at syn- chronous speed, generates e. m. f . and current in the secondary, of a frequency equal to the primary, but single-phase, due to there being but one closed circuit. This secondary current may be *This particular arrangement is chosen, as it appears to the writer to allow a somewhat clearer conception to be obtained of what takes place in the motor. INDUCTION MOTOR— SINGLE PHASE SECONDARY 517 considered as setting up a single-phase field which is actually fixed in space, but which may be replaced by two oppositely rotating fields, each of half value, traveling around the c'ore at a speed synchronous with the frequency of the secondary current generated. One of these rotating fields travels in the same direction and at the same speed as the primary field set up by the rotor windings. It thus corresponds to the rotating magnetic field set up by the usual pol3^hase secondary winding. The action between this rotating secondary field and the primary is that of a polyphase motor, and torque is developed at all speeds from standstill up to sjmchronous speed, as in the usual polyphase motor, as shown in Fig. 3. The rotor under the action of this torque tends to rotate in the right- hand direction. Considering next the other component rotating field set up in the stator by the single-phase secondary current in the rotor, this rotates in the right-hand direction, or opposite to the primary rotating field. This secondary component field traveling around the stator may be considered as the primaiy field of an induction motor. For a secondary, it makes use of the windings on the rotor core, such secondary circuits being closed back through the primary transformers, supply system, etc. This may not be a very good secondary closed circuit, but it is all that is available and the motor' does the best it can under the circumstances. Let us now consider what torque conditions obtain with this right-hand rotating primary field with its freak secondary circuit. Taking standstill conditions first, it is obvious that as the primary field is on the stator and rotates right-handedly, the turning effort, or torque, will tend to run the rotor in the right-hand di- rection. Therefore, at start, the torque is in the same direction as in the case of the left-hand component. The two torques there- fore add at start. Considering next slow rotation of the rotor, as the primary winding on the rotor moves slowly in the right-hand direction the left-hand rotation of its flux decreases in speed with respect to the secondary winding, so that the secondary or stator frequency is correspondingly reduced. Therefore, the two oppositely rotating secondary component fields rotate more slowly. The left-hand one rotates at the same speed in space as the fundamental primary field, as described before. The right-hand component travels in the opposite direction, while its secondary circuit, namely, the primary coils on the rotor, are rotating in the same direction (right 518 ELECTRICAL ENGINEERING PAPERS hand) , but at a somewhat lower speed. The torque exerted by this right-hand component is therefore still in the right-hand direction — ^that is, the same as that of the other component. O — Torque of left-hand component, in synchronism with primary field and in the same direction. B — Torque cf right-hand component, rotating in opposite direction and in syn- chronism with A, relative to the secondary winding. C — Resultant of A and B. D — ^Torque of the same motor with polyphase secondary. However, when half synchronous speed of the rotor is reached a new condition enters. At this speed the secondary frequency has fallen to one-half that of the primary or supply circuit. The left-hand component of the secondary field is now traveling in the left-hand direction at half the speed it had when the rotor was INDUCTION MOTOR— SINGLE PHASE SECONDARY 519 stationary, but the primary field is also traveling left-handedly at the same speed in space, so that the two fields are still in synchronism. Considering next the right-hand component, this is traveUng at half speed in the right-hand direction, while the rotor is travel- ing at the same speed in the same direction. Therefore, the second- ary circuit for this right-hand field (the winding on the rotor) is now traveling in synchronism with it, and therefore no secondary current or torque can be generated. This, therefore, corresponds to full speed on the usual induction motor. The torque-speed curve up to half speed, therefore, may be indicated by the lower half of the curve shown in Fig. 4. At higher than half speed the stator frequency is still further reduced and the right-hand field travels at a stUl slower speed, while its secondary winding on the totor core is now traveling faster than its primary field, or it is running above synchronism. It therefore develops a negative torque. This negative torque above haK speed should show a negative characteristic somewhat similar to the positive speed- torque curve below half speed. This is indicated in Pig. 4. However, there is this difference; as the rotor speed approaches true synchronism, the frequency of the stator circuit approaches zero, until at full speed of the rotor (synchronous speed) the secondary frequency becomes zero. Under this condition the stator field and currents fall to zero, and there is no torque from either the right or the left-hand component field. Therefore, as this zero frequency is closely %,pproached, both torque ciurves rapidly approach the zero value. By combining the speed-torque curves for both component fields the resultant speed-torque curve may be plotted as in Fig. 5. It will be seen from this resultant curve that stable torque conditions are found at about half speed. Also, the starting torque is good. Above half speed the torque conditions are somewhat indefinite, depending upon individual circuit conditions, etc. In general, the rotor wiU continue to run at full speed, if first brought up to this speed by means of poly- phase secondary operation, and wiU pull considerable load with- out dropping to the lower stable speed. While the torque at start, due to the resultant of the two torque curves shown in Fig. 5, is larger than that due to the left- hand component alone, yet it must be borne in mind that this latter may be much smaller than the starting torque of the motor with polyphase secondary. Therefore, this method of starting 520 ELECTRICAL ENGINEERING PAPERS with one se condary circtiit only is not, in general, an improvement on starting with a polyphase secondary. To illustrate this, another speed-torque curve should be added, representing, in a general way, the conditions with the same motor if a symmetrical polyphase secondary winding is used. This is shown in Fig. 6.* In general, this will show better starting and also higher maximum or pullout torque than with single-phase secondary. To the experienced designer it will be obvious that such a motor is but one form of internal cascade, and a very poor one at that. A motor under such conditions of operation carries exces- sive currents, the primary winding carrying both primary and secondary currents, these being of different frequencies, except at standstill. The power-factor is also comparatively poor. *The curves C and D in Fig. 6 have^been plotted partly from actual test data. SINGLE-PHASE INDUCTION MOTOR 521 A PHYSICAL CONCEPTION OF THE OPERATION ^OF THE SINGLE-PHASE INDUCTION MOTOR FOREWORD — In the training of young engineers, directly from the technical schools, the author has found that very few of them have any conception of the opera- tion of the single-phase induction motor. In attempting to work out a simple method of presenting the problem, the subject matter of this paper was gotten together from time to time, and eventually it was written in its present form and presented before the American Institute of Electrical Engineers in April, 1918. It should be understood that the purport of this whole paper is to illus- trate the principles of operation and not to indicate a method of calculation. The usual methods of treating the single-phase induction problem are so mathe- matical that the average engineer or student cannot follow them. In the method given in this paper a knowledge of the characteristics of the polyphase motor is necessary, as the entire method is based upon the fundamental idea of a pair of polyphase machines operating with variable voltages and opposing torques. From this viewpoint, the various characteristics of the single-phase motor are explained in a non-mathematical manner. This paper is, therefore, almost entirely educatioilal in its purpose. It may be considered, to a certain extent, as supplementary to the author's paper on ' 'The Polyphase Motor," which was published twenty-one years before, and which is reprinted in the first part of this volume. — (Ed.) THE underlying principles and the operating characteristics of the polyphase induction motor are so well understood that it is found desirable to consider the, single-phase induction motor, simply as a special case of the polyphase. On this basis the single-phase motor must be considered primarily as a rotating- flux machine. Starting with the old assumption that a single-phase alter- nating magnetic field may be considered as being made up of two constant fields, each of half the peak value of the single-phase field and rotating at uniform speed in opposite directions, j^then Note — It should be understood distinctly that this paper is not to be considered as a presentation of new material for practically all of the underlying principles are old and relatively well known. It is simply an attempt to describe the operation of the single-phase motor in a way which may be easily understood by those not versed in the mathematics of the subject. 522 ELECTRICAL ENGINEERING PAPERS if the single-phase flux distribution is of sine shape and varies sinusoidally in value, it may be replaced, or represented, by two sine-shaped fields of constant value rotating in opposite direc- tions. This is the simplest case and allows a relatively easy explanation of many single-phase problems. However, when the flux distribution, or field form, due to the single-phase winding, is other than of sine shape, then the oppositely rotating components cannot be considered as of sine shape, but will assume certain varying forms as they rotate, the resultant of each instantaneous pair always giving the single-phase field corresponding to that instant. As other than sine-shape fields tend totvard complications in the physical conception of the single-phase induction motor actions, and lead more or less into the mathematical conception, ■the following analysis ' will be limited essentially to sine-shape distributions. As a starting point and to show reasons for certain later analysis, let us assume a single-phase induction motor operating at no-load, full speed, with its polyphase secondary winding short-circuited. The single-phase primary field, of assumed sine shape, is considered as made up of the two sine-shape equal components of constant value, and of half the peak value of the single-phase field, and rotating synchronously in opposite direc- tions. One of these fields is traveling in the same direction as, and slightly faster than, the rotating secondary. The slip of the secondary with respect to this field is of the same nature as in the ordinary polyphase motor. As the machine is carrying no load the secondary current corresponding to this rotating field is very small, being just large enough to overcome the rotational losses in the motor itself, and its frequency is equal to the slip frequency due to the forward field component. As there is an assumed backward flux or field component of equal value, the rotating secondary winding cuts this at almost double the frequency of the line. Stated exactly, the sum of the backward and the forward frequencies, in the secondary winding, is equal to exactly double the frequency of the primary supply system. The secondary winding cutting the backward field at this high frequency tends to generate a very considerable e. m. f. and, with the winding closed on itself, short-circuit currents will flow, which tend to damp out or suppress the flux which causes them. This secondary current will rise until its magnetizing effect is practically equal and opposite to the magnetomotive SINGLE-PHASE INDUCTlOy MOTOR 523 624 ELECTRICAL ENGINEERING PAPERS force which produces the backward field, which thus becomes almost zero in value. Consequently there are two distinct sets of secondary currents flowing, one of very small value and of a frequency corresponding to that of the forward rotation, and the other of very much larger value and of almost double the line frequency. Actual tests of the secondary circuit of a single-phase induction motor at small load, taken with an oscillograph. Fig. 1, show both of these currents as above described. Magnetomotive Forces and Magnetic Fluxes It is seen from the preceding that, right at the beginning of our analysis, a new condition is encountered, namely, the introduc- tion of a secondary opposing, magnetomotive force which rpacts on one of the primary field components and practically neutral- izes it. Also, there is a mixture of magnetomotive forces and magnetic fields, which is liable to lead to confusion. Obviously the introduction of the opposing secondary magnetomotive force rotating synchronously with the backward component- of the' primary introduces some entirely new features. Therefore, before going any further with the above method, it is desirable to set aside for awhile the viewpoint of two equal oppositely rotating fields and begin with a preliminary study of the magneto- motive forces and the magnetic fields resulting from them. It may be mentioned that while the assumption of twp op- positely rotating component fields, in place of a single-phase field, is well known and has been "used quite frequently, the correspond- ing analysis, from the viewpoint of magnetomotive forces, ap- parently has been but little used. When magnetomotive forces, instead of magnetic fluxes, are considered, then the single-phase primary magnetomotive force, fixed in position, can be replaced by two equal components of constant value, such as would be developed by « direct current, each of half the peak value of the single-phase, and rotating at synchronous speeds in opposite directions. , Returning again to our analysis, let us consider two funda- mental magnetomotive forces, namely, a primary single-phase one, fixed in position and varying sinusoidally and a secondary one of constant value, of half the peak value of the primary which rotates synchronously in one direction and which is in opposition to the primary in the position where the two coincide. Let us assume that the primary single-phase magnetomotive force is split into its two equal oppositely rotating components, SINGLE-PHASE INDUCTION MOTOR 525 then the results may be illustrated as in Figs. 2, 3, 4, and 5. In Fig. 2, C and D represent the two components forming the single-phase magnetomotive force A. At the position chosen, C and D are of equal value and coincide in position and polarity, B, which represents the secondary magnetomotive force, is also of half the peak value of A, but is of opposite polarity. It, therefore, neutralizes one of the components C or D, thus leaving a resultant of half the peak value of A . In Fig. 3, the component D has shifted- -thirty degrees to the left, while C has shifted an equal distance to the right. The .secondary magnetomotive force B is shifted thirty degrees to the left, thus neutralizing D and leaving only the component C. In Fig. 4, D and B are shifted sixty degrees to the left, while C is shifted sixty degrees to the right. In the same way, in Fig. 5, B and D have shifted ninety degrees to the left and C has shifted a corresponding amount to the right. Fig. 4 Thus from the above it is seen that a single-phase magneto- motive force, fixed in position and varying sinusoidally, and a constant magnetomotive force of half the peak" value of the single-phase, which is in opposition at the point of coincidence of position, and which rotates synchronously in either direction, will give a resultant constant magnetomotive force, rotating in the opposite direction, but which is of the same polarity as the single-phase magnetomotive force at the position of coincidence. 526 ELECTRICAL ENGINEERING PAPERS In other words, a single-phase magnetomotive force, fixed in position, and an opposing constant one of half the peak value rotating in either direction, will give a resultant rotating magneto- motive force equivalent to that of a polyphase induction motor. As a continuation of the above, the resultant magnetomotive force C could be replaced by a magnetic field or flux, resulting from such magnetomotive force. If this magnetic field is plotted to the same scale as the magnetomotive force which produces it, then C, in Figs. 2 to 5, can represent a magnetic field. This field will be constant in value and of half the peak value of the field which the single-phase magnetomotive force alone would set up. Thus according to Figs. 2, 3, 4 and 5, by the introduction of an "opposing" magnetomotive force, equal in value to one of the component magnetomotive forces of the single-phase and rotating synchronously with it, one of the two components of the mag- netic field can be suppressed and only the other component left. Fig. 5 the resultant is thus a rotating magnetic .field, just as in the polyphase induction motor. However, a further modification of this should be considered. Assuming again, that the single-phase primary magnetomotive force is replaced by its two equal rotating components, as in Figs. 2 to 5, then by the addition of an opposing magnetomotive force, similar to B in the same figures, but of less value than the com- ponent D, then the resultant of this opposing magnetomotive force and the component £> is a reduced magnetomotive force of the same polarity as D. There will then remain two magneto- motive forces, each of constant value, one of half the peak value of A and the other of some srnaller value, depending upon the opposing force B. These two rotating magnetomotive forces can, therefore, set up two oppositely rotating fields of unequal value. These are illustrated in Figs. 6, 7 and 8r In Fig. 6, B is assumed at some less value than the component D. The resultant of D and B is shown as E. Therefore, at this SINGLE-PHASE INDUCTION MOTOR 527 position C and E represent the two resultant magnetomotive forces and the two component fields. In Fig. 7, the conditions are shown for thirty degrees shift and here again E and C repre- sent the two fields. In Fig. 8 the shift is for sixty degrees. Thus by the introduction of a constant "opposing" magneto- motive force of less than either of the components of the single phase, two oppositely rotating fields of unequal value may be set up. As extreme cases of this, if the constant opposing mag- netomotive force is made zero in value, the magnetic field corres- ponding to its position and rotation will rise to the full value of the oppositely rotating field; and, on the other hand, if the constant opposing magnetomotive force is made half the peak value of the single phase, the correspondingly rotating field becomes zero. Both of these cases are in accordance with the 9arlier assumptions. The above conditions of the single-phase primary magneto- ^^^'^ B I ^•'^^ ^ I I Fig. 6 Fig. 7 motive force and a constant secondary one, in opposition, which may be of half the peak value, or some less value down to zero, and which rotates synchronously in one direction, resulting in two magnetic fields which may be of equal or unequal value, and which rotate synchronously in opposite directions, all form essential parts in the physical conception or visualization of the actions of the single-phase motor which will be given below. It should be observed that in the above method of considering the production of a rotating field in the single phase induction motor, the two primary components of the single-phase magneto- motive force and the secondary damping magnetomotive force all rotate synchronously, and such rotation is independent of the speed of the secondary core. In some methods of considering the single-phase induction motor problem, the single-phase primary winding is assumed to generate a magnetomotive force in the secondary which, by rotation of the core, is carried around until it generates a second magnetic field or flux at right angles to the •'i28 ELECTRICAL ENGINEERING PAPERS original primary flux, thus giving the equivalent of a polyphase magnetic field. - However, the above method does not involve such method of treatment. It should also be recognized that the foregoing analysis only covers no-load conditions and that with the addition of load new conditions are brought in to the problem. These, however, will be brought out later, for the no-load conditions requires further consideration, especially as regards the generation of the primary counter e. m. f. by the above described rotating fields. As already shown, there may be a single magnetic field rotating synchronously, or there may be two component fields of equal value rotating in opposite directions, or there may be inter- mediate conditions of oppositely rotating fields of unequal value, depending upon the value of the damping or opposing secondary magnetomotive force. Fig. 8 Counter E. M. P. Generation and Excitation Considering next the counter e. m. f . generated in the primary, we should first look into the e. m. f. conditions produced by two oppositely rotating fields of equal values. If the secondary circuits are open, the two component fields are both present and are concerned in the generation of the counter e. m. f. This is true whether the secondary is stationary or is rotated at full speed. If, however, the secondary is closed upon itself, then when running at speed, one of the component fields is practically damped out and the other must generate the entire primary counter e. m. f. Thus, two entirely different conditions are encountered, depending upon whether the secondary is open or closed. To explain this properly, some further analysis is re- quired', as follows: In the first place, it may be stated that the e. m. f., produced in the primary winding by cutting its two component fields, is the same as that generated by the single phase sine shape field, varying sinusoidally and acting on the primary winding as in a SINGLE-PHASE INDUCTION MOTOR 529 transformer. Herein lies a simple illustration of the equivalence of the transformer and the flux cutting methods for calculating e. m. fs. In Figs. 9, 10 and 11, are shown several positions of the two oppositely rotating fields and their relation to the primary winding. In Fig. 9 is shown the magnetic flux, or field, A , which is set up by a primary winding a. This winding, of course, would FiF. 9 require a tapered distribution to give such a field. This is mentioned incidentally as it has no direct bearing upoii the explanation, except from the mathematical standpoint. Assuming the single-phase field at its maximum -or peak value, then, at this instant, the two component fields, B and C, each of half the peak value, will coincide both in position and polarity. From the transformer method of calculation, the e. m. f. gener- FiG. 10 ated at this instant, in the winding, will be zero, as the rate of chaiige of the flux is zero. Also from the flux cutting method, the e. m'. f. in the primary winding will be zero, for, as is evident from the figure, each belt or group of the primary winding is cutting fields which have equal positive and negative areas or values. Considering liext the conditions in Fig. 10, in which the two rotating components have traveled ninety degrees. The fields 530 ELECTRICAL ENGINEERING PAPERS are shown as B and C. It is evident that the resultant of these two fields is zero in value, that is, the single-phase field is passing through its zero value, and, accordingly, is generating the maxi- mum e. m. f. by the transformer method. Also, considering component B of the rotating fields, obviously, by the cutting method it is generating maximum e. m. f. in the winding a. Also, component C is generating maximum e. m. f. in winding a. However, as one of these fields is positive in this position and is traveling in one direction, while the other field is negative and is rotating in ther)pposite direction, the two e. m. fs. will be in the same direction, and thus will be added. Thus, from the figure, this position will give the maximum e. m. f. in the winding b^^ the cutting method. It can be shown by calculation that this maximum value is the same with either the cutting or the trans- former methods of considering e. m. f. generation. This shows that both of the component fluxes must be taken Fig. 11 into account in generating the total primary e. m. f., and if either component is decreased in value or suppressed, the total e. m. f. generated in the winding will be decreased correspondingly, unless the other component is increased a corresponding amount. Fig. 11 is simply a continuation of the conditions of Figs. 9 and 10, in showing an intermediate position of the component field. The result is the same as if the two fields were momen- tarily replaced by the field D. According to the above analysis, to produce a given counter e. m. f. in the primary, with one of the component fields damped out, the other component must be doubled in value. It was shown before that in the single phase induction motor, running at. full speed with no load, the backward field is practically damped out by the secondary current. Thus with only the forward component field remaining, either the counter e. m. f. will be halved or the forward flux component must be doubled, the latter being the case. This means, in turn, that the primary SINGLE-PHASE INDUCTION MOTOR 531 fnagnetomotive force must be doubled in value." In other words, suppressing one of the two rotating field components results in doubling the no-load excitation of the motor. Furthermore, doubling the magnetomotive force of the primary and thus doub- ling the forward component of the field also doubles the backward component, which, in turn, is suppressed by doubled secondary current. The above conditions of doubled excitation is on the basis of sine flux distribution. With other distributions the same result holds approximately, but not exactly, due to conditions involving the shape, of the field. It is evident from the above that, with the secondary circuits open, the excitation required is of constant value regardless of the speed of the rotor core and windings; also when running at speed, the primary excitation is doubled as soon as the secondary circuit is closed. However, it is not obvious, on first considera- tion, that even with the secondary circuits closed the primary excitation falls to half the full speed value, when the motor is brought to standstill. This involves load conditions which will be treated later, but nevertheless this feature may be brought out at this time. The explanation lies in the fact that at rotor standstill the damping action of the secondary current will be exerted equally on both the forward and backward components of the primary field, so that necessarily these must be maintained at equal value, and, by the above analysis, this requires but half the excitation, compared with the no-load full-speed condition where the backward field is practically completely suppressed. Load Conditions When the single-phase induction motor is loaded, the total ini)ut current can be considered as made up of two components, namely, the no-load (practically all magnetizing) and the load current. This latter is simply the increased current in the pri- mary due to the load and does not entirely represent energy. , Tliis load current, being single-phase, may be represented by two equal oppositely rotating magnetomotive forces in the primary o'f the motor, just as in the case of the no-load current. The fields which these two magnetomotive forces tend to set up are both practically suppressed by two equivalent secondary mag- netomotive forces rotating in opposite directions. The forward secondary component corresponds to the secondary load mag- netomotive force in the polyphase motor and the interaction between this magnetomotive force and the forward primary field 532 ELECTRICAL ENGINEERING PAPERS develops torque just as in the polyphase motor. The backward component, at first thought, would appear to develop an opposing torque, corresponding in value to that of the polyphase motor at approximately 200 per cent slip. This, however, is not the case, for at this slip the ordinary polyphase motor takes an excessive primary current tending to develop a large magnetic field, which is suppressed by a correspondingly large secondary magneto- motive force. In the single-phase induction motor, howfever, the primary backward rotating magnetomotive force component, due to the load current, can be only of the same value as the forward. Tiiis fact must be borne in mind as it is a very import- ant factor in the later analysis. To illustrate the characteristics of thfe single-phase induction 60 5 100 S. £ 120 140 160 200 • — zz. ""^ -~ --, ^ / ) ) B / / /-c / A / ,/ / / / / y / / y y / / / / / / / X / c (i / / ^ 1 C ::;: :::2ir- / Q 10 20 30 40 50 €0 70 80 -TOSQUE Fig. ,12 motor, it may be compared with the action of two polyphase induction motors rigidly coupled together, .and connected to the line to give opposite rotations. Such a set or unit has certain characteristics which are so similar to those of the usual single- phase induction motor that on first consideration one would assume them to be identical. However, a more careful study oi the individual operating conditions shows that the similarity is only a general one, and a number of decided discrepancies are found. The characteristics of the above two-motor unit and the single- phase motor may be compared as follows: (1) The speed torque characteristics of the two motors of the polyphase unit may be represented by A and B in Fig. 12 and SINGLE-PHASE INDUCTION MOTOR 533 their resultant by curve C. According to this latter curve, the resultant torque is zero at standstill, and a slight change in speed in either direction will give an effective torque tending to speed up the unit in whichever way it is started. This, therefore, corresponds to the well known starting characteristics of the single-phase motor. (2) It may also be seen that the maximum torque the unit develops is materially less than that of either of the two corti- ponent motors. This fact is also consistent with single-phase motor operation compared with the same machine on poly- phase. (3) At full speed, according to this resultant curve, the slip 60 140 160 180 200 / ^ M -^ / \ ^ N, v f > / / A J / / / / / / / / / / ( / \ \ / ~— / 10 20 30 40 50 60 70 80 TORQUE Fig. 13 for a given torque is very much larger than that of the corre- sponding polyphase motor. This is not true of the single-phase motor and herein lies one of the discrepancies in this method of illustrating the operation. (4) It is well known that in the polyphase motor the maxi- mum torque it can develop, with constant voltage applied, is independent of the secondary resistance; while, in the single- phase motor, in general, an increase in the secondary resistance will decrease the maximum torque and a decrease will have the opposite effect. This may be illustrated by repeating the curves of Fig. 12 with modified secondary resistance in the two com- ponent motors. In Fig. 13 the secondary resistance is increased and in Fig. 14 is decreased relatively to that of Fig. 12. The 534 ELECTRICAL ENGINEERING PAPERS resultant speed-torque curves for the three figures show that the maximum torques are materially affected by the secondary re- sistance. The same holds true for the single-phase induction motor. 100 120 140 160 — ■ _ — >1 \ y /' ) B / /• y c / /'A / ./ / / / / y / / y / / / y / y / 1 y ^ A y / c y ( ^ / V / 10 20 30 40 50 60 TORQUE Fig. 14 70 80 (5) However, this method of illustrating the characteristics of the single-phase motor torque fails when the conditions of secondary resistance is such that the maximum polyphase torque 20 60 a. S 80 z 100 UJ O £ 120 o. 140 160 180 200 / ~~- ■^ ^ / \ h \ N A / \ \ / 1 / / \ \ / \ \ / \ / " -~^ __^ / 10 20 30 40 50 60 70 80 TORQUE Fig. 15 is developed at about 100 per cent slip. Fig. 15 illustrates this. From this speed torque curve it appears that the unit has a very low resultant torque, but this is not the case in the single-phase SINGLE-PHASE INDUCTION MOTOR 535 induction motor, for with a polyphase motor developing its maximum torque at 100 per cent slip, the same machine on single- phase will give a very considerable maximum torque. Herei again is a discrepancy which the assumed equivalent arrange- ment does not cover properly. (6) , In Fig. 16, the current-torque curve D, for the component motors in the above figures, is shown. This indicates plainly what a wide discrepancy there is between the currents taken by the primaries of the two motors when running at speed. For example, at a given speed a, the current taken by the forward rotating motor is b, while c represents the current taken by the backward motor. Obviously, the current taken from the line, which is the resultant of b and c, is much greater than that re- quired to produce the resultant torque a.nd the power factor of such a unit must necessarily be very poor. However, such is not the case with the single-phase motor, for the inputs and the power factors are not greatly different from those of polyphase motors of the same capacity. Herein lies a radical difference between the single-phase motor \ and the above assumed unit. (7) Another difference be- tween such a unit and the true single-phase motor lies in the no-load or magnetizing input. Obviously, the combined magnetizing components for the two motors will be twice as great as for a single machine, whereas, in the single-phase motor the magnetizing input is practically the sam.e as in the corresponding polyphase machine. Here is another pronounced discrepancy. It is evident from the above that while this method of illus- trating the action of the single-phase motor by means of two polyphase motors, coupled for opposite rotation, is in the right direction, some special modifying conditions must be introduced to account for the discrepancies. The action of this two-motor unit, therefore, will be followed up further, with the introduction of certain modifications derived primarily from consideration of certain characteristics of the single-phase induction motor itself In the first place, curves A , B and C of Fig. 12 were based upon equal and constant e. m. fs. applied to the terminals of both — c n — 1 1 Spee< 'To, ?n a f 7Ue ) 40 \ X \ ff\ tot< J^ ) Rn c« vterf \ /' 100 ' \ / TORQUE Fig. 16 536 ELECTRICAL ENGINEERING PAPERS motors. That this is not a correct assumption can be deter- mined from the operating conditions in the single-phase motor. From the analysis of the component rotating fields it was shown that at full speed the backward component was practically damped out by a secondary magnetomotive force, thus leaving only the forward component, which then rose to practically double value in order to generate the required e. m. f. However, at standstill, the secondary winding holds the same rotational relation with respect to both component fields and, therefore, neither fidd can be damped out more than the other. Conse- quently, at standstill, both component fields are equal in value and the counter e. m. f. of the primary is generated by the two oppositely rotating fields, instead of a single one of double value 20 3 80 100 120 140 160 200 / / - _ rt~~ ^ p<. >"' . / \ 4 ^ \ 1 / / / b ^ ^ / B/ E / y 1>'^ /A. / / / / V / / / j> / / r / / / / / / / / / ( / ^ y / / / - I ( V _v. ^ _ _. 10 20 30 40 50 60 70 80 TORQUE Fig. 17 as is the case at full speed. Therefore, at standstill, the forward field is of only half the value of the full speed field. This corre- sponds to the operation of the polyphase motor at half field strength, that is, with half ihe primary voltage applied, thus re- quiring one-quarter the magnetizing input. The same voltage condition applies also for the backward component at zero speed. It would appear, therefore, that in the unit composed of two polyphase motors coupled together, the voltage applied to the terminals of the forward motor should be at practically full value at synchronous speed and should fall to half value at standstill or 100 per cent slip, and should have practically zero value at ■ 200 per cent slip. Then assuming, as a first approximation, that the decrease in voltage from full speed to 200 per cent slip SINGLE-PHASE INDUCTION MOTOR 537 is. a straight line law, new speed torque curves, corresponding to Fig. 13, but with the torques decreasing as the square of the voltage, can be illustrated as in Fig. 17. Here curves A and\5 correspond to Fig. 12, while. D and E correspond to the above proportionate reductions in voltage. The resultant F of these latter curves is also shown. This new resultant F is similar in general shape to C of Fig. 12, but indicates some quite different characteristics. For instance, at the higher speed values it coincides quite closely with the polyphase speed torque curve, which is actually the case in the single-phase motor. In the second place, with high secondary resistance, as shown in Fig. 15, the speed-torque curves are modified as in Fig. 18, which shows both the former characteristic 60 3 80 1100 il20 140 160 180 200 ^ ^ / / \ ^ ^ / /. )) \ B/ E / \'' 7^ /d \a ( / y y / 1 1 1 / y ^1 / 1 / \ / / / \ \ / f / / V // / / 1 1 v.^ ^^ v(. / 1 1 ^ -A , I 10 20 30 40 50 60 70 80 TORQUE Fig. 18 and the new one. Here the resultant torque, under the new assumption is materially higher and' more nearly conforms with the condition in the single-phase motor. Under the earlier assumption of constant voltage on both motors, it was shown that the magnetizing current would be twice as great as in the single-phase motor. On this new assump- ' tion, however, at full speed, with practically full voltage on one motor and zero voltage on the other, the total magnetizing cur- rent will be only half as great, and will approximate that- of one motor alone, and, therefore, that of the single-phase motor. Furthermore, under the new assumption, the current taken by the primary of the backwardly rotating motor is quite small at high speed and, therefore, the resultant current taken from the 638 ELECTRICAL ENGINEERING PAPERS line is not excessive and is more nearly consistent with actual single-phase motor conditions. Thus, with this new condition of reduced terminal voltage with reduction in speed, practically all the conditions of the single- phase motor are met, except possibly from the quantitative view- point. The two-motor combination thus serves as a very good illustration. There is, however, one further condition which must be rigidly met if the new curves are to be reasonably exact, namely, the primary currents taken by the two motors must be equal, for, as shown in the early part of this analysis, the forward and backward rotating components of the primary current in the single phase induction motor are equal at all times. Conse- quently to duplicate this condition, the primary e. m. fs. im- 3 « 40 ee £0 80 100 c ^ "^ — -£^ N a T" rr- rr^ ^ ^ > Cl.^ 1 ^ ■sJCl Al. \ I r x^ •li)i b\ ^ "^b / ri/ ^ "^ \ / :^ \ V TORQUE Pig. 19 pressed upon the terminals of the two polyphase motors should be varied in such a way that the primary currents will always be equal. In addition, it is assumed that the sum of the two im- pressed voltages is constant. This, however, is only an approxi- mation. The next step is to determine what is the actual law of voltage variation which will satisfy the above conditions of current and voltage. A ready means for obtaining this lies in the speed- torque and current-torque curves of the polyphase motor at constant voltage. From the current-torque curve at constant voltage corresponding curves for any other voltage can readily be plotted by varying the abscissae as the square of the voltage and the ordinates directly as the voltage. This is illustrated in Fig. 19. Here A is the polyphase motor speed-torque curve at SINGLE-PHASE INDUCTION MOTOR 539 constant voltage. B represents the part below the 100 per cent slip line, but turned above the zero speed line for convenience. B can also be considered as the back torque at full voltage, but thrown to the right of the zero torque line for convenience. Curve C represents the primary current for full voltage condi- tions. Then at a speed a, for example, the primary currents corresponding to the forward and back torque will be b and c respectively. Assume next that the voltage is halved for both rotations, then the new speed-torque curves will be Ai and Bi in which the tor- ques are reduced as the square of the voltage. The new current curve will be Ci. The currents for speed a will now be 61 and Ci, or half of b and c, as they are varied as the voltage. The above figure is simply to illustrate the rule for variation of the primary current with the voltage, in the polyphase motor, and does not represent the actual conditions which we are after; for in the above the voltage reductions are the same for both the forward and the back torques. But, according to our former analysis, this condition of equal voltages, for the two rotations, holds only for the 100 per cent slip point. For other speeds the two voltages are reduced unequally, but with the sum of the two approximately constant according to the assumptions. If, for any speed a, we let x represent the percentage of voltage reduction for the forward torque, then \ — x will represent the corresponding reduction for the back torque. Let If be the primary current, corresponding to the forward torque for this speed at full voltage, and /;, the current for the back torque at the same speed and also for full voltage. Then IjX will represent the primary current at the reduced voltage for the forward rotation and h (1 — *;) will be the primary current for the back rotation. One of the conditions of our two-motor unit, to make it correspond with the single-phase motor, is that these two primary currents must be equal. Therefore, Ijx =^ h {I — x), and x = . ]_ J and {I- x) = j ■ f_ . •■■ The above allows the determination of the percentage x of full voltage which must apply for each speed between zero and synchronism, when the values of the current // and h for full voltage are known. A second method of determining the percentages of voltage 5 to ELECTRICAL ENGINEERING PAPERS for the two rotations is available when the speed-torque curve o-f the motor on single phase has been determined, by test or otherwise. By our former assumption this single phase torque is the difference between the speed torque curves for the forward and backward rotations with the respective voltages reduced the proper percentages. These torques for any given speed vary as the square of the terminal voltage. For example, calling T/ the forward torque, at full voltage-and speed a, and Tb the back torque, and Ti the single phase torque for the same voltage and speed, then T/x^ — Ti, {1 — x)^ — Ti, from which x may be determined, with 7/, Th and Ti known. It would appear from the preceding that, if the assumptions made are anyways close to the actual conditions, this method of analysis shows a means for deriviiig the single-phase speed- torque curve from the polyphase curves of the same machine. Methods of calculating the primary current and speed-torque characteristics of the polyphase motor have been developed quite completely, so that it is not necessary at this place to give any details of such methods. The accuracy of the methods for calcu- lating the polyphase curves depends almost entirely upon the correct determination of the reactance and saturation constants. All methods for the direct determination of the single-phase' speed-torque characteristics also involve the use of corresponding' reactance and saturation constants. Therefore, the above method brings in no new and more difficult conditions. The primary object of this paper, however, is not to develop a new method of calculation, but simply to give a better conception of the close relation of the single-phase and polyphase characteris- tics. After development of the above method, an attempt was made to check it by applying certain existing test data, but without positive results, although the indications were quite satisfactory. It was discovered that in all the existing test data at the -writer's command, where the polyphase speed-torque and current- torque curves has been obtained by actual test, constancy of temperature had been more or less disregarded. The effect of change in the secondary resistance on the polyphase speed- torque curve is to change the slips but not the maximum torque. The difficulty, however, in the polyphase tests available was that apparently the resistance had varied very considerably during the tests, especially at the points of high sHp, where the second- ary losses were very large. As a result the speed-torque curves SINGLE-PHASE INDUCTION MOTOR 541 60 5 80 100 corresponded to those of motors in which the resistance increased as the Ipad and slip increased. As a consequence, the torques below the zero speed line were considerably too large, which meant that in applying these curves to the above method, the back torques were presumably entirely too great, thus apparently introducing errors in the derivation of the resultant single-phase curve. The effect of these discrepancies are shown in Fig. 20. Here, A shows the speed-torque curve as it should be at constant tem- perature,, whereas, B shows the curve with the resistance of the secondary increasing with increased slip. The corresponding current-torque curves are also shown. A consideration of these curves would seem to indicate that the resultant single-phase curves derived from A and B should differ somewhat. It was then decided to make a more accurate set of tests on a 10 h.p., 60-cycle four-pole, three- phase motor of the wound- secondary type, so that the > secondary resistance could be | varied if so desired. It was also '^ decided to obtain a test with two similar motors rigidly coupled together, with their individual primary windings in series, but with their secondaries indepen- dent. As already explained, the theory of the foregoing method calls for equal currents in the two oppositely rotating fields. This condition is automatically obtained by coupling two primaries in series with each other.* With this arrangement, if the power factors of the two motors were always equal, then it should be equivalent to the method already described. However, these are practically never equal except at the standstill position, although an analysis of the problem shows that the two primary voltages, with this series arrangement, are not greatly out of phase with each other over a very large part of the working range. The writer has not yet sufficiently analyzed the series arrangement *In reviewing an early draft of this paper, this suggestion, with a number of other most excellent ones, was made by Mr, R. E. Hellmund. However, it developed later that this same suggestion appeared about twenty years ago in Mr.B. A.Behrend'sbook,' 'The Induction Motor. 120 140 160 180 200 N C^ ■\ r — . ) \ 1 ^ >{ B ^ J C»' vei>^ r \, s' 1/ "^ \ // / I 1 / 1 1 y B / / 10 20 30 40 50 TORQUE Fig. 20 60 70 80 542 ELECTRICAL ENGINEERING PAPERS to be sure that it exactly represents all the conditions of the two rotating fields in the single-phase motor, but is inclined to think that such is the case. However, the approximate method developed in this paper lends itself so readily to calculation, that it was considered worth while to check it up carefully by test to see what degree of accuracy could be obtained. The following series of tests was planned: (1) Three-phase speed'torque and primary current curves at 220 volts with one motor alone, with its secondary short- circuited on itself. (2) Single-phase speed-torque and primary current curves on the same motor as (1) at 220 volts and with the secondary short-circuited on itself. (3) Three-phase speed-torque and primary current curves on the same motor and at same voltage, but with external resistance in the secondary circuits. (4) Single-phase speed-torque curves under same conditions as (3). (5) Speed-torque and primary current curves with two similar motors .with their primary windings coupled in series, and with the secondaries independently short circuited on themselves, one of these motors to be that used in tests (1) and (4). (6) Similar tests to (5), but with resistance in the secondaries as in (3). In carrying out these tests, the torque was measured by a special dynamometer brake, the power, absorbing element of which consists of a special separately-excited direct-current machine. Below zero speed, power was supplied to the direct- current, machine in order to obtain negative rotation. Difficulties in obtaining consistent tests, especially at negative speeds, soon developed, due to variations in temperature. With the very heavy currents at low and at negative speeds, the motor would heat so rapidly that all kinds of speed-torque readings could be obtained. Test after test was made and while these would agree very well for the higher speed points where the heating was small, they showed all kinds of inconsistencies for the negative speeds, in particular. The currents for these speeds also showed very wide discrepancies. Eventually it was found that those tests taken with extreme rapidity, and which covered only a comparatively small number of points, would plot in quite reasonable curves above zero speed, so that the writer was enabled thus to obtain quite consistent curves SINGLE-PHASE INDUCTION MOTOR 543 for both torque and current between 1800 rev. per min. and standstill. Not only were the curves, consistent in themselves but those taken with different secondary resistances were fairly consistent with each other. It then remained to obtain rea- sonable readings for the negative speeds. Obviously it was wrong to take a large number of test points and then draw an average curve through them, for it is evident that the er- rors, due to heating, tend to throw the torques and currents to one side of the proper curves. Consequently the correct curves should really be boundary lines rather than averages. It was noted, in particular, that heating did not appear to affect the speed to the same extent as the torque at very large slips, and, consequently, by plotting the current in terms of speed rather than torque, less erratic curves were obtainable, and it was possible to plot speed-current curves which were quite con- sistent for the different conditions of secondary resistance. Furthermore, from the speed-torque and speed-current curves above the zero line, which appeared to be reasonably correct, as they were consistent with each other, it was possible to de- rive the constants for the general equations for speed-torque. It was found that such derived equations fitted these curves quite accurately and, moreover, they held the proper relation of constants for both high- and low-resistance secondaries. The various agreements between the calculations and the tests for the higher speeds were such that one could assume that the derived equations were practically correct and that from them the curves for the negative speeds could be plotted with fair accuracy. In this way the curves for the negative speeds were first obtained and it then remained to check them by actual test. Finally a method of testing was tried which appeared to give quite good results. This consisted in setting the apparatus at about the desired speed and torque conditions ; then cooling the motor down to the required temperature preparatory to obtaining the desired test, the power was then thrown on and readings obtained in the shortest possible time, five seconds, for instance. Allowing the motor to run, additional readings were obtained at five second intervals. A series of consecutive readings, at definite intervals apart, was thus obtained and plotted in a curve. By extending this curve back to the instant of starting, results were obtained which were undoubtedly quite close to those corresponding to the starting temperatures, and were not only quite consistent with each other, but also plotted very close to the negative exten- ,344 ELECTRICAL ENGINEERING PAPERS sions of the calculated curves. As a result of a series of tests extending over several weeks, data was obtained which plotted in curves which agreed fairly well with each other throughout. 20 40 160 60 140 80 120 3 100 "- 100 S 120 "^ 80 140' 160 60 40 180 20 200 ^^ O 1 —° D. b^ ^ ^ o Points on C Points off ( ui-ve i^urve without External Resistance ^^P^ Speed Torque, ^/^ ^ ^ ) ^^>. .Oo ^"*'**^ X V ° ^ 1 100 Per ;ent Slip -, / 1 / / l r / ^ ^ ^ ^ M-,.«-.-xC0 isecutive ids Readini / SSecor ■ / 1 10 20 30 40 50 torque in pounds Pig. 21 60 70 80 Results of Tests Polyphase Speed-Torque, Speed-Current and Current-Torque Curves In Fig. 21 are shown the polyphase speed-torque and primary current both with the secondary short-circuited, and with re- sistance added. In the speed-torque curves the circled points SINGLE-PHASE INDUCTION MOTOR 545 torque test was selected in which no correction had been made for temperature and where the conditions were quite closely comparable with those of the single-phase tests. From the speed-torque and current data of this polyphase test, the re- sultant single-phase speed-torque curve was calculated, making no attempt at corrections of any sort. This speed-torque curve is represented by the small squares in Fig. 23. This lies much closer to the tested single-phase curve, thus indicating that tem- perature is possibly an explanation of a considerable part of the discrepancy between the calculations and the tests. This would TABLE II. Primary Reduced Slip Torque at amperes per leg torque Re- full voltage at full voltage X = sult- ant tor- For For posi- nega- lb For- que tive tive Tf Tb 'f lb l-x ward Back If+lb speeds speeds 0.02 1.98 8.2 64 3 12.5 134.2 0.915 0.085 6.9 0.5 6.4 0.05 1.95 18.9 64.8 18.0 133.8 8.881 0.119 14.7 0.9 13.8 0.10 1.90 33.3. 65.4 28 133.0 0.826 0.174 22.5 2.0 20.5 0.15 1.85 41.5 66.1 37.0 132.3 0.781 0.219 27.5 3.2 24.3 0.20 1.80 53.1 66.9 46.0 131.5 0.740 0.260 29.3 4.5 24.8 0.2.5 1.75 59 8 67.6 53.0 130.8 0.712 0.288 30.3 5.6 24.7 0,30 1.70 65.1 68.4 60.0 130.0 0.684 0.316 30.5 6.8 23.7 0.35 1.65 69.2 69.1 66.0 129.0 0.662 0.338 30.4 7.9 22.5 0.40 1.60 72.2 69.9 71.2 128.0 0.643 0.357 29.9 8.9 21.0 0.50 1.50 76.5 71.4 81.0 125.5 0.608 0.392 28.2 11.0 17.2 0.60 1.40 78.7 72.9 88.0 122.7 0.582 0.418 26.6- 12.7 15.9 0.70 1.30 79.6 74.4 94.0 120.0 0.361 0.439 25.0 14.4 10.6 0.80 1.20 79.6 75.9 99.5 118.1 0.542 0.458 23.4 15.9 7.5 0.90 1.10 79.2 77.8 104.0 112.2 0.519 0.481 21.4 18.0 3.4 1.00 1.00 78.9 78.9 108.2 108.2 0.50 50 19.7 19.7 also indicate that heat effects as referred to in connection with Fig. 20 are not as objectionable as anticipated However, the \yriter does not believe that all the discrepancy is due to heating, but considers that this approximate method of dealing with the problem makes the back torque too small. In the arrangement with two motors in series, as mentioned before, the voltages of the two motors will not usually add up directly to give the line voltage, and the motor which represents the back torque, will have a relatively larger percentage of the total voltage than is ihe case with the above method of considering the problem. 'J1iis will be considered further under the two-motor tests. 546 ELECTRICAL ENGINEERING PAPERS Unfortunately, due to the very short time available, it was not possible to make any extended tests on single phase with a view to correcting for temperature. In consequence, the calculated single-phase speed-torque curve, which is on the basis of constant ternperature, is compared with tested curves in which no temperature correction has been made. It, therefore, is noi- known in this case how much of the discrepancy is due to temperature. In Table II is shown data similar to that of Table I, but for the tests with resistance in the secondary. It will be noted that the resultant of the forward and back torques is considerably lower than in Table I, which is consistent with the fact that in- 20 40 a 60 80 100 10 20 30 -40 50 torque in pounds Fig. 22 60 70 80 creased secondary resistance reduces the maximum torque of the single-phase motor. In Fig. 23 is showri the calculated single-phase speed-torque and the tested torques of the motor with resistance in secondary. Here the circled dots represent the actual test readings and the crosses represent the points obtained from the last column of Table II. The discrepancies are somewhat smaller than in the motor with short circuited secondary. This should be the case, if heating is responsible for any considerable part of the dis- crepancy, for the currents are relatively smaller. In order to get a crude idea as to how much of the difference may be due to this feature of temperature, a polyphase speed-' SINGLE-PHASE INDUCTION MOTOR 547 represent actual test readings, while, the solid line covers the points calculated from the derived equations. In Table I, covering data on the short-circuited-rotor tests, are shown the forward and back torques and the corresponding forward and back currents for the various speeds between zero and 200 per cent slip, as derived from Fig. 21 ; also the calculated values of the ratio of voltages, x and {\ — x), by which the for- ward and back torques should be reduced in order to get the equivalent single-phase speed-torque curve. The corresponding reduced values for the forward and back torques are also given as calculated from the values x and (1 — x). The last column shows the difference between the reduced forward and back torques, which should represent the single-phase torque, accord- ing to the foregoing analysis. TABLE I • Primary Reduced Slip Torque at amperes per leg torque Re- £uU voltage at full voltage X = sult- ant tor- For For posi- nega- lb For- que tive . tive Tf fh V h If+lb 1-x ward Back speeds speeds 0.02 1.98 20. 35.8 19.0 154.8 0.89 0.11 15.8 0.4 15.4 0.05 1.95 41. 36.3 34.0 154.5 0.819 0.181 27.5 1.1 26.4 0.10 1.90 61.7 36.9 55.5 154.0 0.735 0.265 33.3 2.6 30.7 0.15 1.85 71.6 37.7 71.0 153.5 0.684 0.316 33.5 3.S 29.7 0.20 1.80 77.3 38.3 85.0 153.0 0.643 0.357 31.9 4.9 27.0 0.25 1.75 79.4 39.2 96.0 152.5 0.614 0.386 29.9 5.9 24.0 0.30 1.70 79.6 39.9 104.0 152.0 0.594 0.406 28.1 6 6 21.5 0.35 1.65 78.8 40.8 110. 151.5 0.580 0.420 26.5 7.2 19.3 0.40 1.60 77.6 41.6 113.0 151.0 0.572 0.428 25.4 7.6 17.8 0.50 1.50 74.0 43.6 121.0 150.0 0.554 0.446 22.9 8.7 14.2 0.60 1.40 70.0 45.5 128.0 149.0 0.538 0.462 20.3 9.7 10.4 0.70 1.30 65.9 47.8 133.0 147.3 0.526 0.474 18.2 10.8 7.4 0.80 1.20 62.1 50.0 136.5 145.5 0.516 0.484 16.6 11.7 4.9 0.90 1.10 58.8 52.7 139.5 143 5 0.507 0.493 15.1 12.8 2.3 1.00 1.00 55.5 55.5 141.5 141.5 0.50 500 13.9 13.9 In Fig. 22 are shown the single-phase speed-torque and cur- rent-torque curves with short-circuited secondary, as plotted from Table I, and checked by actual test. The circled dots represent actual test points, while the crosses represent points plotted from the last column in Table I. The agreement of test and calcu- lated values are as close as can really be expected considering the difficulties in obtaining the data, and the possible errors. 548 ELECTRICAL ENGINEERING PAPERS Two Motors in Series In Table III is shown the test data and the calculations de- rived therefrom, for two motors with their primaries in series and with their secondaries short-circuited independently. In this test no external resistance was used in the secondaries. Considerable difficulty was encountered in making this test, due partly to bad alignment of the machines, as they were rigidly coupled together. Furthermore, in several of the earlier tests, the effects of temperature were disregarded and all indications were that the secondaries were quite hot during the tests. There was so much discrepancy between the various results that the 20 40 60 80 100 -^ ^ \ r • ^ -A V v\ // ./ V f / V 10 20 30 40 50 TORQUE IN POUNDS Fig. 23 ,60 70 80 writer cannot feel sure of the data shown in this table, although it was obtained under quite careful conditions of test. In the above table the percentage of line voltage applied to each motor is shown. It is of interest to compare these per- centages with those shown in Table I. This is illustrated in Fig. 24. This shows that the percentage of voltage on the forward rotating motor is higher at the higher speeds, than in Table I, but is lower at the low speeds. On the other hand, the voltage . on the backward-rotating motor is higher at all speeds than in Table I. Thus, the back torque has always a higher value than in Table I. Consequently, with the reduced forward torque at the lower speeds and the higher back torque,- the resultant torque SINGLE-PHASE INDUCTION MOTOR 549 II 1° >OO«^Q0WC00000b-'J*COCOO0J • CSlt0OD|T}OlOO>-oooo^-•eo■*p^<^^woo • ■s o eDiOOS(NOJ«D>OOJC0^*-tiOCOOlC0 ■ & ■*ow-^-*mcoiocooi>eoot*'*M o OO>0O0S(NC000»OOOi0OOO - «OcOt-I>t^l>t>l>.I>.(DOiO-*cO<-i - Is .2 = 1° 6 2: i § S § 52 g S 5 S 8 S C: §5 SS S 2 g oooooooooooooooo l-l M 6 2 1 1-1 pq < oooooooooooooooo 42 s2 d 2 1 lOiOOOOiOOOOOOOWiOOOO Ob.OU3(NOO'i*OOt-I>cocqt»(Dwt-uacaoo Ort.H,-l(N(N(NC0i0i0(Dt^000>OO a lOQiOOOOOOOOOOOUSOO Q0t^000sO)0>0>OOOl-^r^^^^C05^ H o « g -^ «" ^' «> » 2 2 S S' S E5 S S' 2 2 : •0 CO ccSio3i«m(MN'-;'-;'-;oooc>o oooooooooooooooo d oooooooooooogooo ooooooogoqoMcop^ooo 650 ELECTRICAL ENGINEERING PAPERS derived from the polyphase curve will naturally be lower than in Table I, which appears to be the case in all the tests made. The data in Table HI indicate that the two motors have their 20 40 60 o- 80 -J 0)«N^r-ieo-^fOooioao si li §••0 o Lieowcoot^c^ooi-HOCMOO'^ 00b-eO**C 11 o p4 t-iOiOOT-HiOOOOiCOiCCOCOO eDir5co(NOOOI>(DCDvOTH- o 'i eg 6 1 S § g « S E: S 2 K S S§ g S 0000000000000 > .J m < 6 1 CSlC0iCt--'-iCD0JT(<0>-tfi^t^r^ 5b2 ELECTRICAL ENGINEERING PAPERS ages applied to the polyphase torques. Part of this difference may be due to temperature conditions. 40 60 ^ 80 t/i 2 100 UJ O o: S 120 140 160 180 ZOO >• ^ P"^^ Two-Motor Unit Forward Motor N / r / f / ,* //* / yi ^^ ^ 10 20 30 40 50 60 70 80 90 100 percent voltage per motor Fig. 26 In Table IV is shown the corresponding data for two motors with resistance in the secondary. Under this condition the various tests made were more consistent with each other and 20 3 « -^= ==^ y X y^ UJ % y / / 80 / / * 10 15 20 torque in pounds Fig. 27 the writer has more confidence in the data than in the case of Table III. In Fig. 26 is shown the percentages of line voltage on each of SINGLE-PHASE INDUCTION MOTOR 553 the two motors, compared with those in' Table II. These show the same differences as in Fig. 24, where there was no external resistance. In Fig. 27 is shown the speed-torque curve for both calculation and test, as taken from Table IV. Here the discrepancies are much smaller than in Fig. 25. Conclusion While the data is not as exact as the writer would desire, yet he feels that the general results obtained from the various tests have indicated that the method of analysis followed in this paper is along proper lines and that this conception of the action of the single-phase induction motor is of considerable assistance in obtaining, a proper understanding of the machine. As stated before, the primary purpose of this paper is not to develop a method of calculation, but is simply to illustrate some of the characteristics of the single- phase motor. It is hoped that' this will bring out more clearly the very intimate relation between the polyphase and single-phase induction motors in their operating characteristics. 554 ELECTRICAL ENGINEERING PAPERS SINGLE-PHASE LOAD FROM POLYPHASE SYSTEMS FOREWORD — This paper was presented at the thirtieth annual convention of the As- sociation of Edison Illuminating Companies, held at White Sulphur Springs, Va., September, 1914. Its purpose was to show some of the possibilities of phase conversion from polyphase to single-phase, in view of the increasing require- ments for single-phase service for electric furnace work and electro-fusion appli- cations, for electric railway service, and for various other special applications. The paper deals with some of the problems of synchronous phase balancers, etc. — CEd.) IN the first place, the broad statement may be made that it is not practicable to transform a polyphase load to single-phase by means of transformers alone. There is a definite, positive reason for this, namely, a single-phase load represents power which is pvilsating or varying periodically from zero to a maximtim value, whil^ a balanced polyphase load represents continuous power of constant value. It is obviously not feasible to transform from constinuous power to pulsating, or vice versa, without some means of storing and restoring power, which is not practicable with trans- formers. ^Keeping the above statements in mind, it is obviously a waste of time to attempt to accomplish the result by special transformer connections or arrangements. However, many attempts have been made to produce this result with transformers alone, and some with superficial evidence of success — that is, in some cases, it has been possible to load the three phases equally in current when delivering single-phase load. But balanced currents in this case do not mean balanced power loads, nor do they, as a rule, mean less total loss in the generator windings. In fact, the equality of the currents in the different leads is obtained simply by out-of-phase currents, part of them usually being leading and part lagging. The resultant re- actions and unbalancing effects of these leading and lagging cur- rents have precisely the same effect on the generating system as the single-phase alone would have. On the basis therefore of storing and restoring power in order to obtain balanced three-phase loads when delivering single-phase, various possible methods of accomplishing this result may be con- sidered, aU involving rotating machinery, that is, mechanical inertia. The obvious method is by means of a motor-generator in which a three-phase motor drives a single-phase generator, the entire single-phase load being transformed from electrical to SINGLE-PHASE FROM POLYPHASE 555 mechanical, and then back to electrical. Where entire inde- pendence of the single-phase and three-phase currents is desired this, of course, is the ideal method. On the other hand, it is possibly the least efficient method. But where both change in frequency and change to single-phase load are involved without dis- tortion of the polyphase load conditions, then double transforma- tion of power appears to be necessary, such as from electrical to mechanical and back to electrical, or from electrical to sqme other form of electrical power, involving a second complete transforma- tion. The motor-generator is an example of the first, while trans- formation from three-phase to direct current by a rotary conver- ter, and from direct current to single-phase of another frequency by a second converter, is an example of the second. Where the power-factor of the load is low, as in some electrical furnace systems, one advantage of the motor-generator method is that the power-factors of the supply system and the load are ab- solutely independent of each other. However, where the transformation from three-phase to single-phase is at the same frequency, it would appear that part of the single-phase load could be delivered directly from one phase of the three-phase system, while the other part of the load could be taken from the other phases and re-transformed in phase by rotating apparatus to that of the single-phase load, so that only part of the load would thus need transformation. For instance, assume that one-third of the single-phase power is taken from one phase, and the other two phases supply power to a suitably wound motor, which drives a single-phase generator having the same phase relation as the third circuit of the three-phase system. Obviously, the generator could feed its single-phase load in parallel with the other single-phase circmt. The three generator circuits would thus be equally loaded and the single-phase generator of the motor-generator set would not be transforming the full single- phase load. This illustrates the principle of transforming from three-phase to single-phase without transforming the whole load, but this particxilar arrangement of apparatus is not a very practical one. But the question naturally arises whether this cannot be done in comparatively simple manner by means of a single machine, con- nected across the three-phase circuit, which will serve to transfer power from part of its circuits to others at a different phase relation. This principle has been utihzed in the past to transform from single-phase to polyphase, and in the same apparatus the- 556 ELECTRICAL ENGINEERING PAPERS Operation has proven to be reversible. It may, therefore, be con- sidered as settled that such transformation is possible and practic- able. Fundamentally, the action of phase balancing is as follows : — When a single-phase load is taken from a polyphase circuit, it tends to distort the phase relations in the latter circuit. Any synchron- ous or induction type polyphase motor connected to a distorted polyphase circuit will act in such a way as to have a balancing effect on its supply system. Any such motor will naturally tend to do this, for the motor, with its own balanced phase relations will tend to take current and load in accordance with the supply voltages^that is, it will tend to take more from the higher volt- ages, and if the power taken from the higher circuits exceeds the load or losses of the motor itself, then the excess is fed back into FIG. 1— SCHEME OF CONSTRUCTION FOR CONVERTING PROM SINGLE-PHASE TO BALANCED THREE-PHASE the lower voltage circuits. It thus has a balancing action on the supply circuit. This is the natural tendency of all polyphase synchronous and induction types of rotating machines when con- nected to a supply circuit. However, in the motor itself, this tendency to correct the unbalancing of the supply circuit will be accompanied by a corresponding tendency inside the motor itself to distort its own internal phase relations until they match those of the supply system. But if the distortion of the phase relations inside the motor can be prevented or neutralized in any maimer, then the motor will transfer loads between its phases or circuits to Such an extent that it will correct the unbalancing of the poly- SINGLE-PHASE FROM POLYPHASE 557 phase system. In other words, if balanced three-phase potentials are held at the point of delivery of single-phase load, then the three-phase supply system, up to that point, will be balanced. The operation of the various phase-balancing methods therefore lies in correcting the effects of the internal phase distortions in the phase-balancing motor, whether it be of the induction or of the synchronous type. The action of a phase-converting device in a simple form can probably be shown best by an arrangement used in railway work for converting from single-phase to balanced three-phase, and from three-phase to single-phase when acting regeneratively. Fig. 1 illustrates ^such an arrangement, consisting of a trans- former, a phase splitter, and single and three-phase circuits. The transformer is connected across the single-phase circuit, which, for simplicity, also is shown as one phase of the three-phase circuit. The phase splitter has one phase connected across the same phase PIG. 2— VOLTAGE CONDITIONS IN THE CIRCUITS INDICATED IN FIG 1 WHEN TRANSFORMING SINGLE-PHASE TO THREE-PHASE as the transformer; while its other phase, which is wound in 90- degree relation to the former, has one end connected to some in- termediate point of the transformer, and its other is connected to the third phase of the three-phase circuit. The voltage relations, both without and with load, when transforming to three-phase, are indicated in Pig. 2. 'In this 558 ELECTRICAL ENGINEERING PAPERS (Hagram, ab represents the single-phase e. m. f. delivered to the transformer.^ The line fc represents the e. m. f. generated in phase 2 of the phase splitter, this being 86.6 percent of ab. Therefore, with fc at right angles to ab, lines ac, and ab are equal, and a balanced three-phase circtiit is obtained at the three-phase termin- als. Next, assuming that a three-phase load is carried, then, due to internal distortions, fc is both reduced in value and shifted in phase to the position fd. The three-phase voltage relations are then indicated by ab, ad and bd. To correct this distorted condition, assume (1) — that the e. m. f . across phase 1 of the phase converter is increased sufficiently to increase the e. m. f . of phase 2, so that it will be represented hyfe, instead oifd, the increase being such that a line connecting c and e will be parallel with ab. Then assume (2) that the connection at / is moved along ab to a point g such FIG. 3— VOLTAGE RELATIONS WHEN TRANSFORMING THREE- PHASE TO SINGLE-PHASE that fg equals ce. This brings terminal e to the position c, and the internal phase relations will then be such that balanced e. m. f.'s, corresponding to ab, ac and be, will be delivered to the three-phase circuit when carrying load, and the three-phase circuit will neces- sarily carry balanced three-phase load, although the source' of power is single-phase. In Fig. 3 is shown a similar arrangement, except that the transfer of power is from three-phase to single-phase, using the SINGLE-PHASE FROM POLYPHASE 559 same apparatus as in Fig. 2. As in Fig. 2, ab, ac and be represent three-phase balanced voltages, or the no-load condition. With load, the conditions are the reverse of those in Fig. 2. The voltage fc is shifted in phase with respect to ab, but in the opposite direction. Also ab is shortened with respect to fc. The unbalanced phase relations can therefore be represented by the triangle ai, bi, d. Therefore, if aibi is to be maintained at the value ab, then fd will be increased proportionately to fdi, and the relations are represented by the triangle abdi. This triangle therefore has to be corrected to correspond with the balanced diagram abc. This can be done by (1) reducing the e. m. f. of phase two of the phase converter (by reducing phase one, for instance), and by (2) — • moving /to g. This brings di in coincidence with c and a balanced three-phase condition then results. It is obvious in Fig. 2 that the addition of an e. m. f. at the terminal d corresponding in value and direction to the line cd would have corrected to a balanced condition for the assumed load and power-factor. Also, in Fig. 3, a correcting e. m. f. dic would have accomplished the desired result. In the actual dia- grams, instead of supplying this correcting e. m. f . directly, it was obtained indirectly by combining two right angle e. m. f.'s of suit- able value and direction, these two being readily obtainable in the arrangement shown. However, the illustration shows how a single correcting e. m. f. of proper phase and value can correct from a distorted three-phase system to a balanced system. Instead of the above special arrangement for changing from three-phase to single-phase, any standard type of three-phase motor, either synchronous or induction, coidd be used for phase balancing by the addition of a suitable correcting e. m. f. in one of the phases, and if this correcting e. m. f. is of such value and direction as to maintain balanced e. m. f.'s across the three termi- nals, then the phase-balancing motor wiU correct the single-phase load. If an induction motor is used as a phase balancer under the above conditions, then it will simply serve as a phase converter, but has no abihty to correct or adjust the power-factor. If the phase balancer is of the synchronous type, however, it can be adjusted and controlled to act as both a phase converter and a power-factor corrector. If the single-phase load to be carried is at a relatively low power-factor, then it will exert a demagnetizing effect upon the phase balancer which must be taken into account 560 ELECTRICAL ENGINEERING PAPERS when the e. m. f phase relations are adjusted for proper balancing. This means that the field excitation of the phase balancer must be increased sufficiently to overcome the demagnetizing tendency of the single-phase load. This increase in field excitation will tend to increase the e. m. f.'s of all the armature circuits but, as one winding, when balanced conditions are obtained, will carry practically all the wattless current corresponding to the single- phase load, while the others will be carrying power only (on the asstimption that 100 percent power-factor is maintained on the three-phase system) the effect of the internal self-inductions of the phase balancer will be such that the resultant e. m. f.'s of some of the windings will be increased to a greater extent than others when the field excitation is increased. Therefore, when correcting for inductive loads, a different value and direction of the correct-^ ing e. m. f. is necessary than would be required for single-phase loads without power-factor correction. It is obvious from the above that what is needed for obtaining balanced conditions and corrected power-factor on the polyphase system when carrying a low power-factor single-phase load, is a suitable synchronous motor acting as a phase balancer in connec- tion with some auxiliary means for introducing a correcting e. m. f . which should vary in value and direction with the load and power-factor. There are various ways by which this result can be accom- plished. To illustrate: It may be assumed that the desired correcting e. m. f. may be obtained by means of a small synchron- ously running booster which is connected in series with one phase of the phase balancer. The value of this e. m. f . can be varied by vary- ing the field excitation of the booster field. The phase relation of this booster e. m. f. can be regulated in various manners, as, for instance, by mechanically shifting the field structure circumfer- entially with respect to the armature. Or, the armature of the booster might have two fields side by side, but with their poles displaced circumferentiaUy 90 degrees with respect to each other. Then, by separate adjustment of the excitations of the two fields up and down, or reversed, the e. m. f. generated by the booster armatiore can be given any desired direction or value. Or, instead of two fields side by side, a single field structure can be used in the booster, with two exciting windings overlapping or displaced 90 degrees with respect to each other, like the primary windings of a two-phase induction motor. By proper adjustment of the excit- SINGLE-PHASE FROM POLYPHASE 561 ing current in these two windings, the same restilts as with two fields side by side may be obtained. With the booster e. m. f . thus under control, it is obvious that any desired phase or voltage correction can be obtained in the phase balancer. There are various other ways of obtaining the corrective e. m. f., such as by induction regulators, etc., but the above is sufficient to illustrate the general arrangement or method of operation. Mr. E. F. W. Alexanderson* has also proposed a method of accomplishing this resvdt. The very considerable complication of such methods of phase balancing nnay be necessary where widely fluctuating loads and non-related variations in power-factor are encountered. In such cases, automatic voltage regtolations can be used in connection with the main and the booster fields to obtain the desired cor- rective action. However, combination of the synchronous ma- chine and its booster, or boosters, requires, as a rule, considerably less total apparatus than a straight motor-generator, and the losses should also be materially less. However, where the single-phaSe load conditions are not too widely fluctuating, it is possible to use much simpler arrangements. In electric futnace work the single-phase load and power-factor may be almost constant when the load is on. In such cases, phase splitting may be accomplished in a fairly simple and effective manner by a single synchronous machine, either with or without a small additional autotransformer, and with suitable taps and switches for varying certain voltage relations. In synchronous phase balancers there is a very considerable magnetic action on the field poles and structure by the armature winding when carrying load, unless the field poles are equipped with ample cage dampers similar to those required on the fields of large single-phase generators. If these dampers are of proper proportions, the pulsating effect of the armature on the field can be suppressed with comparatively small loss in the dampers. However, the alternative of such machine, namely, the straight motor-generator, must also have heavy dampers on its single- phase element, so this does not change the relative efficiencies of the two methods. When power-factor correction is required, as well as phase balancing, then the size or capacity of the 'phase balancer will depend' to a certain extent upon the amount of power-factor ♦Phase Balancer for Single-Pftase Load on Polyphase Systems," by Mr. E. F. W. Alexan- derson, General Electrical Review, December, 1913. ,5«2 ELECTRICAL ENGINEERING PAPERS correction. As it may be of interest to know what capacity of phase balancer is reqiiired in terms of single-phase load, the ap- proximate curves shown in Fig. 4 have been worked out for dif- ferent power-factors, showing the capacity (three-phase) re- quired in phase balancers for each 1,000 k.v.a. single-phase load taken off. The ordinates represent power-factors of the single-phase load, while the abscissae show the k.v.a. ratings of the phase balancers required at various three-phase power-factors. The phase balancing k.v.a. is given in terms of three-phase capacity — ■ that is, the capacity which- the machine would have as a three- phase generator, with a cirrrent rating corresponding to the largest of the unbalanced currents in its three phases. In other words, this rating is on the basis of maximum local losses, instead of averaging, and thus represents the most severe condition. The capacity of the booster or other apparatus for supplying the correcting e. m. f. is not included. This can be assumed roughly as about 15 percent of that of the phase balancer, whether it is a separate piece, such as a separate booster or transformer, or is obtained in the balancer windings. ~~^ ^ K "" "^^ ^c. a. ^ \ \ \ i \ ^. N s. 2 x*f? sj \ \ \ \ \ \ X \ \ \ \ K -Q--60 -^ ^», N \ \ N \ \ ■^^ v \ \ V \ \ \ \ X N \ N \ \ \ \ \ ■ 10 DO HOD 1 1400 1 <.v.a ^haseE liOO alancirig Cap: 1600 city I r/00 1800 FIG. 4— CURVES SHOWING THE BALANCING CAPACITY REQUIRED (THREE-PHASE) FOR EACH 1 000 K.V.A. OP SINGLE-PHASE LOAD These curves show that usually there is considerable saving in capacity of apparatus in the use of a phase balancer, as com- pared with a straight motor-generator where power-factor cor- rection is not important. For example, assume a single-phase load is at 70 percent power-factor, while the corresponding three- SINGLE-PHASE FROM POLYPHASE 563 phase balanced power is to be held at the same power-factor. From the table, the approximate capacity of the phase balance is 1,000 k.v.a. Adding 15 percent for the booster, gives 1,150 k.v.a. as the total balancing capacity required. Comparing this with a straight motor-generator, the driving motor will have a normal capacity of 700 kw approximately, while the 1,000 k.v.a. single- phase generator would approximately correspond in capacity to a 1,500 k.v.a. three-phase machine — thus reqmring a total of 2,200 k.v.a., compared AArith 1,150 k.v.a. for the phase balancer. The latter means, therefore, materially less expensive apparatus — also more efficient. It may be noted throughout that where there is no correction of power-factor, the balancer capacity in k.v.a. will be equal to the k.v.a. of the single-phase load. If, however, in the above example, 70 percent single-phase power-factor is to be cor- rected to 90 percent in the three-phase circuit, then the balancer capacity will be 1,390 k.v.a. Adding 15 percent for the correct- ing booster gives 1,600 k.v.a. against 2,200 k.v.a. for the motor- generator. The phase balancer therefore apparently does not correct for power-factor as advantageously as the straight motor-generator. Also, where automatic correction of power-factor is desirable, the motor-generator arrangement is somewhat less complicated. 564 ELECTRICAL ENGINEERING PAPERS THE TECHNICAL STORY OF THE FREQUENCIES FOREWORD — This paper was presented before the Washington Section of the American Institute of Electrical Engineers in January, 1918. It covers briefly the history of the various frequencies used in America and the engineering and technical reasons which have influenced their ultimate choice or rejection. The author had in mind the preservation of this in more or less historical form, in order that it should not eventually be entirely lost. Since the publication of this paper, the author has received many favorable comments on it, as being the only reliable source of information on the subject which is now available. It has been reprinted in its entirety in a number of technical papers and the material drawn from it has been used in a number of technical lectures by various engineers and educators. — (Ed.) THE STORY of how and why the various commercial fre- quencies came into use and then dropped out again, in most cases, is not primarily the story of the frequencies themselves, but of the various uses to which the alternating current has been applied.* In other words, fundamental changes in the application of alternating current have led to radical changes in the frequencies. Some of the applications which have had a determining factor on the frequency of the supply system are as follows; incandescent lighting, transformers, transmission systems, arc lighting, induction motors, synchronous converters, constructional conditions in rotating machinery, and operating conditions. A brief consideration of these items individually. STORY OF THE FREQUENCIES 565 from the present v^iewpoint, indicates that while some of them had, at one time, very considerable influence in determining frequency conditions, yet, in a number of cases, the original reasons have disappeared through improvenients and refine- ments, as will be described later. At various times the following standard frequencies have been in use in this country, namely, 1333^, 125, 83J/^, 66 2/3, 60, 50, 40, 30 and 25 cycles per second. These did not appear chronologically in the order given above, and a few odd fre- quencies in a few special applications are omitted. In the following, the various frequencies will be considered more or less in the order of their development and basic reasons will be given for their choice, and the writer will endeavor to show why certain of them have persisted, while others have dropped out. It will also be shown why the commercial situa- tion has first tended strongly toward certain frequencies and afterwards swung toward others. 133 AND 125 Cycles In the earliest alternating work, the whole service consisted of incandescent lighting, and the electric equipment was made up of small high-speed belted single-phase generators and house- to-house distributing transformers. As the transformers were of small capacity and as their design was in a very crude state, 't was believed that a relatively high frequency would best meet the transformer conditions. A choice of such an odd frequency as 1333^ cycles per second, is due to the fact that in those early days (1886 to 1893) frequencies were usually designated in terms of alternations per minute. One of the earliest com- mercial generating units constructed by the Westinghouse company had a speed of 2000 rev. per min. and had eight poles. This presented a fairly convenient constructional arrangement for the surface-wound type of. rotating armature, which was the only one recognized at that time. The speed of 2000 rev. per min., with eight poles, gave 16,000 alternations per minute, or 133§ cycles per second according to our present method of designation. Thus the earliest frequency in commercial use in this country was fixed, to a certain extent, by constructional reasons, although the house-to-house transformer problem ap- parently indicated the need for a relatively high frequency. The Thomson-Houston company adopted a standard fre- quency of 15,000 alternations per minute, (125 cycles) insteac* 566 ELECTRICAL ENGINEERING PAPERS of the Westinghouse 16,000, but the writer does not know why this difference was made. However, the two frequencies were so close together that practically they could be classified as one. At this time, it should be borne in mind, there were no real transmission problems, no alternating-current arc lighting, no induction motors and the need for uniform rotation of the genera- tors was not recognized. The induction motor, in its earliest stages, came in 1888 and considerable work was done on it in 1889 and 1890, but it required polyphase supply circuits and com- paratively low frequency and, therefore, it had no connection whatever with the then standard single-phase, 133^ and 125 cycle systems. The synchronous converter was also unheard of (one might say almost undreamed of) at that time. 60 Cycles In 1889 or 1890 it was beginning to be recognized in this country that some lower frequency than 125 and 133g cycles would be desirable. Also about this time direct-coupled and engine-type alternators were being considered in Europe and it was felt that such construction would eventually come into use in America. It was appreciated that in such case^ 133| cycles would present very considerable difficulties compared with some much lower frequency, due to the large number of poles which would be required. For instance, an alternator direct driven by an 80-rev. per min. engine would require 200 poles to give the required frequency and such construction was looked upon as being practically prohibitive. About this time Mr. L. B. Stillwell, then with the Westinghouse company, made a very careful study of this matter of a new frequency, in connec- tion with the possibilities of engine type generators, and after analyzing a number of cases, it appeared that 7200 alternations per minute (60 cycles per second), was about as high as would be desirable for the various engine speeds then in sight. Trans- former constructions and arc lighting were also consideredin this analysis. While it was deemed that a somewhat higher frequency might be better for transformers, yet a lower fre- quency than 60 cycles was considered as possibly better for engine type generators. A compromise between all the various conditions eventually led to 60 cycles as the best frequency. However, while this frequency originated about 1890, it did not come into use suddenly, for it was impossible to introduce suck a radical change in a brief time. Moreover, the direct-coupled STORY OF THE FREQUENCIES 567 or engine-type generator was slow in coming into general use and, therefore, there was not the necessity for the introduction of this low frequency in many of the equipments sold from 1890 to 1892. However, by 1893, 60 cycles became pretty firmly established and was sharing the business with the 133^ -cycle systems. It should be borne in mind that, at this time, the adoption of this frequency was not considered as a direct means for bringing forward the polyphase induction motor, for the earlier 60-tycle systems, like the 125- and 133|-cycle, were all single-phase Also, it was then thought that the polyphase motor would possibly require a still lower frequency and, more- over, the polyphase system was looked upon as in a class by itself, suitable only for induction motor work. At that time the introduction of polyphase generators for general service was not contemplated. This followed about two or three years later. In 1890 the Westinghouse company, which had been de- veloping the Tesla polyphase motor, laid aside the work, largely on account of there being no suitable general supply systems for this type of motor. The problem was again revived in 1892, in an experimental way, with a view to bringing out induction motor which might be applied on standard frequencies such as could be used in commercial supply circuits for lighting and other purposes. It should be understood that at this time such cir- cuits w^re not in existence but were being contemplated. In 1893, after the polyphase motor had been further developed up to the point where it showed great commercial possibilities, the best means for getting it on the market were carefully considered. It was decided that the best way to promote the induction motor business was to create a demand for it on commercial alternating- current systems. This meant that, in the first place, such sys- tems must be created. Therefore, it was decided to undertake to fill the country with polyphase generating systems, which were primarily to be used for the usual lighting service. It was thought that, with such systems available, the time would soon come when there would be a call for induction motors. In this ■way experience would be obtained in the construction and opera- tion of polyphase generators and the operating public would not be unduly handicapped in the use of such generators, compared with the older single-phase types. An early example of this new practise yvas in the 2000-kw. polyphase generating units used for lighting the Chicago World's Fair in 1893. Here, the single-phase type still persisted, as each 568 ELECTRICAL ENGINEERING PAPERS generator unit was made up of two similar frames placed side by side, but with their single phase armatures d splaced one-half pole pitch from each other so that the combined machine de- livered two single-phase currents displaced 90 degrees from each other. It was considered that each circuit could be regulated independently for lighting service, and polyphase motors could be operated from the two circuits. These generators (at that time the largest in this country) were designed in 1892 and were of 60 cycles. These, therefore, indicate the tendency at that time toward loWer frequency and .'polyphase gfeneration, although commercial polyphase motors were not yet on the market. 25 CYCLES At the same time that 60 cycles was selected as a new standard it was recognized that at some future time there would be a place for some much lower frequency, but it was not until two years later that this began to narrow down to any particular frequency. In 1892 the first Niagara electrification, after several years consideration by eminent authorities, had centered on polyphase alternating current as the most desirable system. The engineers of the promoting company had also worked out what they considered the most suitable construction of machine. This involved 5000-h. p. units at 250 revolutions per minute. Prof. George Forbes, one of the engineers of the company had furnished the electrical designs for a machine with an external rotating field and an internal stationary armature. His design used eight poles, thus giving 2000 alterations per minute, or 16| cycles per second. Quite independently of this, the Westinghouse com- pany, in 1892, had been working on the development of synchron- ous converters, using belted 550-volt d-c. generators with two- phase collector rings added. The tests on these niachines had shown the practicability of such conversion and had even proved at this early date, that the converter copper losses were much lower than in the corresponding d-c. generators. Thus it is an interesting fact that the first evidence of this important principle was obtained from a shop test rather than by calculation. The writer, from an analysis of the tests, which were made under his immediate direction, concluded that the armatiire copper losses must be considerably lower than in the same machine used as a d-c. generator. He also brought the matter to the attention of Mr. R. D. Mershon, then with the Westinghouse company, and the problem was then worked out mathematically by him STORY OF THE FREQUENCIES 569 and the writer, in two quite different ways, but with similar results, showing that the converter did have actually very much reduced copper losses'. As a result of this work of the Westinghouse company on the synchronous converter, it was decided that, to make such ma- chines practicable, some suitable relatively low frequency was required. This appeared to be about 30 cycles. About this time the construction of the Niagara generators was taken up with the Westinghouse company to see whether it would con- struct these machines according to the designs submitted by the promoting company's engineers. These designs were gone over as carefully as the knowledge of such apparatus, at that time, permitted, and many apparent defects and difficulties were pointed out. The Westinghouse company then proposed, as a substitute, a 16-pole, 250-rev. per min. machine (the speed being definitely fixed at 250 rev. per min.). This gave 33f cycles or as near to the Westinghouse proposed 30 cycle system, as it was possible to get. Then many arguments were brought forward, pro and con, for the two machines and frequencies. Prof. Forbes' preference for 16f cycles was based partly on the pos- sibilities it presented for the construction and operation of com- mutator type motors, just as with direct current circuits. The Westinghouse contention was that this frequency was too low for any kind of service except possibly commutator type' ma- chines. Tests were made with incandescent lights and it was found that at 33^ cycles there was little or no winking of light, while at 16f cycles, the winking was extremely bad. Tables were also made up, showing the limited number of speed com- binations at 16f cycles for induction motors, in case such should come into use. This showed how superior the 33^ cycles would be as regards such apparatus. It was also brought out that synchronous converters, when such became commercial, would be much better adapted for the higher frequency, as the choice of speeds would be much greater. From the present viewpoint the arguments appear to have been much in favor of the West- inghouse side of the case. As a consequence of all this discussion the suggestion was advanced by some one, that a 12 pole, 250-revolution machine, (that is, 3000 alternations, or 25 cycles), might meet sufficiently the good qualities of both of the proposed frequencies and would thus be a good compromise. In consequence a 12-pole, 25-cycle machine was worked up by the Westinghouse company and 670 ELECTRICAL ENGINEERING PAPERS eventually this frequency was adopted for the Niagara genera- tors. Afterwards, while these generators were being constructed it was brought out pretty strongly that the great advantage of this frequency would be in connection with synchronous con- verter operation, but that it was also extremely well adapted for slow-speed engine type generators, which were then coming into use. In consequence of the prominence given this frequency it was soon adopted as a standard low frequency, especially in those plants where synchronous converters were expected to form a prominent part of the system. However, while 60 and 25 cycles came into use, as described above, it must be recognized that they had competitors. For instance, 66| cycles (8000 alternations, or one-half of 16,000) was used to a considerable extent by one of the manufacturing companies. Also 50 cycles came" into use in certain plants and, to a certain extent, is still retained, but has become the standard high frequency of Europe. ' Instead of 25 cycles, the Westing- house company advocated 30 cycles for some of its plants, largely because with the 25 per cent higher speeds permissible with such frequencies, the capacities of induction motors could be cor- respondingly increased and also incandescent lighting was more satisfactory. However, it was soon recognized that the 66f and 30 cycle variations from the two leading frequencies of 60 and 25 cycles were hardly worth while, and they were gradually dropped, except in plants already installed. A brief attempt was made at a somewhat later period to place 40 cycles upon the market as a substitute for both 25 and 60 cycles. This was done under the impression that 40 cycles would give a universal system for arc and incandescent lighting, transmission,, induction motors, synchronous converters and. about everything else. This fre- quency possessed many merits and it was thought, at One time, that it might win out, but apparently the other two frequencies were too well established, and the 40 cycle system eventually lost ground. The problem of the frequencies finally narrowed down to the two standards, and these two were accepted because it was thought that they covered such entirely different fields of ser- vice that neither of them could ever expect to cover the whole. In other words, two standards were required to cover the whole range of service. It was recognized that 25 cycles would not take care of alternating-current arc lighting and that it was questionable for incandescent lighting in general. In other ways, STORY OF THE FREQUENCIES 571 such as- suitability for engine-type construction, application to induction motors and synchronous converters and transmission of power to long distances, it met the needs of an ideal system, as then understood. Also, in parallel operation of engine-type alternators, which was one of the serious problems of those days, the 25-cycle machines were unquestionably superior to the 60- cycle ones, due to the lesser displacement of the e. m. f. waves with respect to each other with a given angular variation in the engine speeds. However, although the 25-cycle system pre- sented so many advantages, it could not take care of the lighting business, and, therefore, could not entirely dominate the situa- tion. As regards 60 cycles, it was felt that this could handle the direct lighting situation in a very satisfactory manner and was pos- sibly better suited for transformers than 25 cycles, although there were differences of opinion in this matter, especially when it came to the larger capacities.. It was reasonably well adapted for induction motors in general, but not for very low speeds. In matters of transmission and in the operation of synchronoiis converters it was thought to be vitally defective. From the above consideration it would appear that the 25- cycle systems presented the stronger showing as a whole and, therefore, there was a decided tendency toward this frequency, except in those cases where lighting directly from the alternating- current system was considered of prime importance. In those systems, such as many of the Edison companies, where low- voltage three-wire direct current was used from synchronous ■converters, the tendency was almost solidly toward the 25-cycle system. In those days the central station, which had gotten itself committed to the 60-cycle system so deeply that it could ^not change, was looked upon with commiseration. Sixty-cycle plants were looked upon, to a certain extent, as a necessary evil. In fact, so strong was the tendency toward 25 cycles that in many cases 25-cycle plants were installed for industrial purposes, where 60 cycles would have been better. The 25-cycle synchronous converter development advanced by leaps and bounds and the machines were so good in their operation that it was believed that 60-cycle converters could never be really competitive with them. On the other hand, in those large plants, which were so "un- fortunate" as to have 60 cycles installed, many apparent make- shifts were adopted to meet the various service requirements. 572 ELECTRICAL ENGINEERING PAPERS In arc lighting, incandescent lighting, transformers and inotors there was no need for makeshifts. However, in conversion to direct current, one of the greatest difficulties appeared. There were many who advocated motor-generators for this purpose, largely because the 60-cycle converter was thought to be im- practicable, in spite of the fact that the manufacturing companies were putting them on the market. The. 60-cycle converter at that time bore a bad name. It is now recognized that many of the faults of the early 60-cycle synchronous converter operation were not in the converters themselves, but were, to a consider- able extent, in the associated apparatus. Low-speed . engine- type, 60-cycle generators were not always adapted for operation of synchronous converters. In fact, in numerous cases such generators would not operate in an entirely satisfactory manner in parallel with each other, and yet when it was attempted to operate synchronous converters from these same generators the unsatisfactory results were not blamed upon the generating system but upon defects of the converters themselves. Unfor- tunately, defects in the generating and transmission systems usually appeared in the converters as sparking and flashing, and such troubles naturally would be credited to defects in the construction of the converters themselves. In fact, in those days, 60-cycle converters were expected to do things which now are considered as absurd. For instance, in one case in the writ- er's knowledge a 60-cycle synchronous converter was criti- cized as being a very badly designed piece of apparatus, due to serious flashing at times. Investigation developed that this converter was expected to operate on either one of two indepen- dent 60-cycle systems with no rigid frequency relation to each other. The converter in service was thrown from one system to the other indiscriminately, and sometimes it flashed in the trans- fer and sometimes it did not. The machine was considered to be "no good" because it would not always stand such switching. At one time the writer stood almost alone in his belief that the 60-cycle synchronous converter presented commercial pos- sibilities sufficient to make it a- strong future contender with the 25-cycle machine, provided proper supply conditions were furnished and certain difficulties in the proportions of the con- verter itself were overcome. One basis for his contention -was that in some of the 60-cycle plants, where the generator rotation was quite uniform, the converters were evidently much superior in their operation to other plants, using slow-speed engine-type STORY OF THE FREQUENCIES 573 ■generators with considerable periodic variations. In such plants the hunting tendency of the converters was very greatly re- duced, with consequent improvement in sparking and general operation. It was early recognized that hunting was a very harmful condition, both in 60- and 25-cycle synchronous con- verters, but whereas it was a relatively rare condition in 25-cycle plants it was much more common with 60 cycles. However, the operating public was not particularly concerned whether the trouble was in the generating plant or in the converters themselves, as long as such trouble existed and was not overcome. Very early in the synchronous converter development it was found that hunting would produce sparking or flashing at the commutators of the converters. However, even in those plants where there was no hunting apparent, there was difficulty at times due to flashing, iespecially with sudden change of load, which resulted in temporary increase in the d-c. voltage. This was a difficulty which was inherent in the converter itself and could not be blamed entirely upon the generating or transmitting conditions, for 25-cycle machines were practically free from this trouble under similar conditions of operation. Investiga- tion developed the fact that this flashing trouble was due largely to unduly high value of the maximum volts between commutator bars. This difficulty was recognized long before it was over- come, simply because certain physical limitations in construction had to be removed. There were two ways in which the maximum volts per bar could be reduced, namely, by increasing the number of commutator bars per pole and by decreasing the ratio of the maximum volts to the average volts per bar, that is, by increas- ing the ratio of the pole width to the pole pitch, but both of these involved structural limitations in the allowable peripheral speeds of the commutator and the armature core. Here is where a little elementary mathematics comes in. The per- ipheral speed of the commutator is directly proportional to the distance between adjacent neutral points on the commutator, and the frequency. Therefore, with, a given frequency the distance between the adjacent neutral points is directly propor- tional to the peripheral speed.. 'Thus, a commutator speed of 4500 ft. per min. which was then considered an upper limit, the distance between adjacent neutral points on a 60-cycle converter is only 7i in. (19 cm.) This distance is thus fixed mathematically and is independent of the number of poles or revolutions per minute, or anything else, except the peripheral 574 ELECTRICAL ENGINEERING PAPERS speed and the frequency. With this distance of 7| in., (19 cm.), about the only choice in commutator bars per pole was 36, giving an average of 16f volts per bar on a 600-volt machine, and nearly 20 volts per bar with momentary increase of voltage to 700, which is not uncommon in railway service. However, it is not this average voltage which fixes the flashing conditions, but it is the maximum voltage between bars, and this is dependent upon the average voltage and upon the ratio of the pole width to the pole pitch. Here is where one of the serious difficulties came in. As mentioned above the pole pitch is directly dependent upon the peripheral speed of the armature core and the frequency. Therefore, in a 60-cycle machine, if the peripheral speed is fixed, the pole pitch is at once fixed. For example, with an armature peripheral speed of 7200 ft. per min., which was considered high- at that time, the pole pitch becomes 12 in. (30.48 cm.), regardless of any other considerations, and here was where a most serious difficulty was encountered. If a sufficiently wide neutral zone for commutation was allowed the interpolar space became so wide that there was not enough left for a good pole width. For instance, if the interpolar space was made 6 in. (15.24 cm.) wide, in order to give a sufficiently wide commutating zone to prevent sparking or flashing, due to fringing of the main field, then this left only 6 in. for the pole face. With this relatively narrow pole face the ratio of the maximum volts to the average volts was so high that with the 36 commutator bars per pole the machine was sensitive to arcing between commutator bars thus resulting in flashing. By widen- ing the pole face this difficulty would be lessened or overcome but with the fixed pole pitch of 12 in. (30.48 cm.) the neutral zone would be so narrowed as to make the machine sensitive to sparking and flashing at the brushes. Thus, no matter which way we turned we encountered trouble. Obviously there were two directions of improvement, namely, by increasing the number of commutator bars, thus reducing the average voltage, and by increasing the pole pitch, thus allowing relatively wider poles with a given interpolar space. These two conditions look simple and easy, but it took several years of experience to attain them. When we have reached apparent physical limita- tions in a given construction, especially when such limitations are based upon long experience, we have to feel our way quite slowly toward higher limitations. For instance, in the case of the 60-cycle converters we could not boldly jump our STORY OF THE FREQUENCIES 575 peripheral speeds 20 to 25 per cent higher and simply assunie that everything was all right. We first had to build apparatus and try it out for a year or so. Troubles, due to peripheral speed, do not always become apparent at once, and thus time tests are necessary Therefore, while the peripheral speeds of the 60-cycle synchronous converters were actually increased 20 to 25 per cent practically in one jump, yet it took two or three years of experimentation and endurance tests before the manu- facturers felt sure enough to adopt the higher speeds on a broad commercial scale. Thus, while the change from the older more sensitive type of 60-cycle converter to the later type occurred commercially within a comparatively short period yet the actual development covered a much longer period. Let us see now what an increase of 25 per cent in the peripheral speeds actually meant. As regards the commutator, the number of bars could be iricreased 25 per cent, that is,' from 36 to 45 per pole, which was comparable with ordinary d-c. generator practise. In the second place, an increase of 25 per cent in the peripheral speed of the armature core meant a 15-in. (38.1-cm.) pole pitch, where 12 in. (30.8 cm.) was used before. Assuming, as before, a 6-in. (15.24-cm.) interpolar space, then the pole face itself became 9 in. (22.8 cm.) in width instead of 6 in. (15.24 cm.) or an improvement of 50 per cent. In fact, this latter improve- ment was so great that some manufacturers did not consider it necessary to increase the number of commutator bars, although in the Westinghouse machines both steps were made. The above improvements so modified the 60-cycle converter that it began to approach the 25-cycle machine in its general characteristics. It was still quite expensive compared with the 25-cycle, due to the large number of poles, and its efficiency was considerably lower than its 25-cycle competitor, on account of high iron and windage losses. However, due to the need for such a machine it was gradually making headway, in spite of handicaps in cost and efficiency. Almost coincident with the initiation of the above improve- ments in the 60-cycle converter, came another factor whic^ has had much to do with the success of this type of machine. This was the advent of the turbo-generator for general service. As stated before, one of the handicaps of the 60-cycle converter was in the non-uniform rotation of the engine type generators which were common in the period from 1897 to about 1903 or 1904. But, about this latter date, the turbo-generator was making 576 ELECTRICAL ENGINEERING PAPERS considerable inroads on the engine-type field and within a rela- tively short period it so superseded the former type of unit, that it was recognized as the coming standard for large alternating power service. With the turbo-generator came uniform rotation and this at once removed one of the operating difficulties of 'the 60-cycle converters. However, in the early days of the turbo- generator, 25 cycles still was in the lead and.many of the earlier generators were made for this frequency, especially in the larger units. But it was not long before it was recognized that 60 cycles presented considerable advantage in turbo-generator design due to the higher permissible speeds. In the earlier days of turbo-generator work, this was not recognized to any extent, as the speeds of all units were so low that the effect of any speed limitations was not yet encountered. For instance, a 1500-kw , 60-cycle turbo-generator would be made with six poles for 1200 revolutions, while a corresponding 25 cycle unit would be made with two poles for 1500 revolutions. This slightly higher speed at 25 cycles about counterbalanced the difficulties of the two- pole construction compared with the six-pole. Howev-er, before long, more experience enabled the six pole, 60-cycle machine to be replaced at 1800 revolutions, and a little later by two poles at 3600 revolutions. This, of course, turned the scales very much in the other directiori. In larger units, however, the advantage still appeared to be in favor of 25 cycles, but in the course of development, 1500 revolutions was adopted quite generally for 25-'cycle work, and this was the limiting speed, as such machines had only two poles, or the smallest number possible with ordinary constructions. On the other hand, for 60 cycles, 1800 revolutions was adopted quite generally for units up to almost the extreme capacities that had been considered, consequently the con- structional conditions in the large machines swung in favor of 60 cycles. Therefore, with the coming of the steam turbine and the development of high-speed turbo-generator units, the tendency has been strongly toward 60 cycles. This, with the greater perfection of the 60-cycle converter, had much to do with directing the practise away from the 25 cycles. However, there were other conditions which tended strongly toward 60 cycles. In the early development of the induction motor, the 25-cycle machines were considerably better than the 60-cycle and possibly Httle or no more expensive. However, as refinements in design and practise came in, certain important advantages of the 60-cycle began to crop out. For instance, STORY OF THE FREQUENCIES ' 577 with 25 cycles there is but little choice in speed, for small and moderate size motors. At this frequency a four-pole motor has a synchronous speed of only 750. The only higher speed per- missible is 1500 revolutions with two poles, and it so happens that in induction motors the two-pole construction is not mate- rially cheaper than the four pole, consequently the principal ad- vantage in going to 1500 revolutions was only in getting a higher speed where such was necessary for other reasons than first cost. However, in 60 cycles the case is quite different, where a four- pole machine can have a speed of 1800 revolutions, synchronous, a six pole 1200, an eight pole 900 and a ten pole 720 revolutions. In other words, there are four suitable speed*- combinations where a 25 cycle motor had only one. Moreover, with the advance in design it developed that these higher speed 60-cycle motors could be made with nearly as good performances as with the 25-cycle motors of same capacity, and at somewhat less cost. However, leaving out the question of cost, the wider choice of speeds alone would be enough to give the 60-cycle motor a pronounced prefer- ence for general service. However, there is one exception to the above. Where very low-speed motors are required, such as 100 rev. per min., the 60- cycle. induction motor is at a considerable disadvantage com- pared with 25 cycles, or this has been the case in the past. It is partly for this reason that the steel mill industry, through its electrical engineers, adopted 25 cycles as standard some ten or fifteen years ago. At that time, it was considered that in mill work, in general, there would be need for very low-speed motors in very many cases. However, due to first cost, as well as other things, there has been a tendency toward much higher speeds in steel mill work, through the use of gears and otherwise, so that part of this argument has been lost. However, there still remain certain classes of work where direct connected very low-speed induction motors are desirable and where 25 cycles would ap- pear to have a distinct advantage. In view of the above considerations, steel mill work has hereto- fore gone very largely toward 25 cycles, particularly where the mills installed their own power plants. However, in recent years there has been a pronounced tendency toward purchase of power, by steel mills, from central stations, and the previously described tendency of central stations toward 60 cycles has forced the situation somewhat in the steel mills, particularly in those cases where the central power supply company can furnish 578 ELECTRICAL ENGINEERING PAPERS power at more reasonable rates than the steel mill can produce in its own plant. This, therefore, has meant a tendency toward 60 cycles in steel mill work, even with the handicap of inferior low- speed induction motors. But, on the other hand, remedies have been brought forward even for this condition. The great diffi- culty in the construction of low-speed, 60-cycle induction motors is in the very large size and cost if constructed for normal power factors, or the very loW power factor and poor performance if constructed of dimensions and costs comparable with 25 cycles. In the latter case the extra cost is not entirely eliminated because a low power factor of the primary input implies additional gener- ating capacity, or some means for correcting power factor on the primary system. However, in some cases it is entirely practi- cable to correct the power factor in the motors themselves by the use of so called "phase advancers" of either the Leblanc or the Kapp type. Such phase advancers are machines connected in the secondary circuits of induction motors and so arranged as to furnish the necessary magnetizing current to the rotor or second- ary instead of to the primary In this way the primary current to the motor will represent largely energy and the power factors can be made equal to, or even much better than in, the corre- sponding 25-cycle motor; or, in some cases, the conditions may be carried even further so that the motor is purposely designed with a relatively poor power factor, in order to further reduce the size and cost, and the phase advancers are made correspondingly larger. In those cases where the cost of the phase advancer is relatively small compared with the main motor, tliere may be a considerable saving in the cost of the main motor and then add- ing part of the saving to the cost of the phase advancer. One difSculty in the use of phase advancers is found in the variable speeds required in some kinds of mill work. In those cases where flywheels driven by the main mdtors are desirable to take up violent fluctuations in load, it is necessary to have considerable variations in the speed of the induction motor, in order to bring the stored energy of the flywheel into play. Unfortunately this variable speed in the induction motor is one of the most difficult conditions to take care of with a phase advancer, so that here is a condition where the 60-cycle motor is at a decided disadvantage. Thus it may be seen from the above that even in the steel mill field, where the induction motor has the most extreme appli- cations, there is quite a strong tendency toward 60 cycles, due to the purchase of power from central supply systems. STORY OF THE FREQUENCIES 579 There remains one more important element which has had something to do with the tendency toward 60 cycles, namely, the transmission problem. In the earlier days of transmission of alternating current, 25 cycles was considered very superior to 60 cycles due to the better inherent voltage regulation con- ditions. At one time, it was thought that 60 cycles had a very limited field for transmission work. However, a number of power companies in the far west had installed 60-cycle plants, principally for local service and with the growth of these plants came the necessity for increased distance of transmission through development of water powers. At first it was thought they were badly handicapped by the frequency, but gradually the apparent disadvantages of their systems were overcome and the distances of transmission were extended until it became apparent that they could accomplish practically the samfe results as with 25 cycles. Part of this result has been obtained by the use of regulating synchronous condensers. It is a curious fact that the possibility of synchronous motors used as condensers for correction of disturbances on transmission . systems, has been known for about 25 years, but it is only within quite recent years that they have come into general use as a solution of the trans- mission problem, and largely in connection with 60-cycle plants. In 1893 the writer applied for a patent on the use of synchronous motors as condensers for controlling the voltage at any point on a transmission system by means of leading or lagging currents in the condenser itself. A broad patent was obtained, but there was no particular use made of it until it had practically expired. Another improvement came along which still further helped to advance 60 cycles to its present position, namely, the use of commutating poles in synchronous converters. The principal value of commutating poles in the 60-cycle converters, has not been so much in an improvement in commutation over the older types of machines, as in allowing a very considerable reduction in the number of poles with corresponding increase in speed, resulting in reduction in dimensions. As a direct result of this increase in speed the efficiencies of the converters have been increased. If, for instance, the speed of a given 60-cycle con- verter can be doubled by cutting its number of poles to one-half, while keeping the same pole pitch and the same limiting per- ipheral speed, then obviously the amount of iron in the armature core is practically halved and, at the same magnetic densities the iron loss is also practically halved. Also with the same 580 ELECTRICAL ENGINEERING PAPERS peripheral speed and half diameter of armature the windage losses can be decreased materially. Thus the two principal lo"sses in the older converters have been very much reduced. There have also been reductions in the total watts for field excitation, and in other parts, so that, as a whole, the efficiency for a given capacity 60-cycle converter has been brought up quite close to that of the corresponding 25-cycle machine, even when the latter is equipped with commutating poles. This gain of the higher frequency compared with the lower is due to the fact that the lower-frequency machine was much more handi- capped in its possibilities of speed increase, and furthermore, the iron losses and windage represented a much smaller propor- tion of the total losses in the low-frequency machine. This improvement in the efficiency of the 60-cycle converter together with the lower losses in the 60-cycle transformer as compared with the 25-cycle, has brought the 60-cycle equipment almost up to the 25-C3^cle, so that the difference at present is not of controlling importance. This development has given further impetus toward the acceptance of 60 cycles as a general system. Formerly a serious competitor with the 60 cycle converter was the 60-cycle motor-generator. This was installed in many cases because it was considered more reliable and more flexible in operation than the synchronous converter. Both of these claims were true to a certain extent. However, with improve- ments in the synchronous coverter the difference in reliability practically disappeared, but there remained the difference in flexibility. In the motor-generator set, the d-c. voltage could be varied over quite a wide range, while in the older 60-cycle rotaries the d-c. voltage held a rigid relation to the alternating supply voltage. However, with the development and perfection of the synchronous' booster type of converter, flexibility in voltage was obtained with relatively small increase in cost and minor loss in economy. This has been the last big step in putting the 60-cycle converter at the front as a conversion apparatus, so that today it stands as the cheapest and most economical method of converting alternating current to direct current. Moreover, while the 25-cycle synchronous converter has appar- ently reached about its upper limit in speed, there are still possibilities left for the 60-cycle converter. In line with the above it is of interest to note that for units of 1000 kw. and less, the 60-cycle converter has nearly driven the 25-cycle out of business from the manufacturing standpoint. STORY OF THE FREQUENCIES 581 For the very large size converters, 25 cycles still has the call, but largely in connection with many of the railway and three- wire systems, which have been installed for many years; that is, the growth of this business is in connection with existing genera- ting systems. However, the 60-cycle converter, in large capacity units, is gaining ground rapidly and it is of interest to note that the largest converters yet built, namely, 5800 kw., are of the 60-cycle type. One most interesting point may be brought out in connection with the above described "battle of the frequencies", namely, it was fought out in the operating field, and between conditions of service, and not between the manufacturing companies. This is a very good example of how such matters should be handled. Here the engineers of the manufacturing companies were expending their efforts to get all possible out of both frequencies, and consequently development proceeded apace. When 60-cycle frequency seemed to be overshadowed by its 25-cycle competitor, the engineers took a lesson from the latter and proceeded to overcome the shortcomings of the former. It was no innate preference of the designing engineers that has brought the higher frequency to the fore; it was the recognition that it had greater merits as a general system, if its weak points could be sufficiently strengthened; and, therefore, the engineers turned their best efforts toward accomplishing this result. It must not be assumed, for a moment even, that because 60 cycles appears to be the future frequency in this country, that 25 cycles was a mistake. Decidedly it was not. In reality it formed a most important step toward the present high develop- ment of the electric industry. Many things we are now ac- complishing with 60 cycles would possibly never have been brought to present perfection, if the success of the corresponding 25-cycle apparatus had not pointed the way. The success of the 25-cycle converter, and the high standard of operation at- tained, gave ground for belief that practically equal results were obtainable with 60 cycles. Therefore, the 25-cycle frequency served a vast purpose in electrical development; it was a high class pacemaker, and it isn't entirely out-distanced yet. There has been considerable speculation as to what two stand- ard frequencies would have met the needs of the service fc the best manner, and would have resulted in the greatest develop- ment in the end. It has been claimed by some, that 50 and 25 cycles would have been better than 60 and 25. In the earlier 682 ELECTRICAL ENGINEERING PAPERS days possibly the former would have been better, but as a result both standards might ha'^e persisted longer. In any case, the general advantages would have been small. In one class of ma;- chines, namely, frequency changers, consisting of two alternators coupled together, the 25-50 combination would certainly have been advantageous. Again it has been questioned whether 30 and 60 cycles would not have been a better choice. This was the original Westing- house choice of frequencies, but not on account of frequency changers. As stated before, it was felt that 30 cycles could do about all that 25 cycles could, and would give an advantage of 25 per cent higher speed in motors and converters, with corre- spondingly higher capacities. Also for direct coupled alterna- tors, the two-to-one ratio of frequencies would fit in nicely with engine speeds, in most cases. Possibly, from the present view- point, the choice of thirty cycles, would have longer retained the double standard. Something further may be said regarding the 40-cycle system, brought out by the General Electric Company. This contained many very good features, for the time it was brought out. It was then believed that if the 60 cycle frequency was retained, the double standard was necessary. The 40-cycle system was an attempt to eliminate this double standard. It apparently furnished a better solution than 60 cycles then promised for the synchronous converter problem, and was a fair compromise in about everything else. But it came too late, for the 25-cycle system was too firmly entrenched, and for further development, the designing engineers preferred to expend their energies in seeing what could be accomplished with 60 cycles, as this seemed to present greater possibilities than either 25 or 40, if it could be stiiEciently perfected. Thus the 40-cycle system probably missed success due to being just a little too late. As to 50 cycles, it was stated that this is still in use to a limited extent. Most of the 50-cycle plants in this country are in Cali- fornia. Such plants were started during the nebulous period of the frequencies, and have persisted, to a certain extent, partly because certain 60-cycle apparatus could be easily modified to meet the 50-cycle requirements. Also, as 50 cycles is the standard in many foreign countries to which this country exports equipment, the use of 50 cycles in some home plants has not been unduly burdensome from the manufacturers' standpoint. In addition to the preceding, there have been certain classes STORY OF THE FREQUENCIES 583 of "electric service which have depended upon frequency, but which have not been a determining factor in fixing any par- ticular frequency. Among these may be considered commutat- ing types of a-c. apparatus. The first a-c. commutating mo- tors of any importance, which appeared, were, of course, the 25- cycle, single-phase railway ' motors. These as a rule have operated from their own generating plants, or from other plants through frequency-converting machinery. One exception in the railway work may be noted in the use of 15 cycles on the Visalia plant in California. There is a pretty well defined opinion among certain engineers experienced in such apparatus that some low frequency, such as 15 cycles, would present very considerable advantages in the use of single-phase railway motors in very heavy service, such as on some of the western mountain roads. Here the problem is to get the largest possible motor capacity on a given locomotive, and the main advantage of the lower fre- quency would be in allowing a very materially higher capacity within a given space. This does not imply reduced weight or cost compared with the 25 cycles, but simply means greater motor capacity. With the modern, more highly developed, single- phase types of railway motors, it would appear that there may be very considerable possibilities in 15 cycles. Outside of the railway field, there has been more recently a development of various types of a-c. commutating apparatus, principally in connection with heavy steel mill electrification work. Such apparatus has been largely in the form of three phase commutating machines and these have been used prin- cipally in connection with speed control of large induction mo- tors. As these regulating machines are usually connected in the secondary circuits of induction motors, the frequency supplied is represented by the slip frequency. Consequently where the slip frequency never rises to a large percentage of that of the primary system, such commutating motors are applicable without undue difl5culties. Such motors, presiimably are better adapted for 25-cycle mill equipments than for 60-cycle, but due to the ten- dency, already described, for steel mills to go to 60 cycles on pur- chased power, it has be'en necessary to build these three-phase "commutating motors for the regulation of 60-cycle main motors, in many cases. There is still another class of service, which has come in re- cently, where the choice of frequency is of much importance, but where there is no great necessity for adhering to any standard. 584 ELECTRICAL ENGINEERING PAPERS namely, in heavy ship propulsion by electric motors. As each ship equipment is a complete system in itself, and as it cannot tie up with other systems, there is not any controlling need for main- taining any definite frequency or voltage. Except in similar vessels, there is little chance for duplication in parts, as the various equipments vary so much in size and capacity. In conse- quence it has been found advisable, at least up to the present time, to design each propulsion equipment for that frequency which best suits the generator and motor speeds, taking into ac- count the various operating conditions and limitations, such as the different running speeds, steaming radius, etc. In conse- quence, different maufacturers bidding on such equipments may specify different frequencies, depending upon the construc- tional features of their particular types of apparatus. At the present time with the relatively small amount of experience ob- tained with the electrical propulsion of ships, it looks as if it would be a considerable handicap to attempt to adopt some standard frequency for all service. Later, with wide experience, it may be possible to adopt some compromise frequency, which will not unduly handicap any of the service. Conclusion It has been the writer's intention to show that, as a rule, the, choice of frequency has been a matter of most serious considera- tion, based upon service conditions at the time. Moreover, in view of the wide range of conditions encountered, it is surprising how few frequencies have been seriously considered in this coun- try. Occasion has arisen, times without number, where an obvious solution of a given problem would lie in modification of the frequency to allow the use of apparatus and equipment already designed, but the engineers of the manufacturing or- gahization have steadily held out against such policy, regardless of the apparent need of the moment. The swing of the pendulum from 60 cycles to 25 cycles and back, has covered a period of many years and, therefore, cannot be considered as a fad of the moment, but is the result of well defined tendencies, backed by the best engineering experience available. As a rule no manu- facturer has made any particular frequency his "pet," but all have worked to develop each system to its utmost. DEVELOPMENT OF THE A.C. GENERATOR 585 THE DEVELOPMENT OF THE ALTERNATING- CURRENT GENERATOR IN AMERICA FOREWORD — The following article, which just appeared in the Electric Journal, contains a fairly complete brief history of the evolution of the alternating- current generator in so far as the Company with which the writer is connected is concerned, as drawn from the writer's memory principally. Reference is made incidentally to the work of other manufacturing companies, but this cannot be very complete, as the writer nat\lrally does not have the necessary material available for describing such developments, except in a very general way. Therefore, the inadequacy of this history, out side of the Westinghouse part, should be charged to the lack of data rather than to lack of respect for the work of others. — (Ed.) IN the early days of the alternating-current generator, it was constructed in almost as many types as there were designers. The principal endeavor of each designer appeared to be toward the development of a new alternator which would bear his name. A few of these early types were of the rotating field construction, while a much greater number were of the rotating armature type. Some had iron core armatures, while others had coreless armattires, and there were many discussions as to whether the core or the coreless type was superior and would survive. Many of the early predictions wotdd now form quite interesting reading, in view of the fact that present practice is so far removed from the early anticipations. Here and there among the early machines was one which contained some of the important elements of recent ap- paratus, but in many cases such machines disappeared in the general course of development, the meritorious features being instdficient to save the type. Surface Wound Armatures In America, the principal early type of alternator had a rotating armature with surface windings and an external cast iron multipolar field. This type was used very considerably or, in fact, almost exclusively, from 1886 to 1890. This was the type built by the Westinghouse and the Thomson-Houston Companies. There were only minor differences in the construction of the ma- chines built by these two companies which, however, at that time, appeared to be very great. These differences consisted principally in the way the end windings of the armature coUs were supported, in the construction of the end bells and ventilating openings in the armature core, in the method of attaching the armaturecore to the 686 ELECTRICAL ENGINEERING PAPERS shaft, in the winding of the field coils in metal bobbins, etc. Both machines had surface windings with concentric coils, one layer deep in the radial direction. In the Westinghouse construction, the end windings were turned down toward the shaft and were supported by radial wooden clamps, as indicated in Fig. 1. In the Thomson-Houston armature, the end windings were arranged in an axial instead of a radial direction, and were supported by bands or external clamps. This construction is also indicated in Fig. 1. The Slattery machine, which was also on the market at that time, was of the same general type as the above machines. Presumably these two different methods of end winding were used on accovint of the patent situation. At that time there was much discussion of the respective merits of the two constructions. These early machines were built principally for frequencies of 15,000 and 16,000 alternations per minute (125 and 133 cycles per second) . In those days, everything was rated in alternations per minute, as this represented the product of the number of poles by the number of revolutions. Such high frequencies were selected, mainly, on account of transformer conditions, and not alternator design. Practically aU alternating service consisted of house to house lighting, and in relatively small units, and the higher fre- quency was supposed to be of great advantage in transformer design and operation, which presumably was the case with the very small amount of data and experience available at that time. About the only commercial voltage for alternating work at that time was 1000 or 1100 volts. This was supposed to be ex- cessively high and dangerous, and there was much question whether such an excessive voltage should be permitted. This matter was actually taken before a number of the state legislatures for the ptupose of obtaining laws prohibiting or limiting the use of such voltage. Another reason why no higher voltage was used was in the construction of the alternators and transformers. With the experience and materials available at that time, together with the high speed rotating armature construction and the surface windings, even 1100 volts was a very serious problem in the gen- erator. About 1889 or 1890, there appeared some slight demand for higher voltages, and a few 2000 or 2200 volt surface-wound alternators, of the then standard type, were bttilt. However, even then it was recognized that the surface-wound type of alternator was not well adapted for higher voltages, and there was much question whether a different type winding shotdd not be developed DEVELOPMENT OF THE A.C. GENERATOR 587 for 1100 volts. This gradually led to the next big step, namely, the development of the "toothed" type of alternator with one big tooth per pole, in distinction from the slotted type of armature with a nimiber of slots per pole, which was a considerably later development. Toothed Armatures The first commercial toothed type of armature appears to have been gotten out by the Westinghouse Company. These first ma- FIG. 1— SURFACE WOUND ARMATURE WITH RADIAL CLAMPS (UPPER) AND WITH AXIAL CLAMPS (LOWER) chines were radically different, in details of construction, from the later toothed armature types of machines which came into general use. The first toothed armatures were small air-gap machines. In the surface-wound armatures, the clearance between the arma- ture surface and the field poles was comparatively small, although the total air-gap (iron to iron) was large on account of the surface winding. In constructing the new toothed armature, the actual clearance (iron to iron) between armature and field was kept about the same as in the surface-wound alternators (bands to iron), but this clearance actually represented the total air-gap in the toothed type. Moreover, in sinking the windings below the surface, it was endeavored to maintain a practically uniform outside stirface, so that overhanging tooth tips were used with relatively narrow slots for putting in the windings. The general construction was similar to Fig. 2. On account of the small clearance, and consequent higher magnetic conditions, it was found necessary to use lamin- 588 ELECTRICAL ENGINEERING PAPERS ated poles with these machines in order to avoid excessive field heating. The self-induction of the armature windings on these machines was very high compared with the old surface-wound armatures and therefore, in order to o]Dtain passably good regulation, fewer armature turns had to be used, with correspondingly higher induc- tions, and this made the use of solid poles impracticable on account of heating. Furthermore, on acc^ount of the overhanging tooth [Za FIG. 2. tips, the small air-gap and the high induction per pole, this early type of toothed armature was very noisy. In one instance, it was credibly reported that one of these machines could be heard two miles away on a quiet night. However, several machines of this construction were put out by the Westinghouse Company, and operated for many years. Meanwhile, the possibilities of the toothed armature con- struction in the old cast iron field were being given consideration. The writer made a special study of this matter, and finally decided that, in order to make this construction possible with solid cast iron poles, it would be necessary to work at relatively low induc- tions per pole, and with a very large air-gap, (fully as large as on the old surface-woimd machines) and with a shape of tooth tip which did not have such great width compared with the pole tip as shown in Fig. 2. This meant that a pole tip and air-gap afe shown in^Fig. 3, should be used. With this arrangement, the armature DEVELOPMENT OF THE A.C. GENERATOR 589 self-induction would still be relatively high, and the regulation correspondingiy bad, necessitating some form of compounding for regulating the voltage, similar to the compounding of a direct- current generator. This armature construction was worked out in detail for a 37.5 kilowatt field (that is, for the standard field of the 3 7 . 5 kw surface-wound type of machine) . The armature teeth FIG. 3. were similar to those in Fig. 3, in shape, and the air-gap or clear- ance from iron to iron, was made ^ inch on each side of the machine. The field was also compounded. When this machine was put on test, it was found at once that it could be loaded to 60 kilowatts without undue heating of the armature and field iron, and the problem of perfecting this machine then became one merely of increasing the amount of armature copper to carry the cttrrent at the 60 kilowatt rating. This, therefore, represented a big step in the development of the American type alternator. It was found that all the other Westinghouse standard cast iron machines of the rotating armature type could readily be changed in line with the above improvement. Compounding Alternators The compounding of the 60 kilowatt machine was not a new feature, for already some of the laminated field toothed-armature type of machines had been compovmded, in order to improve their regulation. Two different methods of compounding alternators had been developed by the Westinghouse and Thomson-Houston Companies, respectively. In the Westinghouse armature construe- 590 ELECTRICAL ENGINEERING PAPERS tion, the armattire discs were punched in single pieces, with spokes, and were threaded directly on the armattire shaft, no spider being used. This construction is illustrated in Fig. 4. In the assembled armature, the spokes were therefore of laminated material. These laminated spokes were utilized as the core of a compounding trans- former. One lead from the armature winding was carried around the spokes of the armature before passing to the collector ring. This winding formed the primary of a series transformer. The secondary was also wound on the spokes, and the two ends were carried to the bars of a rectifying commutator on the shaft. The number of commutator bars was equal to the number of poles. The, alternating oirrent from the secondary winding was by this means changed to a pulsating direct current. In the Thomson-Houston method of compounding, the main armattire cvirrent was carried directly to a rectifying commutator, and, after being commutated, was passed to the field-compound winding, and back to the commutator, and then to a collector ring. FIG. 4— SKETCH OF COMPENSATED TYPE OF WINDING The maia armature current therefore passed directly through the field, while in the Westinghouse method the secondary cturent from a series transformer was passed through the field. Both methods represented series compounding, and gave practically equal results, but there was much discussion as to the merits of the two methods. Both of these methods deUvered pulsating direct current to the field winding. There was considerable inductive e. m. f. set up in the field windings by this pulsation, and this tended to cause inductive discharges across the rectifjdng com- DEVELOPMENT OF THE A.C. GENERATOR 591 mutators. In the Thomson-Houston method this trouble was overcome to a considerable extent by the use of a non-inductive shunt in parallel with the rectifying commutator, i.e., across the compound winding. In the Westinghouse method a similar result was accomplished by saturating the series transformer (or armature spokes) to such a high point that the inductive kick from the field could readily discharge through the secondary winding of the transformer without giving high enough voltage to flash across the commutator. FIG. 5— EARLY WESTIMGHOUSE 69 KIL0W.\TT ALTERNATOR WITH COMPENSATING WINDING The details of this method of compounding have been gone into rather fully, as this compotmding was, at that time, an im- portant step in our progress. The fact that of all these machines were built for single phase, allowed us to use such compounding. With the advent of polyphase generators, such methods of com- pounding soon disappeared, principally for the reason that a great majority of the early polyphase machines handled separate single- phase loads on the different phases, and it was not practicable to compound for these independently. The above toothed type generator came into use about 1890 and lasted for several years, or practically until polyphase gener- ators actually came into fairly general use before true polyphase loads became common. These toothed type generators allowed 592 ELECTRICAL ENGINEERING PAPERS the use of relatively high voltages, as far as the armature winding was concerned, so that 2200 volts became comparatively commpn, and even 3300 volts or higher was used in some cases. In fact, the hmit in such machines appeared to be at the collector rings, rather than in the armature winding. Something may be said regarding the type of winding used on the armatures of these machines. In the Westinghouse con- struction the armature coils were machine-wound and taped before placing on the armatvLre core. Each coil was made wide, enough to slip over the top of the armat\u-e tooth, as shown in Figs. 6 and 7. This made the coil considerably wider than the body of the armature tooth, so that, after slipping over the tooth top the coil had to be reduced in width by special clamping tools. Supporting wedges were then driven in between adjacent coils. Something may be said regarding the temperatures of these early alternators, both of the surface-wound and of the toothed types. In those days temperatiu-e measurements were- very crude compared with present practice, which is admittedly still only approximate. In some of the surface-wound armatures excessively high temperatures must have been encountered in many instances, judging from the appearance of the insulation on the individual wires, after a year's service, for instance. However, FIG. 6— SKETCH SHOWING METHOD OF PUTTING MACHINE WOUND COILS ON THE POLES it was difficult to obtain reasonably correct temperature of the armature windings, for the actual temperature of the conductors was undoubtedly reduced very greatly before the armature could be brought to a standstill. Even after this, temperature rises of 50 or 60 degrees C. were not considered as excessively high. With- out doubt, some of these early machines, at times, attained actual internal temperatures of 120 to 130 .degrees C, or even higher, with insulation on the conductors consisting of untreaded cotton fibre. No overloads were possible, for each size of machine was rated in as many "lights'" as it could carry on a shop test without DEVELOPMENT OF THE A.C. GENERATOR 393 breaking down. The first few minutes, while starting up a new alternator in the testing room, were always anxious ones for the operators, especially so if any "improvements" had been made on the armature winding. Any defect in winding, or wrong con- nection usually resulted in a stripped armattu-e and much fl3^ng copper. If nothing happened within the first few minutes after the machine was put on load, the attendants all came out from behind posts and other protections and went on with their work. When the toothed armattue came into use the above condi- tions were much alleviated. Defects in construction, or short- circuits, could not strip such armatures, and thus the danger and excitement were removed. However, it was found that the first short run did not tell the story of excessive heating as promptly as in the case of the surface-wound type. Experience showed that the toothed construction apparently covild stand a severe shop test and still go wrong under similar loading within a short time after being installed. It was found that a given size of conductor would not carry as much current in the concentrated coils of the toothed construction as was the case in surface-wound coils. However, the method of testing the temperature did not show this, as the main part of the toothed armature coil was so embedded and so covered with insulation that the thermometer readings did not indicate nearly the true temperatures. The size of wire and the amount of copper in the coils then had to be increased until the machines did stand up in service. The true explanation of the discrepancies was not well understood at that time. In these toothed alternators, as in the surface-wound machines, the first machines were rated in "lights," but gradually the kilowatt rating came into use and became standard practice. Introduction of Polyphase Alternators In 1892 and 1893, polyphase alternators began to be con- sidered seriously. In 1889 and 1890, a few such alternators had been built for the operation of Telsa induction motors. These early polyphase alternators were of the surface-wound, rotating armature type. These machines were very special in construction, and, Uke the Telsa motors, did not find much of a market. How- ever, in 1892- and 1893, it began to be recognized that the best w2fy to encotirage the development of the induction motor would be by creating a demand for it, and it was decided that a good way to create a demand would be by encom-aging the general adoption of 594 ELECTRICAL ENGINEERING PAPERS polyphase alternators and supply circuits, with the idea that, when a suitable supply circuit was available, there was eventually bound to be a demand for motors to operate upon such circuits. With this general policy in view, there was great activity in the develop- ment of polyphase generators. This very quickly led to a very considerable departure in armature construction from the usual toothed armature as used in single-phase machines. The poly- phase winding, requiring two or more coils per pole, naturally FIG, 7— VIEW OF ARMATURE IN PROCESS OF PLACING COILS AND CLAMPING THEM INTO SHAPE IN THE SLOTS tended toward the slotted armature construction with two or more slots per pole. It was soon recognized that, in general, the larger the nimiber of slots per pole and the smaller the number of con- ductors per slot, the better would be the general characteristics of the machine, so that the construction naturally tended toward the modern slotted type. Moreover, practically aU the development in pol5rphase alternators was at relatively low frequency, com- pared with former practice. It so happened that there had been a well-defined tendency toward lower frequency in the period from 1890 to 1892. This tendency was largely independent of the induction motor problem for, at the time it became most pro- nounced, there was no true induction motor problem. It was becoming recognized that 125 to 133 cycles per second was too high for certain classes of work and for engine type generators, and that, in general, a very considerably lower frequency must event- ually be adopted. A great many lower frequencies were tried by the different manufacturing companies, ranging from 50 to 85 cvcles. However, 60 cycles seemed to have the preference at the time polyphase alternators began to come in. The early polyphase generators were mostly of the rotating armature type, and usually with a fairly large number of slots per DEVELOPMENT OF THE A.C. GENERATOR 595 pole. One notable exception was the "monocycle" machine which usually had only two slots per pole, one large slot for the main armature winding, and one smaller slot for the so-called "teaser" winding. Also, the early two-phase alternators of the "inductor" type, built by the Stanley-Kelly Company, frequently had only two slots per pole. However, it may be said that, from 1893 to about 1898, the great majority of the American built alternators were of the rotating armature type with distributed armature windings. The principal exceptions were the Stanley inductor type of machines and a few special "rotating field" machine's, as distinguished from the inductor type. The rotating armattire machines were usually of 1100 or 2200 volts, although a few of considerably higher voltage were con- structed. A few cases may be cited where special constructions were used. For instance, the principal lighting plant at the Chic- ago World's Fair in 1893, consisted of a large ntunber of Westing- house "twin" type generators. Each tinit had two single-phase, standard toothed tjrpe armatures side by side on the same shaft. The teeth of the two armatures were staggered 90 electrical degrees with respect to each other, so that the two together cotald deliver currents having 90 degrees relation to each other. The object of this construction was to obtain polyphase current with standard single-phase types of machines without any radically new development. This type of unit did not persist and, in fact, was simply an expedient for this particular occasion. The First Niagara Generators Also, in 1892 and 1893, the first large Niagara electrical de- velopment was worked out. The advisory engineers of this plant proposed 5000 horse-power generators, having stationary internal armatures and rotating external fields to obtain large flywrheel capacity. In fact, the construction was not unHke the usual rotating armature machine of that period, as far as general ap- pearance of the armature and field cores and windings were con- cerned. However, the method of supporting and rotating the heavy external field at a speed which, at that time, was con- sidered excessively high, required an "umbrella" type of field support, which gave these machines a distinctive appearance. This type of construction did not persist, although these early machines are still in operation. 596 ELECTRICAL ENGINEERING PAPERS A further distinctive feature in these first Niagara machines was in the frequency employed. A speed of 250 revolutions per minute was decided upon. The engineers of the power company proposed eight-pole machines, giving 2000 alternations per minute or 16 2-3 cycles per second. The Westinghouse Company pro- posed, as an alternative, 16 poles, giving 33 1-3 cycles, the advan- tages claimed for this frequency being that it was better suited for motors and rotary converters, which were then promising to become of importance. One advantage claimed for the 16 2-3 cycle machine was that it would permit the use of commutator type alternating-current motors. After much discussion, and weighing and balancing of all the various arguments for and against these two frequencies, it was finally decided to use 12 poles, giving 3000 alternations per minute, or 25 cycle polyphase current and,, as far as the writer knows, this was the origin of the present 25 cycle standard. Considering what a radical departure from ordinary construc- tion was made in these first Niagara generators, it is self-evident that many curious and interesting conditions developed during their design, construction and tests. As far as the writer knows, these were the first large alternators which were deliberately short-cir- cuited at their terminals when running at full speed and at normal field charge. There were no instruments available to measure the first current rush, but it was obvious that this current was far greater than the steady short-circuited current of the machine under similar field charge, for there were ample evidences of a ter- rible shock at the moment of short-circuit. It was suspected at that time that the first rush of current was only limited by the armature impedance, and not by the so-called synchronous im- pedance which fixes the value of the steady short-circuit current. This also was the earliest machine of which the writer pre- determined the field form and wave form by analysis of the flux distribution. Later, when making shop tests on one of these machines, the e. m. f . wave form was measured directly by rotating the field at normal field charge at such an extremely low speed that a voltmeter connected across the armature terminals showed such gradual variations in' e. m. f . that readings taken at regular intervals could be plotted to form the voltage wave. Slow rotation was obtained by means of a steel cable wrapped about the outside of the external field and with one end of the cable attached to a small diameter spindle around which it was wrapped at a very DEVELOPMENT OF THE A.C. GENERATOR 597 slow rate. This was a very crude method, but the wave form thus obtained checked very accurately with tests made some years later. Also, the early Niagara machines embodied one of the first distinct attempts to ventilate alternators artificially. Early belted machines had had small ventilating bells on each end. But these Niagara machines were designed primarily with a view to setting up an abnormal air circtilation by means of special "scoops" or ventilators on the umbrella supports. Very much thought and discussion were given to this subject of artificial ventilation. The restilts of our tests led to the arrangement of the scoops so that they acted as exhaust pipes. Also, water cooling of the armature spider was tried on some of these early machines, but proved ineffective, due to the fact that the cooling medittm was applied too far away from the point of development of the larger part of the armature iron and copper losses. Influence of Direct-Current Design It must be kept in mind that the general trend of direc- currenf development had a certain influence on alternating-current generator work. For example, there had been a slow, but positive tendency in direct-current generators, toward the engine type construction. Also, from 1890 to 1893, direct-current generator armature construction had changed from the surface wound to the slotted type. This doubtless had some influence in .changing al- ternator design toward the slotted type, especially when the poly- phase type of windings came into use. Also, there was a pro- nounced tendency toward the engine type, slow speed alternator, accompanying direct-current practice. Practically all of these early engine type alternators, except the inductor type, had rotating armatures. Meanwhile, an interesting development took place in the armature construction of some of these machines. In most of the smaller belted machines, open armature slots were used with machine-woimd armature coils. However, many of the early larger machines, especially of the engine type, were built for relatively low voltage, such as 440 volts, two or three phase. This admitted in many cases of simple bar windings with one or two conductors per slot. This allowed partially closed armatilre slots with shoved-through straight conductors, and bolted-on end windings, giving a very strong substantial type of winding for re- sisting the rotational stresses. The partially closed slot became a 598 ELECTRICAL ENGINEERING PAPERS sort of standard in Westinghouse machines, and enditred for a number of years, and was even carried into the stationary armature type of machine when rotating fields came into general use. This partially closed slot arrangement was a very good one as long as the generator voltages were relatively low. The same may be said of the rotating type of armatiire as a whole. However, when high voltages came into more general use, a different construction was preferable. In reviewing the period of the rotating armature, slotted types of machines, the monocyclic system shoxold be briefly described. Apparently this was gotten out with the idea that it avoided the patented features of the Telsa polyphase system. The armature circuits on this monocyclic system were so arranged that, when carrjring load, one phase carried nearly all of the energy load, while both phases supplied magnetizing current for the operation of induction motors. Dtiring the period when this machine was in vogue, single-phase lighting work represented the principal service, while induction motor loads were relatively small. With increased use of polyphase loads, and with the elimination of the patent situation, the monocyclic system gradually dropped out. It was early recognized that a stationary armature winding would be an ideal one in some respects, but it was thought that any rotating field construction was bound to be a difficult and expen- sive one. The inductor type construction was supposed by some engineers to overcome the objections to the rotating field, but many others considered that this type was not a final one, as it did not use the magnetic material in the machine to the best advan- tage. In the earlier alternators, with insufficient ventilation through the armature core, relatively low magnetic densities were necessary to avoid excessive iron heating, and the inductor alter- nator, with its non-reversal of armature flux, was worked at almost double the induction of the rotating armature type of machine, and thus the disadvantages of the non-reversal of flux of the in- ductor type were masked. In other words, the inductor alter- nator waas worked weU up toward saturation, while the other types were worked at only about half satviration. However, with improvements in ventilation due to radial ventilating ducts, im- provements in iron by better annealing and painting of the lam- inations, etc., the flux densities in the rotating armature machines were gradually increased until high densities, approaching satura- tion, were reached. A corresponding increase in flux densities DEVELOPMENT OF THE A.C. GENERATOR 599 in the inductor type was not possible, on account of saturation. Therefore the rotating armature type of machine, in the later designs, was much more economical than the inductor t3^e, al- though the latter had a very considerable advantage, especially at high voltages, in its stationary armature construction. Due to the merits of the stationary armature construction, the present rotating field type of machine was gradually evolved, which pos- sesses the advantages of the stationary armature of the inductor type machine and the reversing fltix of the rotating armatiore al- ternator. It was the development of this type of machine which sounded the death-knell of the inductor type. However, the Westinghouse Company, about 1897, decided to bring out a line of inductor type alternators to meet market conditions, although such decision was contrary to the recommendations of the design- ing engineers of the company, whose recommendation in particular was in favor of the rotating field construction as a more permanent type. However, as the rotating field type was not yet established, except in a very minor way, and as the inductor type had been on the market for years, it was decided to build the inductor type, although the design adopted was somewhat different from the Stanley type. Three sizes of these machines were built, two belted and one engine type, but the inductor type, as a commercial pr6position, soon died out. One of the interesting peculiarities of the inductor type alternator, as usually built, was in the enormous stray field appear- ing in the shaft, bearings, bedplate, and sometimes in the engine- governing mechanism in engine type units, necessitating in at least one case, the use of brass governor balls. In the usual con- struction of inductor alternator, there was but one exciting wind- ing. The magnetic circuit and the field winding were arranged as in Fig. 8, which shows both the Stanley and the Westinghouse constructions. The normal or useful path of the magnetic flux is indicated by the dotted Hnes a, a. Obviously, the field coil which set up flux through these paths could also send magnetic fluxes through the shaft, bearings and bedplate along the dotted lines h, b. Moreover, if the two bearings were not connected by a magnetic bedplate, as might be the case in engine type machines, then, in two-crank engines the engine cylinders and other parts became opposite poles of a very powerful electro-magnet, when the field coil was excited. The stray magnetic field set up in engine type units was sometimes so strong as to interfere with the 600 ELECTRICAL ENGINEERING PAPERS governing mechanism. Also, with a strong unidirectional flux between the bearings and shaft, each bearing became part of a small imipolar generator, of which the bearing surfaces formed the brushes. .In some machines, quite heavy currents were generated in the bearings, sufficient to ' ' eat away ' ' the bearing surfaces or to pit them so that bad bearing operation resulted. As this was primarily a magnetic trouble, insulating the bearings from their pedestals would not stop the action. To overcome this trouble, FIG. 8— SKETCH OF MAGNETIC CIRCUIT OF AN INDUCTOR ALTERNATOR the Stanley Company added a ' ' bucking ' ' coil placed around the shaft at one side of the generator, this coil being in series with the main field winding and magnetizing in the opposite direction.. The ampere-turns of this bucking coil being made equal to those of the field coil, the resultant ampere-turns between the two bearings would be zero. Obviously, in an alternator with a bedplate and two bearings in which the armature frame rested directly on the bedplate, a single bucking coil at one side of the machine would not neutralize the stray field through both bearings. Rotating Field Generators Considering next the rotating field type of machines, possibly the earliest example was the Niagara type, mentioned before. This had an internal stationary armature, with windings on its outer periphery like the ordinary rotating armature. Outside this was the rotating field, consisting of a heavy forged steel ring with inwardly projecting poles. However, this type of construction was relatively expensive, and was never adopted generally. The more modern DEVELOPMENT OF THE A.C. GENERATOR 601 rotating field type of alternator, with external stationary artnature, was a rather gradual development and, during this period, there was much heated discussion as to the relative advantages of the rotating field and rotating armature types. The rotating field gradually superseded the rotating armature construction for a num- ber of reasons, the principal one having to do with the armature windings and voltages. In the rotating armature, the end wind- ings were more difficult to support than in the stationary armature. Also, with the gradual advent of higher voltages, the stationary winding proved to be far superior. However, as a .goodly pro- portion of the alternators built during this transition period were of the engine type and for low voltage, in which heavy bar wind- ings could be used, (such being conditions imder which the rotat- ing armattu-e made its best showing,) this type persisted for several years after the rotating field type became commercial. Gradually increasing voltages, however, necessitated the use of stationary armattire machines, for at least part of the business. The manu- facture of two types of apparatus for the same general purpose could not persist, and eventually that type was adopted exclusive- ly, which aJlowed both high and low voltages. By 1900, the rot- ating field alternator had come into very general use, and the rotating armature type was disappearing. This rotating field type has persisted until the present time, although many nndnor modi- fications have been brought out from time to time, due largely to change in speed conditions, etc. In the rotating field development, the tendency for a number of years was strongly toward the engine type construction and relatively low speeds in many cases. The construction was carried to the extreme, in some cases, where the usual flywheel capacity required for the slow speed engines was incorporated in the field structure of the alternator itself. In some cases, this meant enormously large machines for the output. A prominent example of this is found in the seventeen 6 000 kilowatt engine-type ma- chines designed in 1899 and 1901 respectively, and installed in the Fifty-ninth and Seventy-fourth Street power stations of the Interboro Rapid Transit Company of New York City. As an indication of the changes taking place in the electrical field, it may be stated here that arrangements have been made recently to take out a number of these machines and install in their place 30 000 kw tvirbo-generator units. The existing engine type machines are probably in as good condition now as when first installed, and are 602 ELECTRICAL ENGINEERING PAPERS being replaced simply because they occupy too much space in proportion to their output. The rotating field alternator of the early days was not radically different from the rotating field alternator of today, the principal FIG, 9— SHOP VIEW OF 5 000 HORSE-POWER TWO-PHASE NIAGARA ALTERNATOR differences being in the type of armattire windings, methods of ventilation, etc. Field Construction In the types of field windings there has been but little change. In many of the old stationary field machines of large capacity, the field windings consisted of strap wound on edge, one layer deep. For smaller machines, either square or round wire was commonly used. In the latter rotating field machines, similar constructions are used. In the construction of the field itself, there have been some variations and modifications. In many of the older ma- chines the poles were laminated as at present. The method of DEVELOPMENT OF THE A.C. GENERATOR 603 attaching the poles varied in different constructions. In many of the earlier Westinghouse rotating fields the laminations were punched with two or more poles in one piece, the poles having no overhanging tips, and the field coils being held in place by metal wedges between pole tips, fitted into notches or grooves at the pole tips, each pole being attached to the field ring or yoke by means of bolts or dove-tails. This latter construction possesses numerous advantages, in that cheap dies can be used, and the same pole punchings can be used for a number of different designs, in which either the diameter or the number of poles is varied. Water Wheel Type Generators With the advent of the tvirbo-generator on a large scale, the engine type rotating field alternator almost disappeared from the manufacturing field, except in the smaller size units. However, during this period there has been a gradual development in the use FIG. 10— STATOR OR ARMATURE OF NIAGARA ALTERNATOR, of water powers, and water-wheel driven generators have come into much greater prominence in the past few years. In this line of development, speeds and capacities, unheard of in the earHer days, have become accepted practice with the development of both high-head and low-head water powers. In the former the ten- dency has been toward very high speeds for a given capacity such 604 ELECTRICAL ENGINEERING PAPERS as the 17 000 k.v.a., 375 r.p.m., Westinghouse machines, built for the Pacific Light & Power Company, and the 10 000 k.v.a., 600 r.p.m., Westinghouse generators, built for the Sao Paulo plant in Brazil. Typical examples of low-head, slow-speed .practice are found in the 60 cycle, 75 r.p.m., 96 pole, 2 700 k.v.a. Westinghouse generators for the Stevens Creek development, and the 25 cycle, 58 r.p.m., 52 pole, 9 000 k.v.a. General Electric generators for the Keokuk plant. The former are abnormal in the very large number of poles required for moderate output, while the latter are ab- normal in the very low speed. Both of the above machines are of the vertical type, and are examples of a very pronounced tendency toward vertical machines, which has been apparent in the later water wheel practice. On account of the high speeds of some of the modern rotating field alternators, mechanically stronger spiders have come into general use. Even .in moderate speed units the usual high run- away over-speed of 100 percent has necessitated the use of very substantial spiders. During the past few years, some very interesting spider con- structions for the rotating fields of large high speed alternators have been built to meet the severe speed requirements. Some of these have been made up of cast steel centers or spiders with cylindrical rims built up of overlapping laminated punchings ,thoroughly bolted together and attached to the spider by dove-tails. The outer periphery of the laminated ring carries dove-tail grooves for poles. In another construction, the entire spider consists of thick rolled iron plates, bolted together, and with dove-tail grooves on the out- side for the poles. In still other constructions, the rim of the spider consists of a heavy steel ring in one or more sections to which the cast spider is bolted. Usually with this cast rim the poles are bolted to the spider. In some cases the rim forms an integral part of the spider itself, being cast with the spokes and hub. The type of construction adopted in each case is, to a large extent, dependent upon the stresses to be taken care of, so that no one type seems to fit all cases to best advantage. The Problem of Ventilation In the later rotating field alternators the problem of, ventila- tion has received much consideration, especially in the case of machines operating at abnormal speeds. In very high speed machines of very large output the armature and field cores have DEVELOPMENT OF THE A.C. GENERATOR 605 a ratio of width to diameter which is relatively much greater than in ordinary machines, and this has necessitated abnormal conditions of ventilation. Something may be said here regarding the general problem of ventilation of alternators and its^ influence on the evolu- tion. Back in 1891 or 1892, radial ventilating ducts or passages came into use commercially on certain direct-current machines. The results being quite satisfactory, it was natural that alternators should have the same method of ventilation. The use of such ducts was in reality one of the great steps forward in the evolution of dynamo-electric machinery, although but little recognition has been given to this fact in electrical literature. The use of radial ventilating ducts has continued to the present time with little change except in the construction of the spacers themselves, which have been many and varied in design and materials. With the change from the rotating armature to the rotating field construc- tion of alternators this feattire was retained in full. In some of the earlier Westinghouse rotating field machines the field structure also had numerous ventilating ducts, principally for the ptirpose of supplying ample air to the armature ducts. Also, about ten years ago, special ventilating end bells and vanes began to be used on rotating fields, in order to set up an extra air circulation through the armature end windings, etc., due largely to the fact that the slow-speed engine-type machines of that period did not have much natural blowing action. Following this, and partly as an out- growth of turbo-generator enclosing, came the semi-enclosed alternators, mostly for high-speed water-wheel driven units, and this practice is not uncommon at present. The proper ventilation of an alternator or, for that matter, of any dynamo-electric machine, is very much of a problem, for no two cases, in different sizes or types of machines, are quite alike. The problem Hes first, in furnishing the proper quantity of air to carry away the heat developed, and in then distributing such air in proper proportion through the complex multiple paths in the machine. The proper distribution of the ventilating air is usually the most serious part of the problem. The present solutions of the problem are based largely upon past experience, and no really work- able rules have yet been developed. In arriving at the present practice many disheartening experiences have been undergone by all designing engineers. The writer has known many cases where totally unexpected resvilts, both good and bad, have been devel- oped and, in some of these cases, no logical explanation was forth- 606 ELECTRICAL ENGINEERING PAPERS coming, so that the results could not be taken advantage of, with any assurance, in future work. This has been one of the most discouraging features in the general problem of ventilation. Armature Windings Something might be added here on the subject of armature windings. There have been probably as many types of armature windings developed as there have been types of alternators. The windings for the earliest smooth body and the toothed armature constructions have already been described. In the early West- inghouse polyphase alternators, two-phase was used mostly, due principaUy to the fact that single-phase lighting circuits formed the principal load, and, with two-phase machines, there were only two circuits from a machine instead of three circuits with the three- phase winding. Moreover, many of these very early polyphase t* ^J "flVn D D aD~rr~" D_il_ fig: 11— various types of early stationary armature windings alternators were used in reality as straight single-phase machines, taking current off one phase only. For this purpose a closed coU armature winding, like that of a direct-current generator or rotary converter, with four taps for taking off the two phases, gave about the most economical type of winding, as far as arma- ture copper losses were concerned. When such an armature is used for single-phase it can deliver seven-tenths as much output DEVELOPMENT OF THE A.C. GENERATOR 607 as a single-phase machine as it can give two-phase with the same total copper loss per coil. It was partly for this reason that many of the early Westinghouse polyphase machines had a single closed coil winding. Another reason for such winding was that there were no definite phase groups and no high potential between phases. Furthermore, the arrangement of the end windings was such that the coils tended to interlock and support each other, thus assisting in resisting centrifugal forces. This winding was used mostly for two-phase machines, but was also used to a con- siderable extent on three-phase armatures. With the advent of the rotating field type of machine, this closed coil type of armature winding was not used to any great extent, open-coil two-phase and star-connected three-phase taking its place. Delta-connected three-phase was used in very rare cases, as there was danger of circulating current with such wind- ings. In the construction of armature windings possibly more radical changes have taken place than in the types of windings. Many of the larger low speed rotating armature alternators had bar windings with separate end connectors, soldered or bolted on. Many of the earlier stationary type armatures had either bmlt-up bar or strap windings, or concentric type windings in which each phase winding was arranged in a concentric group, and the groups of the different phases overlapped each other. Some of these were made with partially closed slots and others with open slots. The built-up bar or strap windings were frequently of the partially closed slot type, while the concentric windings were more usually of the open slot type. Gradually, however, both these types of windings were superseded by the "duplicate coU" type of winding, similar in appearance to the usual direct-current armature and induction motor primary windings. This later type of alternator winding was arranged in two layers of coils at the ends, in either one or two layers in the slots. The two-layer, two-coil per slot arrangement is now practically the standard. These types are illustrated in Fig. 11. In the rotating field machines, partially closed slot construc- tion was carried to comparatively high voltages. For instance, the 6 000 kilowatt, 75 r.p.m., 11 000 volt, three-phase generators bmlt' for the Manhattan Elevated Railway in 1900 had three bars side by side, in each slot, with soldered-on end connectors. 608 ELECTRICAL ENGINEERING PAPERS As the parlialh' t'loscd slot and the open slot eonstructions are radieally different from each other, something should be said regarding the reasons whieh pronipted the use of cither type. As already indicated . the partially clc;sed slot type came in with the larger rotating-annaturc low-\'oltage alternators in which bar windings could Ije used. This construction ga\'e good mechan- FIG. 12— BAR AND END CONNECTOR TYPE OF WINDING WITH PARTIALLY CLOSED SLOTS ical support for the bars in the slots, tluis avoiding the use of bands. ]\Ioreo^■cr, with the very narrow slot ojienings at the top of the ,sl(jts, there was very little "bunching" of the magnetic flux at the armatvire tonth tips with the consequent low pole face losses, even with \"ery small air-gaps, and high gap flux densities. The disadvantages of the ]jarLiall\- cl(jsed slot is found largely in the tyiX' of windings ri.'(juired. DEVELOPMENT OF THE A.C. GENEILiTOR 609 In these early machines, it was found practicable, in general, to use complfeteh' formed and insulated coils with such slots, and therefore, either hand windings or built-up types of windings were used. While these were possible and practicable in a manufactur- ing establishment, yet such types of windings are usually difficult to repair by the ordinary operator inexperienced in the refinements of amiature winding. When it comes to repairs, the usual ma- chine-wound coil, which is completely insulated before being placed on the armature core, is very superior but, in general, this type of winding requires an open slot construction . riowe\'cr, when FIG. 13— DUPLICATE COIL TYPE OF WIXDIXG, TWO COILS PER SLOT the stationary armature construction came into general use, the advantages of the overhanging tooth tips in supporting the coils largely disappeared. There remained therefore the disadvantages of the flux bunching, against the advantage of coil construction, if ofjcn slots were used. However, the use of laminated poles, and the judicious proportioning of the air-gaps and fliLx densities, to a great extent eliminated the losses due to open slots. In conse- 610 ELECTRICAL ENGINEERING PAPERS quence, the open slot construction and the duphcate type of armature coil, have apparently come to stay, in this country. Various attempts have been made to obtain the advantages of both the open and the partially closed slot arrangements. Probably all large manufacturers of alternators have tried some form of magnetic wedge, instead of the usual fibre or wood wedges which serve to retain the coils in the slots. Another arrangement is the equivalent of combining two or more open slots in one, with an over-hanging tooth tip, which covers the slot with the exception of the widths of one coil. Two or more completely insulated coils are fed successively into the slot opening and arranged side by side. This does not give any narrower slot opening than with the open slot constructionj but the number of openings is reduced to one-half, or one-third. This construction is used rather extensive- ly in the rotors of large induction motors, but apparently is but little used in generators. Bracing of the end windings against short-circuit shocks has been a comparatively recent practice. The necessity for such bracing has been dependent to a considerable extent upon the output per pole, and the old time machine seldom had such a large output per pole that the short-circuit current-rushes were suffi- cient to cause dangerous distortions of the. end windings. How- ever, such bracing was used on the Niagara machines, previously described, and also on the Manhattan generators above referred to. These, however, were very rare instances. However, with the recent high-speed, high-output water-wheel generators, the outputs per pole have become such that some form of end bracing has become rather common. Modem Westinghouse machines of this kind are braced to stand a dead short-circuit across the terminals without damage to the windings. Under this condition, these large machines may give a momentary current rush of from ten to twenty times the rated full-load current. However, the bracing required on the end windings of such machines is of relatively much less importance than on turbo-generators of corresponding capacity, due to the fact that, in the former class of machines, the end windings are relatively short compared with those of ttirbo-generators. The above description brings us practically up to date, as far as the ordinary synchronous alternator is concerned. No de- scription of the development of the turbo-generator has yet been given. This forms a rather distinct development which should DEVELOPMENT OF THE A.C. GENERATOR 611 follow at this point presumably, but it is thought advisable to interpolate here some description of the problems of parallel operation, e. m. f. wave form, regulation, etc., which came into prominence and were practically taken care of previous to the advent of the turbo-generator on a large scale. Parallel Operation of Alternators One of the great problems which developed in the operation of alternators was that of the parallel running of two or more units. At one time this was a very serious question, but in recent years, it is very seldom heard of. Considering the almost universal FIG. 14— DETAIL VIEW OF THREE-PHASE CONCEXTRIC WINDI.\'G practice of paralleling alternators, which holds at the present time, one might be led to wonder why there ever was any trouble. Far back, in the days of the high-frequency surface-wound alternators, paralleling was attempted in many cases and, not infrequently, with considerable success. However, a failure in an attempt to parallel, in those days, usually meant the destruction of the ap- paratus. Those old time surface-wound alternators usually had very low self-induction, so that, in case of sudden short-circuit, an enormous current could flow, sufficient usually to strip the ami- 612 ELECTRICAL ENGINEERING PAPERS ature winding fron: the core, by bursting the bands, or otherwise. A failvire in an attempt to parallel two machines was practically equivalent to a short-circuit, and this usualljf meant destruction of the apparatus. However, if once paralleled successfully, the machines usually thd not act badly. One favorable condition, not then appreciated, was that all these early machines were l)elt- FIG. 15—75 K.V.A., THREE-PHASE. 60 CYCLE, : MH) \'(>LT. 15(1 R. P. .\I. ROTATING FIELD ENGIXE TYPE ALTERNATOR driven. It may be said, however, that in those days parallel opera- tion, while considered possible, was also considered more or less risky. In the period immediately following the surface-wound alternator, parallel operation was very much the exception, rather than the rule and, when engine-type alternators came into use, paralleling was considered for several A'cars as very questionable. At this time the situation was as follows; — Belted alternators could be paralleled in many cases. Direct-coupled alternators. DEVELOPMENT OF THE A.C. GENERATOR 613 if flexibly driven, could be paralleled almost a§ well as belted machines, while direct-coupled or engine type generators, without flexible coupling or drive, could not be relied on to parallel with- out hunting. It thus became recognized that some flexibility between the generator and its prime mover was an important adjunct to parallel operation. This led to the consideration that the engine might be back of the difiiculty in many instances, and it was then assumed that inequalities in the regular rotation restilt- ing from insufficient flywheel or from hunting governors, tended to cause hunting in the generators. Investigation showed that such conditions did tend to produce hunting, but that the magnetic conditions in the machine itself would oftentimes maintain, or even accentuate, the hunting. Obviously, therefore, the trouble was both in the prime mover and in the generator. It was noted fur- ther that if the angular fluctuations in the driving power were relatively small, hunting usually would be very small, or would not be apparent at all. It was further recognized that, with belt or flexible drive, which tended to smooth out the speed fluctuations due to the prime mover, the hunting tendency tended to disappear. Attention was then turned toward improvement of the prime movers, especially in engine-type machines, in order to reduce fluctuations in angular velocity by means of heavy flywheels, and by means of dampers of some sort, such as dashpots, on the governing mechanism of the engine. Much improvernent was accomplished in this way. The Introduction of Dampers During this period many attempts were made to lessen the tendency of the alternator to maintain hunting. Investiga- tion showed that, during hunting, the magnetic flux in the fleld poles shifted back and forth across the pole faces in time with the hunting, while such action did not occur when there was lio hunting. This at once led to the theory that a low resist- ance winding on the pole face, or imbedded in the poles, would prevent or oppose this flux shift, and thus assist in overcoming hunting. However, about this time, rotary converters were coming into use, and it was found that, in such machines, hunting was usually more severe than in alternators, so that, in this country, the first true appUcation of damping windings or devices to stop hunting were applied on rotary converters. It was also noted at this time that soHd pole generators and rotary con- verters did not hunt to the same extent as did laminated pole ma- 614 ELECTRICAL ENGINEERING PAPERS chines, ami it was correctly assumed that the solid pole faces gave an effect similar to that of low resistance damping windings. How- ever, as it was desirable to use laminated pole tij)s, copper dampers on the poles gradually came into use. Some of these early dampers were \'ery crude in fomi and type compared \\'ith i)resent construc- m^ ^^^^^^& FU;. 16— \'ARIOUS FOR.MS OF DAMPERS lions. Howc\'er, imperfections in the construction of the dami)ers were Ijalanced to s(jme extent b}' the large section of copper used and consetiuent low resistance. The earliest fonn of damper used in this c(.)untr\' consisted of cojiper rings siuTounding the poles and FIG 17— GRID DAMPERS OX FIELD I'OLES co].)]jer ti] IS ( A'crhanging the beveled pole edges. This was the fonn most comni'inly used on converters. On alternators, in some cases the dam].)er o insisted simjjly of a low resistance ring around each pdle. In still (ither cases the damper consisted of a heavy copper plate covering the pole face. This latter construction was only DEVELOPMENT OF THE A.C. GENERATOR G15 possible in machines with large air-gaps and very narrow or partial- ly closed armature slots. These crude fomis of dampers were gradually superseded by the so-called ' ' grid ' ' damper which con- sisted of a copper grid surrounding the pole and with rilis which lay in slots in the pole face. These various types of dampers are shown in Fig. 16. In \'ery few cases were these old types of dampers so interconnected as to form a complete cage \A'inding around the field. Many tests were made at various times to determine the effect of interconnecting the grids on the different poles to fonn one complete cage. As a rule, there was no appreciable gain, and it was then assrimed that such interconnectirju had no material advantages. HowcA'cr, it later de\'elopcd that the reason why interconnection of the dampers did not impro\'e the damping action A-ery materially, was due largely to the very great amount of copper in those parts of the grid dampers lying Ijetween the poles. The grid damper was \'ery cffcctiA'c, fjut was expensive in material, and was not easily apjilicd on poles with o\-erhanging FIG. 18— CAGE \VI.\DI\G TYPE OF DAMPERS pole tips. This type of damper was gradually suijcrseded by one similar to the usual cage winding on the secondaries of induction motors, and this is the tj'pe which is in most general use at the present time. This construction has practically the same effect- iveness as the old grid type, but is much more economical in material and, being placed in partiaUy closed slots, it does not as greatly affect the iron losses in the machine, as was liable to be the case with the open slots, generally used with the grid damper. With the gradual hitroduction of dampers and improvements in angular rotation of the prime movers, hunting troubles in alter- nators practically disappeared, and parallel operation presented «16 ELECTRICAL ENGINEERING PAPERS • no difficulties, except under very abnormal conditions. Apparent- ly these dampers or "amortisseurs," as they are sometimes called, were first proposed by the French engineer, Maurice LeBlanc, about 1891. However, they were "rediscovered" in this country by engineers who were not familiar with the above engineer's work. Voltage Wave Form The e. m. f. wave form of alternating-current generators has been a matter of much discussion since the early days of alter- nator design. The old surface-wound machines gave a very close approximation to a perfect sine shape, due to the arrangement of the winding and to the very large air-gap. The first toothed ar- matures, with their very small air-gaps, gave e. m. f . waves which ~r — ^ LJ I I JUT I I L 1S~LJ TZSZ FIG. 19— VOLTAGE WAVE FORMS Of early toothed armature machines and of later toothed armatures with larger air gaps and beveled poles. departed very mdely from a true sine. In fact, this had about the worst wave form of toy of the alternators which have been put out by the Westinghouse Company. Its shape was somewhat like that shown in Fig. 19, as would now be expected when the configuration of the armature tooth tips is taken into account. The later toothed armatures with large air-gaps and beveled tooth tips gave much better wave shapes. With the advent of the true polyphase windings and the slotted armatures with several slots per phase per pole, fairly close approximation to sine shaped e. m. f . waves became common. In the first Niagara Falls 5 000 horse-power, two-phase alter- DEVELOPMENT OF THE A.C. GENERATOR 617 nators, the voltage wave was slightly flattened on the top due ta the fact that the pole face width was somewhat greater than the width of each phase group in the armature. When very high voltages came into general use, and especially in machines with smaU pole pitch, the number of armature slots per phase per pole was reduced to a minimum in order to lessen the total insulation space. In extreme cases, but one slot per phase was used, giving but two slots per pole for two-phase and three slots per pole for' three-phase. Such windings required special shaping of the field pole tips in order to approximate even roughly a smooth wave form of the sine shape. Later practice, however, has tended ' toward the equivalent of at least two slots per phase per pole, in order to obtain better results. Sometimes the desired result is obtained by the use of one extra idle or "hunting" tooth per phase. In the early days of parallel operation of engine type alter- nators which, as described before, represented the most difficult conditions, great stress was laid upon the question of wave form in some of the discussions of parallel operation, and particularly, in the operation of rotary converters without hunting. Grad- ually, however, this question disappeared and it became recog- nized that all the cases of hunting encountered could be explained in some other way than by the e. m. f . wave forms, and it is now generally accepted that about the only effect on parallel operation due to wave form lies in possible circulating currents of higher frequency than the fundamental. At the present time, a very close approximation to the sine shaped wave is considered preferable for general purposes, es- pecially in transformation and transmission work. There have been some instances of telephone disturbances due to wave form but, as a rule, some local peculiarities of the distribution circuits have been involved in this trouble, for, in other cases, similar or even worse shaped waves have given absolutely no telephone dis- turbances. Regulation and Compounding Something should be said on the subject of regulation of alternators, for this is a very important characteristic, and has had- considerable influence on types and designs . The old surf ace- wound alternators had extremely good regulating characteristics' due to their low armature self-induction and low armature reac- 618 ELECTRICAL ENGINEERING PAPERS tion consequent upon their large air-gaps. The writer does not know what value the current rose to, on steady short-circuit, compared with the normal rated current, but it was probably four or five times full load. The ctirrent rush on short-circuit was probably five times as great as the steady value. It is not to be wondered at that such armatures not infrequently wrecked them- selves in case of a dead short-circmt. In the later toothed arma- ture types, the armature self-induction and reaction on the field were very much larger, proportionately, than in the surface- wound machines. This, however, spoiled the regulation and some • method of compounding was used, as already described. This compounding was common practice until larger capacity machines, especially the engine type, came into use. Even some of these latter were compounded by commutating the armature current (either directly or from a series transformer) and compounding the exciter field by means of the commutated current. A few of the smaller size alternators were both self-excited and compounded by commutating derived alternating-current circuits from the armature. This, however, was found to be very delicate, as the excitation and compounding were greatly affected by changes in the power-factor of the load, and by changes in speed. One early attempt was made to compound single-phase alternators to correct for power-factor. In this case the com- mutated armature current was sent through the series or compound winding of the exciter. The brushes on the alternating-current commutator were so set that at 100 percent power-factor they were commutating about the middle of each voltage wave. In con- sequence, the current delivered to the brushes was not a true direct current but consisted of a double number of half waves, half of which were inverted, and the direct-current component of this commu±ated current was small and had but little compound- ing effect. However, with change in power-factor of the load, the phase of the- current shifted, so that at some reduced power-factor, commutation occiurred at the zero point of the current waves and the resultant current was all effective for magnetizing the exciter field. The total commutated voltage was very low and the com- mutator bars were shunted by a resistance so that there was no bad sparking, even when commutating at the middle of the current wave. This method did actually compound fairly well for change in power-factor, but the field for such method proved to be very limited, for compounding of alternators fell into disuse shortly after this. DEVELOPMENT OF THE A.C. GENERATOR 619 The usual method of compounding on the early alternators was simple series-current compounding, just as in direct-current apparatus. Where the commutated current was supplied directly to the field compoiind winding, voltages of about 30 to 60 volts were most common at rated full load. With much higher than 60 volts, there was a liability of short-circuiting the compounding by arcing between bars on the commutator. There was also a liability of arcing or flashing when the phase of the current shifted due to change in power-factor. When polyphase rotating armatures came into use, similar methods of com.pounding were resorted to. However, the second- ary current was a resultant of the two, or three primary currents, for each of the primary phases was carried around the compensat- ing transformer (or spokes of the armature) and the secondary winding carried a current in phase with the resultant of the primary ampere-turns. In the case of three-phase windings, the direction of one lead was reversed around the compensating transformer. Some curious conditions arose from the phase relations of the secondary current when parallel operation was practiced. It was necessary, when paralleling the main winding, to parallel also the compound winding. As the compounding current from each machine pulsated from zero to maximum value in each alternation, it was necessary to so parallel the terminals that all the commut- ated currents had zero value at the same instant, otherwise, the brushes on one commutator would, at times, short-circuit the cur- rent from the other commutator. , With the advent of larger belted machines, and of engine-type machines in particular, the compounding of polyphase machines was more or less unsatisfactory and was practically abandoned. To compensate for the lack of compounding, bettei inherent regu- lations were aimed at in the designs. This meant, primarily, machines which wovild give comparatively large currents on steady short-circuit, three to four times full load being rather common, and even six times ftill load being attained in some machines. The momentary current rush at the instant of short-circuit must have been excessive on some of these machines, due to their very low armature self-induction. However, due to the relatively small ampere-turns per pole, no very destructive distortions were found in practice. This characteristic of the short-circmt currents was carried into the rotating field construction, and even into the early turbo-generator work. 620 ELECTRICAL ENGINEERING PAPERS This practice of giving the alternators good inherent regula- tion was expensive in a number of ways, as it usually meant higher iron losses and less output than was possible otherwise, with a given size machine, or a given amount of material. Even at this early date, it was recognized that some form of automatic field current regulator which would maintain the terminal voltage con- stant, regardless of the inherent regulation would be a very useful piece of apparatus. Some form of regulation which would take care of change in power-factor, as well as load, was the aim of many designers. Among the different schemes brought out, the Rice method of compounding, brought out by the General Electric Company, is of interest. This was used principally with rotating field alternators. In this scheme, the exciter was usually placed on the same shaft as the alternator field, and, in such case, had the same number of poles as the alternator. The leads from the alternator armature were carried through the exciter winding in such a way that a lagging current, carried by the alternator, tended to strengthen the field of the exciter by shifting the arma- ture reaction with respect to the exciter field poles. In this way a compounding action on the exciter was obtained which was prac- tically in proportion to the demands of the alternator field with varying power-factor. In the case of engine-type machines of comparatively low speed, the exciter was geared to the alternator shaft, so that it ran at a considerably higher speed and the number of poles in the exciter was correspondingly reduced. This method of compounding was effective, but the whole combination was apparently unduly complicated and expensive. Furthermore, it did not give the desired compensation under all conditions of operation, as it would not correct for changes in speed. A later method of compensation for power-factor was devised by_^ Alexanderson, and was used on a Hmited number of General Electric machines. In this scheme a derived current from the alternator itself was commutated in such a manner that compensa- tion, proportional to the power-factor, was obtained. This was a purely self-excited alternator scheme and, like all self-exciting schemes in such apparatus, it was sensitive to speed changes, probably to a much greater extent than the Rice arrangement above described. A fundamental defect in all self-exciting com- pensated alternator schemes lies in the fact that stability of ex- citation is dependent upon having considerable saturation in the DEVELOPMENT OF THE A.C. GENERATOR 621 alternator magnetic cvirrent and, coincidently, if there is such saturation, the compotind current has no direct relation to the load or power-factor. Thus such machines are either sensitive to speed changes, or their compounding is only approximate. Following these schemes came the use of automatic regulators of which the Tirrill is best known. This regulator acts directly on the exciter field by short-circuiting a resistance in series with the field winding, the range of exciter voltage being controlled by the length of time the rheostat is short-circuited. Instead of cutting the resistance out in steps, which tends to give sluggish action in the fields, the Tirrill regulator cuts the whole resistance out each time, and the length of time is varied. This results in quick action. As the regulator tends to hold constant voltage at the alternator terminals, or on the line, change in power-factor or in speed does not modify the action. This type of regulator has proven very effective, especially in the case of alternators sub- jected to sudden and violent changes in load, power-factor and speed. With the advent of larger alternator units, in proportion to the changes in load, the inherent regulation has been made relat- tively poorer, primarily because better machines otherwise are thus obtained. The short-circuit currents are reduced, and relatively lower iron losses, and lower temperatures or, higher out- puts with a given temperature, are obtained. This has been car- ried further in turbo-generator design than in any other class of alternators, due partly to fundamental limitations in design. However,' this poorer inherent regulation has proven to be of no practical importance, where suitable automatic regulators have been used with the machines. One fallacy which was frequently found in the past, and which still persists to some extent, is that alternators should have equal inherent regulation to parallel properly. This is based partly on the feeling that the field currents of the alternators should vary over equal range when carrying their proper proportion of load, together with the knowledge that the variations in field current are dependent, to some extent, upon the inherent regulation. However, the fact that the shape of the saturation curve, in a given alternator, may have much more influence on the excitation, especially at high saturations, is usually overlooked. 622 ELECTRICAL EXGIXEERIXG PAPERS Turbo-Generators The advent of the turbo-generator has had a predominant influence on alternator design. After the turbo-alternator onee became established commercially in this country, it quickly revo- lutionized conditions by driving the large engine-type alternators out of the field. The evolution of all electrical apparatus has been comparatively rapid, but that of the turbo-alternator has possibly exceeded anything else in the electrical field. This evolution therefore merits a fairly complete description. The first turb(.)-alternators built by the Westinghouse Com- pany, were installed in the power plant of the Westinghouse Air Brake Company abovit 1808. These were three rotating amiature machines of .lOO kilowatts cajxicity, which ran at a speed of 3 600 r. p. m., giving 7 200 alternations per minute, or 60 cycles per ul' FIG. 20— FIELD OF E.\RLY ROT.ATIXG .ARM.\TURE TURBO-CENERATOR second. They \\xTe coupled to Parsons turbines, built l)y The Westinghouse Alachine Company. The Parsons Company in England had been building rotating armature alternators for a number of years, and the Westinghouse Compan}- simply followed the Parsons' precedent. These first machines were operated for se\'eral years, but it was obvious, soon after their installation, that the rotating armature type of machine would not serve for DEVELOPMENT OF THE A.C. GENERATOR r.23 general turbo-alternator purposes. It was evident that, for voltages even no higher than 2 200, the rotating armature con- struction, at the necessary turbo-generator peripheral speeds, would become almost impracticable. Attention therefore was soon turned toward a 3 600 revolution, two-pole, rotating field type. FIG. 21.— ARM.\TURE FOR TURBO-GEXERATOR OF THE TYPE SHOWN IX FIG. 20 and a very large number of possible constructions were con- sidered. Finalh' one like that shoWn in Fig. 23 was worked out and built in 1899. This had the field windings completely em- bedded in a number of parallel slots, with sujjporting metal wedges FIG. 22 — 1 000 KW OPEN- TYPE TURBO-GENERATOR at the tops of the grooves or slots. One machine of this tj-pe was built and tested. It operated in a satisfactory manner, except as regards windage and noise. The machine was not closed at the ends, like modern turbo-alternators, and thus any noise generated in the machine could be readily transmitted to the outside. The noise was caused largely by the two flat sides of the rotor. It was 624 ELECTRICAL ENGINEERING PAPERS SO shrill and penetrating that it was very disagreeable to be around the machine, and was even painfiil to the ears after a short time. This construction was therefore abandoned temporarily, but after a few months it was taken up again and a new rotor was built which was entirely round, as shown in Fig. 24, but was otherwise very similar to that shown in Fig. 23. This new rotor, although noisy compared with modem machines, was so quiet, compared with the first construction, that it was immediately adopted as a standard construction. This is the now well-known parallel slot construction which has been used very extensively by the Westing- house Company, although many very radical changes have been made in the constructive features of the rotor itself. This type of rotor was used originally only for the 400 kilowatt size at 60 cycles. In the earlier machines of this type a number of very curious conditions developed. In the first machines the rotors were built of a number of thick discs or "cheeses" side by side, which were put on the shaft at high pressure. The two end discs were thicker than the others in order to accommodate the grooves in which the rotor end windings lay. The discs were made of high grade forg- FIG. 23— EARLY TWO-POLE ROTATING FIELD ings. After some of these machines had been in operation for a considerable period it was found that some of the discs in the field core were traveling axially, i.e., quite appreciable gaps or spaqes were showing between adjacent discs. In one instance they trav- eled to such an extent that the field windings were stretched longitudinally at the openings between the discs, until the con- ductors were actually attenuated to an extent visible to the eye. Obviously, the stretching force must have been enormous. DEVELOPMENT OF THE A.C. GENERATOR 625 Eventually, the construction was changed on these two-pole rotors to a single disc of forged steel. 'Still later, steel castings were used qtiite extensively instead of forgings although, later still, the castings were abandoned in favor of forgings. There was much adverse opinion regarding the advisability of using castings for the 3 600 revolution machines, as some engineers held that they were more liable to contain flaws than would be the case with forgings. An interesting fact in connection with this is that, while a number of these early high speed machines "exploded," gener- ally during nmaways, yet in no instance was a cast steel field wreck- ed from this cause. This, however, does not constitute a proof of the superiority of cast steel, for it so happened that aU ^the serious runaways were on machines with forged rotors. However, PIG. 24— ROUND TYPE TWO-POLE ROTOR the record is a clear one as far as cast steel fields are concerned, for, of all the sizes and speeds of steel rotors which the Westing- house Company has put out, not a single cast steel disc has burst. Present speed and output requirements have now carried the construction up to a point where special forged materials are the accepted practice. Soon after the two-pole, 400 kilowatt rotating field machine was put on the market, a four-pole, 750 kilowatt, 1 800 revolution machine was built. The rotor of this machine had four silent poles bolted on. These poles were provided with overhanging pole tips, and the field winding consisted of four coils wound with strap-on- edge. In fact, this first construction was very similar to the present type of rotor fields now used for other than turbo work. This construction proved difficult and expensive, but was applied to a number of six-pole, 1200 revolution machines. However, 626 ELECTRICAL ENGINEERING PAPERS the parallel slot construction used in the two-pole machines was so satisfactory that it was soon adopted for the four and six-pole machines, as shown in Fig. 25. In the six-pole machine it was not possible to make the poles integral with the central core, on account of the inability to machine the parallel slots in the sides of FIG. 25— PARALLEL-SLOT FOUR-POLE AND SIX-POLE FIELD CONSTRUCTION the poles, or to put in the windings. Therefore, separate poles were constructed, with parallel slots, and these were first woimd and then bolted into place on the central core which, in this case, was made integral with the shaft. The four-pole machine was con- structed for 750 and 1 000 kilowatts capacity, and the six-pole con- struction was made for 1 500 to 3 000 kilowatts. Meanwhile, there had grown up some demand for moderate capacity 25-cycle machines at 1 500 revolutions. These were constructed along exactly the same lines as the two-pole, 3 600 revolution machines above described. In this early work one order for four 5 500 kw, four-pole, 1 000 revolution machines was taken. This was entirely beyond the constructions undertaken before by the Westinghouse Company. The parallel slot type of rotor was adopted. An attempt was made DEVELOPMENT OF THE A.C. GENERATOR 627 to get forgings in a single piece large enough for these rotors, but they were found to be glass hard and brittle, except at the outer surface. As very large steel castings were frowned upon, it was decided to make these rotors of discs turned out of very thick steel plates, somewhat like the early 400 kw machines already de- scribed. Parallel slots were used as in the smaller four-pole machines. This construction proved to be feasible but was very expensive, and shortly after this, large cast steel discs were used, two discs side by side being used to form one rotor. This con- struction was satisfactory, and was used for many years. ? FIG. 26— TWO-POLE FIELD OF THE BOLTED ON CONSTRUCTION Shortly after turbo-generators came into general use, there- was considerable complaint regarding the noise due to windage. All these machines were equipped with some form of ventilating device, which either formed part of the normal construction of the rotor or consisted of some special blowing device at the ends of the rotor. Both the high speed and the large quantity of cooling air 628 ELECTRICAL ENGINEERING PAPERS required, tended to make a noise which was very objectionable. A series of experiments with covers over various parts of the machines, showed that, by completely enclosing the two ends of the machine and by enclosing the field frame except at the top and bottom, (in a horizontal machine) the windage noise could be so deadened as to be practically unobjectionable. However, the tests also showed that artificial ventilation was necessary under this condition. This very quickly led to the practice of enclosing and artificially cooling turbo-generators, which practice has been maintained to this day. The first Westinghouse enclosed ma- chines were built about 1903. The use of artificial cooling marked a great step in advance in turbo-generator work, for the results indicated that, by supplying a sufficient quantity of air and properly distributing it through the machine, very marked increase in capacity was possible, and a point was soon reached where the possible capacities were beyond the mechanical limitations of the construction. This led to radical modifications in the type of rotor, with a View to taking advantage of the increased capacity. Apparently all manufacturers did more or less development work along such lines. In the Westing- house constructions, the use of a thorough shaft was found to be one of the serious limitations, and this led to types of rotors with- out any through shaft. In the two-pole machines, this was particularly important, and the problem was especially difficult with the parallel-slot construction, provided ample space was al- lowed for the field winding. The old through-shaft two-pole con- struction lost considerable winding space, due to the shaft space, as shown before in Fig. 24. Attempts to construct such a machine with the shaft forming part of the core, resulted in still less ef- ficient use of the possible winding space. It was obvious that if the whole possible winding space were taken up with slots, then the capacity of the field winding would be greatly increased. In consequence, a rotor construction, such as shown in Fig. 26, was designed and constructed. In this, bronze end supports or "heads" were bolted to each end of the field core, and the shaft proper was attached to these bronze heads. Bronze, or a similar non-magnetic material, was necessary to prevent magnetic short- circuiting of the field fiux. This design was constructed and tested on a 1 000 k.v.a., 3 600 revolution machine, and then was built successively for 1 500, 2 000, 3 000, 4 000 and 5 000 k.v.a. machines, all at 3600 r. p. m. The same construction was also applied to DEVELOPMENT OF THE A.C. GENERATOR 629 two-pole machines of 25 cycles, up to 12 000 k.v.a. capacities. This construction of rotor has given an extremely good account of itself. However, it proved to be expensive on small capacity ma- chines, as the bronze heads formed an undue proportion of the cost of material. For higher capacities of 3 600 r. p. m. machines, increase in capacity is obtained largely by increasing the length of the rotor core, and thus the bronze heads form a relatively lower percentage, and the construction becomes more reasonable in cost. From the preceding, it may be seen that only two types of turbo-generators have been used very extensively, namely, the parallel-slot type and the radial-slot type. Each of these types has some very pronounced advantages. The principal advantage of the parallel-slot type is in the arrangement and support of the field coils. Each coil can be wound directly in place, with the conductor under tension, and the finished winding is completely encased, and is thoroughly protected against dirt, movement of the conductors, etc. Against this, the radial-slot machine allows more room for copper, and is magnetically more economical in material. However, the field windings are more difficult to apply and must be supported at the ends by auxiliary means, such as separate steel rings. The enormous increase in output of turbo-generators, within very recent years, has made the electric and magnetic proportions of the rotors a feature of first importance in the design, so that the radial-slot type for two-pole machines has become the standard construction, almost universally. This will be referred to again under the four-pole construction. While the two-pole parallel slot construction was being de- veloped for larger capacities, the four-pole construction for 60 cycle machines has been pushed up to capacities of about 12 000 k.v.a. with the parallel slot, cast steel rotors. In order to do away with the through-shaft construction, the rotor was made of two castings or discs, each of which was cast solid with the shaft, as shown in Fig. 27. The two discs, after machining, were bolted together by a number of very heavy bolts located near the pole tips and, in some cases, shrink links were placed in the pole face, connecting the two halves together. The parallel grooves were then machined in the steel core, just as in the through-shaft type. In this four-pole construction, the problem of armature ventila- tion was comparatively simple. Air-gap ventilation (that is, all air through the armature core supplied from air-gap) was easily 630 ELECTRICAL ENGINEERING PAPERS accomplished, due to the open spaces between the poles, which could admit an ample air supply. However, the same construction tended toward high windage losses due to air "churning." >-- y ^ ^^ r FIG. 27— FIELD CONSTRUCTION WITH TWO HALVES HELD TOGETHER BY HEAVY BOLTS This problem of ventilation has had much to do with the evolution of turbo-generator design.* In the two-pole, parallel slot machine for 3 600 r. p. m., in which the diameter of the rotor is relatively smaU, the amount of air which can be forced into the air-gap from each end is rather limited. Assuming, for example, a rotor diameter of 24 inches, which is almost as large as we can go for a 3 600 r. p. m. machine, then, with an air-gap (iron to iron) of 54 -inch, which is also a fairly large gap, the total cross-section of the air inlet at the air-gap at both ends of the rotor will be 112 sq. in. With the very high air velocity of 10 000 ft. per minute, this means a total air supply of less than 8 000 cu. ft. per minute. This may be sufficient for a moderate capacity turbo-generator, but for machines of high capacities, such as 3 000 to 5000 k.v.a., this is not nearly enough cooling air. Obviously, either much larger inlets through the air-gap are required, or some additional method of cooling is necessary. Larger air-gaps usually mean either more expensive machines, or reduced output with a given machine, due to lower flux densities. Therefore, the tendency, in machines of the very high capacities, and very high speeds, has been toward a combination of air-gap with other methods of ventilation. In the 25 cycle, two-pole machine with a maximum speed of 1 500 r.p.m., rotors of larger diameter are possible and, as a rule, much larger air-gaps are practicable than in 60 cycle machines. In conse- quence, air-gap ventilation comes nearer being practicable but in *A more complete exposition of the subject of ' 'Turbo-Alternator Ventilation, is contained in the paper on page 312. DEVELOPMENT OF THE A.C. GENERATOR 631 the larger capacities, e\'en this is insufficient and auxiliary methods have been necessary in some cases. This need for auxiliary methods of ventilation led to the axial method of ventilating amiature cores in distinction from the radial method, in which the air was carried out through numerous radial air ducts or passages. In the axial method, a large number of ventilating holes are arranged in the annature core parallel to the. axis of the niachine. These form A^entilating paths in parallel with the air-gap path. With the small diameter long cores necessary for 3 600 r. p. m., high ca]jacity machines, the develop- ment of this mcthijd of \'entilation \\-as contemporaneous with the development of the higher capacities. The same has proved to be the case for the later tyijcs of Westinghouse four-pole, 60 cycle, 1 800 r. p. m. machines, which departed very considerably in rotor construction from the four-pole cast steel tyi)e already described. As the capacities of the ,^ 600 r. p. m., 60 cycle machines were gradually jnished up, a corres])(jnding dc\'elopmcnt occurred in the 1800 r. p. m, machines. At 10 000 to 12 000 k.^-.a., the four-pole cast steel construction was ajjparently api^roaching its limits. FIG. 28— MODERN ROT.aLTIK'G FIELD ON BAL.-WCINO WAYS For larger sizes, therefore, a different construction was adopted which allowed more suitable material to be obtained. For the largest diameters and highest speeds, a plate construction was adopted by the Westinghouse Company, in which the end discs and the shaft ends were forged as units, and the intermediate discs were made of rolled plate material, the whole construction being bolted together pemianently to form a solid core. This core was then slotted with radial slots, and the usual radial slot type of field winding is used. A similar construction was adopted on the larger 25 cycle machines. For intermediate capacities, both 60 and 25 632 ELECTRICAL EXGIXEERING PAPERS cycles, solid discs arc used in some cases instead of the plate construction. This brings th<_' lar<.;cr turlio de\-clopnient u|) to the present date. In the coniparati\'ely small ()() cycle turtxj-generators, where the parallel slot construction with the bronze driving was relatively- expensive, as already described, the later development has been towards core and shaft forged in one jjiece, and with radial slots, FIG. 29— STATOR OF 625 K.V.A.. 2 300 VOLTS, 3 600 R. P. M.. TURBO-GEN'ERATOR With axial ventilation and central duct. Supporting ring brjth inside and outside the end ^-indings. Typical method of bracing smaller machines. and CA'cntualh' this construction may be carried up to the largest practicable size of 3 600 r. p. m. machines. It is difficult to predict the liniit in capacity which ma^- 1 ic reached e\-entually in 3 600 r. p. m.. generators, but 6 250 k.v.a. a]j]jears to Ijc practicable. Some special radial-slot machines had been developed for the New Haven Railroad about 1907. As these machines were de- signed to deliver 25 cycle single-phase current, and as the pulsating annature reaction of such machines would he rclativeh- high, the rotors were designed with laminated cores, with a A'iew to lessening core losses. The rotors were made of single disc laminations shrunk on the shaft. The discs were provided with radial slots. The con- struction was very similar to the later radial-slot rotors, except that the rotor end windings were also embedded in slots, and DEVELOPMENT OF THE A.C. GENERATOR 633 supported by wedges embedded in the periphery of the core, whereas, in the later radial-slot rotors, the end windings are sup- ported by external rings. These early radial-slot rotors showed very considerable overheating in single-phase operation, and it was found necessary to apply a very complete cage damper em- bedded in the periphery of the rotor . Later experience showed that the solid-core parallel-slot rotor with an equal damper applied to its siu-face was just as effective, and many of the later single-phase machines were built in this manner. However, some recent 11250 k.v.a. single-phase generators are being built of the plate construc- tion already described. Regulation and Short-Circuit Currents of Turbo-Alter- nators Like the ordinary synchronous generator, the modern turbo- alternator is designed with a comparatively high inherent regula- tion. In fact, in order to avoid excessive short-circuit currents, the inherent regulation must be made comparatively poor by making the armature self-induction as high as practicable. Even under the best condition, such machines are liable to give 12 to 15 times rated current during the first current rush. Furthermore, the solid plates or discs, of which most turbo-rotors are now made, tend to prolong the period of maximum short-circuit current. The consequence of these conditions is a tremendous racking force acting on the end windings druing a short-circuit current rush, which tends to distort the winding badly unless it is very strongly braced. The Westinghouse Company encountered such a diffi- culty on some of their earliest turbo-alternators and there has been a practically continuous development along the lines of more substantial bracing which has kept pace with the increased require- ments of the higher speeds and the higher capacities. The bracing used on the modem machines is designed to resist distortion of the end windings, under dead short-circuit, without reactances inter- posed, and each new size as it is developed is given such a short- circuit test. A 20 000 k.v.a. 60 cycle, 1 800 r. p. m. high voltage alternator was recently subjected to such short-circuit tests at full voltage without injury. The preceding gives a brief history of the development of the turbo-generator, insofar as carried out by the Westinghouse Com- pany. The General Electric Company went through a correspond- 634 ELECTRICAL ESGLXEERIXC PAPERS ing course of development, in general, although not in the specific constructions descril)e(l, and a nunil")er of interesting types or con- structions have been brought out. The gradual increase in speed has undoubtecily had much to do with the e\-olution of their various types, just as in the case of the Westinghousc c\-olution. One of the most rachcal steps which the General Electric Company has made in the ]jast few years is in the change from the vertical to the horizontal type of machines. Presumaljh- the A'cry high speeds which later came into use have had much to do with this change. In the earlier turbo-generator practice, the speeds of the General Electric Compan}-'s machines were rclati^X'ly lower than the FIG. 30— SAME .M.'\CHI\E .\S SHOWN IX FIC. _"> BEFORE WINDING Westinghousc, prcsumaljly on account of tlie type of steam turbine used. Many of the early rotor constructions were of the salient pole type for four poles and higher. Gradualh' these were super- seded b}' constructions leading uj) to a radial-slot type in which the slots were formed Ijy teeth inserted in dovetail grooves in the rim of the spider. This tyijc was \'ery similar in appearance to the later types, except that the slolted-in-poles, following the West- inghouse No. 38 motiar with cast-in poles. The Westinghousc followed in its No. 56 and No. 68 motors with bolted-in poles Both of these motors had ventilated armature windings and curved field co'ls, this latter iiractice being deri^-cd from the Walker and Lorain motors, lioth of which comijanies had been taken over 1)V the Westinghousc. In the No. 68 motor the alternate FIO, 15— WESTIXC.HOUSE XO. 68 .MOTOR comers of the pole tip laminations were cut away, in order to give a higher degree of saturation with heavy load and thus lessen the field distortion and reduce loss in the ijole face. This had been com- mon practice for some time in the railway- generators. Various detail improvements were alsincnts. This agitatii:in bore fruit, and a weight cuttint; cam]jaign lie,i,'an which has resulted in the adojition of extrcmclx' light weight cars, trucks and motors. The question of car and truck design may not be dis- cussed here, alth(ju,^'h it ](joks now as if the weight-cutting cam- paign has gone ]jast the best limit. Howc\xt, a large part of the reduction in the wei.L,'ht of motors has been entirely logical and is largeh-' the result of careful design and impro\'ements in ventila- STREET RAILWAY MOTOR IN AMERICA 745 tion. Motors are now built with large fans, mounted on the pinion end of the armature shaft, which pull air through the armature core and oyer l^he surfaee ,pf the armature and between the field windings, which has made an increase of probably 50 per cent in the continuous rating of the motors. In addition to this, the arma- tiire speed has been yery considerably increased and the gears have, in many cases, been changed from 3-pitch to 3J^, 4 and even 43^. Open ventilation of the motors has been a natural consequence of the-great improvement in insulation made in the last few years. The early motors were made open to the weather but this had to be abandoned because of the large amount of insulation trouble. After a 'good many years with the enclosed motor, it gradually became the practice to open the motor up somewhat for better ventilation, and finally fans were installed to create a circulation of air, so that now the continuous rating of railway motors is higher per pound than ever before. 746 ELECTRICAL ENGINEERING PAPERS TECHNICAL TRAINING FOR ENGINEERS FOREWORD — This paper was compiled from two addresses, one given by special re- quest before the Pittsburgh Section of the American Institute of Electrical Engineers in 1916, and the other, covering much the same subject matter, was given before the National Association of Corporation Schools at its annual meeting in Pittsburgh, 1916. On account of the favorable comments on the two addresses, they were afterwards combined and printed in the Electric Journal in the form here given. The author has had a wide experience in the education of so-called 'educated" men. Almost since his entrance into the employ of the Westinghouse Eectric & Mfg. Company, early in 1888, he has given a considerable part of his time to the development of the more promising young engineers with whom he comes in daily contact. Being himself ex- tremely fond of the analytical side of his work, and also recognizing its funda- mental Importance, he has been very free in imparting his miethods, data and experience to his associates and assistants, thus in fact, although not in name, becoming an educator along advanced lines. He always has been in search of young men of the right turn of mind whom he could develop into "stars" in his profession, and many men prominent in the electrical industry today can speak with pride of the training they received while associated with him. Rec- ognzing that the engineering development work of the manufacturing com- panies is becoming increasingly difficult from year to year, he has given special attenton, during the past several years, to the selection and training of grad- uates of the technical schools who show, to an unusual degree, certain character- istics and aptitudes which he believes to be necessary in maintaining the high standard of the Westinghouse Company In the engineering field. In other words, he is applying his analytical methods to men very much as he has applied them to apparatus and principles in the past years. — (Ed.) IN the earlier days of the Westinghouse Electric & Mfg. Com- pany many young technical students were taken directly into the various departments and there trained. But in time the student problem became so large and important :hat an educational de- partment was d3veloped to meet in a systematic manner the grow- ing needs of all departments. This educational department works in conjunction with the other departments in training men and in placing them where they will have opportunities '.n accordance with their special abilities. While the educational department supervises the student course, yet much of the training is through representatives of the commercial manufacturing and engineering departments. The following remarks represent the writer's own personal opinions based largely upon a comparatively wide experience with the young engineers who have entered the student's course during the past five or six years. In that time this company has taken into its educational department over one thousand graduates of tech- nical schools from all over the Un'.ted States and Canada. Of these, several hundred have wished to specialize in engineering, while the aim of the others has been toward the manufacturing and the commercial lines, both of which require good technical training. The electrical salesman of today is quite technical, regardless of TECHNICAL TRAINING FOR ENGINEERS 747 how he got his training. Also the complexities of the electrical business of today require many high-class technical men in the manufacturing departments As to engineering, it goes without saying that those who follow this branch of the electrical business should be technical men, if they are to advance very far. In con- sequence the Westinghouse Company takes on technical graduates almost exclusively for its student's course, regardless of what branch of the electrical business they expect to follow. The writer's personal experience has been very largely with those students who expect to follow the engineering branch of electrical manufacturing. During the past few years he has come in contact with practically all those who leaned toward engineer- ing work. One of the most important considerations in the engi- neering student problem has been that of fitting the men to the kinds of work for which they are best adapted. In former years this was done in a more or less haphazard manner by trying the men out in different classes of work to see whether they would make good. This procedure proved so unsatisfactory that it became necessary to adopt some method of classifying the students ac- cording to their aptitudes and abilities, and then try each one out . on that line of work for which he seemed to be best fitted. Obvious- ly, this method was in the right direction, but the primary diffi- culty lay in determining the characteristics of the individual students. The writer has spent quite a considerable amount of time in the past few years in studying the characteristics of the students to see whether their natural and their acquired abilities can be sufficiently recognized, during the preliminary stages of the work, to allow them to be properly directed toward that field in which they will make the best progress. In this study, in which hundreds of young men wefe analyzed with regard to their char- acteristics, many very interesting points developed, quite a number of which have a direct bearing on the subject of technical training In the first years of this study the results were very discouraging, due largely to the fact that the young men had been brought to us in a wholesale way, regardless of their characteristics' or their suitability for our engineering work. Many of them had no ideas whatever in regard to the land of work for which they were fitted. Apparently, the man who had not, at least partly, made up his mind as to his preferences or his capabilities for some given line of endeavor by the time he had gone through four years of college and then entered our course, had much difficulty in making up his mind 748 ELECTRICAL ENGINEERING PAPERS after he had been with us a year or two. It developed, in many cases, that he was lacking in decision. This was a very predomin- ant fact in the first few years after the writer had gotten into this work more actively. Aftet a careful study of the situation it was recommended that an attempt be made to get a different class of college men, namely, those who had more definite ideas as to what they wanted and what they were fitted for. This policy was tried, and with great improvement in the grade of men obtained. It is principally from the study of these later men that the writer has been able to draw some of the conclusions which are here given. One of the most prominent features which has developed from the study of these young men is that in practically all cases the most valuable aptitudes or characteristics which they have shown were possessed by them long before they entered college. In fact, many of them have apparently possessed such aptitudes, more or less developed, from comparatively early childhood. For example, the best constructing or designing engineers all had a strong ten- dency toward the construction of mechanical toys and apparatus in childhood. In regard to such characteristics, the schools and the colleges have merely directed and developed to a greater extent what is already there. From this viewpoint, therefore, the college simply develops. If the tendency isn't there, it would seem that there is but little use to try to develop or zultivate it. Viewed from this standpoint, quite a large percentage of the young men who take up engineering courses in college are quite unfitted for such work. Therefore, one function of the college should be to sort out and classify the young men according to their characteristics, to dis- courage them from following along any line of endeavor for vhich they have no real aptitudes, and to direct them into more suitable lines. This applies particularly to technical schools. It might be said that in otir present educational system the usual method is to educate the young men and then select the real engineers, this selection being made afterwards through bitter experience. The ideal method, apparently, would be first to select the real engi- neers and then to educate them. In other words, those who show a natural aptitude for engineering should be educated along tech- nical lines. In the technical school one of the first efforts should be toward finding the student's natural aptitudes. Some boys apparently have no leaning toward any special line of endeavor. On the other hand, many boys really have some inherent preference which. TECHNICAL TRAINING FOR ENGINEERS 749 however, may not have been strongly enough developed to stand out prominently. Too often his real preference has been entirely neglected or even discouraged. In the writer's own case, as a boy, he was very frequently and severely criticised for his inclination to ' ' waste valuable time ' ' in trying to make what were called ' ' usele 5S things." However, fortunately for himself, no real pressure was brought upon him to prevent him from following his preferences or tendencies, and eventually the ' ' call ' ' was so strong that it took him into the very work which he wanted above all else. < On the other hand, the boy may express a preference for a line of work for which he is entirely tmfitted. In other words, this preference may not be based upon natural aptitudes or character- istics and is not a real "call." It is these boys, who are unfitted for the lines which they have chosen, who are a real haiidicap on their classmates. The class never moves along faster than its average man,, and very often at the speed of the poorest men. If these poorest men were eliminated, naturally the progress would be much faster. Apparently the present methods of training have not yet overcome this difficulty, although very many teachers recognize the evil, and are attempting to correct it. This will be referred to again later. Coming to the technical training of the students, experience indicates that too much specialization is a mistake. He gets enough of that in after years. Wliat is nee ded is a gppd,jDroadtraining in fundamental principles. In engineering matters, a thorough grasp of such fundamentals is worth more than anything else. By fundamentals is meant basic principles or facts. These should not be confused with theories or explanations of facts. A fact is basic, and does not change, although the theories which explain it may change many times. A thorough knowledge of basic principles will enable a direct answer to be made in many cases, even where the conditions of a problem may appear to be very complex. Take, for example, the perpetual motion fallacy in its various forms. A per- petual motion scheme may be made so complex and involved and may include so many principles and appurtenances that the best analyst may be more or leSs puzzled to explain the various rela- tions clearly. But by appli^ng the principle of conservation of energy no further explanation is necessary. This one fundamental fact covers the whole case. In the same way a thorough grasp of some basic principle will often clear up the most complex problems ' or situations and will allow a conclusive answer to be made: With' 750 ELECTRICAL ENGINEERING PAPERS such a grasp of ftmdamentals, one is not liable to believe that a "pinch" of some wonderful new powder or chemical, mixed with a gallon of water, will give the equivalent of a gallon of gasoline, and at the cost of few cents. And yet this fallacy ' ' breaks loose ' ' period- ically, and is given wide circulation in the hews of the day. What' is needed in such cases is a little knowledge of fundamental prin- ciples. This very grasp of fundamentals accustoms the boy to think for himself In other words, it develops his analytical ability. As one educator mentioned to the writer some time ago, " If a boy has analytical ability, there is hope for him; if he has none, he is 'punk.' " By analytical ability is not necessarily meant mathe- matical ability with which some people are inclined to confuse it. By analytical ability is meant the abiUty to analyze and draw cor- rect conclusions, from the data and facts available. This faculty can be ctiltivated to a considerable extent, although, in the writer's opinion, it originates rather early in life. This is considered by many as the first and foremost characteristic that an engineer must have, and therefore the schools should expend their best energies in this direction. Allied with a grasp of basic principles is the requirement of a physical conception of such principles as distinguished from the purely mathematical. This can be cultivated, as the writer's personal experience with many students has indicated. As a con- crete example of the value of a physical conception the following may be cited: — Three electrical engineers, familiar with induction motor design, are given some new problem regarding the action of an induction motor. One of them immediately thinks of a "circle diagram"; the second thinks of a mathematical formula; the third thinks of flvix distributions and conductors cutting them at certain speeds, etc. Assuming equal mathematical sMll for these three men, the one ^Aoth^the physira.l_conception oiJit!Sj:on_ductors cutting fluxes^ has a broader means for_.attackLngJ;he problem than either of the others jcan be said to-have. He can tackle a new con- dition with better chance of success, as he goes back to the funda - merita.l principles o f the apparatus. He thiis may create, con- fidently, new formul.ae-and. diagrams jto meet new conditions and problems. This physical conception is closely related to the development of imaginative powers, and without such powers highly developed no engineer can expect to advance far in his profession. The man TECHNICAL TRAINING FOR ENGINEERS 751 with originality, resourcefiilness or with the constructive facilities well developed, or the man who "can see through things" readily, must have strong imaginative powers. This faculty also should be developed to the utmost, but should also be directed. It begins early in some children, but, unfortunately, instead of being direct- ed, it is too often discouraged, both at home and in the school. If the boy in the public school develops a new method of solving a problem, or reaches any conclusion by other than the well- established routine way, he is criticised more often than encouraged for his departure from the beaten track, or rather his instructor's particular methods. As stated before, the student should be well trained in funda- mentals or basic principles. In many branches of engineering this means that he should have a good training in mathematics. Most of the graduates of the technical schools . are woefully weak in mathematics. Apparently this is not due entirely to lack of mathe- matical ability on the part of the students, but largely to defective training in their earlier work. One great defect in many colleges is due to passing the entrants, in algebira and trigonometry, on the basis of their high school training. In most cases this early training in algebra is very defective, as sufficient skill is not developed in the student and the practical side is largely neglected. Algebra and its applications to geometry, trigonometry, etc., should be taught in a more practical manner in the engineering college course, as a found- ation for the higher engineering mathematics. The higher the struc- ture is to be, the stronger must be the foundation. If the engineer- ing student is not sufficiently practiced in these elementary mathe- matics, then he should be drilled specially as a step to further engineering work. In the practical engineering work, beyond the college, skill in the use o f algebra an d trigonornetry is of relatively much more importance than practice in the higher mathematics, for it is needed one hundred times where the other is used once. In the writer's experience with engineers he has reached the con- clusion that the principal reason why mathematics are not used more in everyday work is because the average engineers have not the necessary skill. Most of them claim that they have become "rusty" in such mathematics through disuse. However, in many cases, this excuse is worse than none at all, for the occasion for such mathematics exists in practical engineering work and has been there all along. 752 ELECTRICAL ENGINEERING PAPERS In the education of the engineer, higher mathematics forms a very valuable part of' the training. One of its uses is to show how one can do without it. In other' words, if' properly taught, it gives a broader grasp of methods of analysis; it tends to fix certain fundamental principles. However, as a tool in actual engineering work it is seldom required, except in rather special lines. The higher inathematics might be looked upon as a fine laboratory in- strument or tool to be used on exceptional occasions, while the or- dinary mathematics shotild be considered as an everyday tool in engineering work, and should be ready at hand at all times. There has been quite a fad for specialization in engineering training.' The writer's personal opinion is that specialization in college training is not advisable, except possibly in- a very general way. There has been- a false idea. in many schools that if a man specialized along some individual line of work it would advance him more rapidly when he leaves school for active work. The writer almost never asks the student in what field he specialized. It is, desired to know whether he is a good analyst, if he is fairly skillful at mathematics, if he has the imaginative faculty and what, goes with it. Has he initiative, resourcefulness, etc. ? Is he a man with a broad grasp of general principles rather than one who has made a special study of one individual subject? In college training the time spent on commercially' practical dfetails is usually largely wasted, as it may give the student en- tirely wrong ideas. When a young man says that he has had a course- in practical design and is positive that he can design, the chances are about ninety-nine out of one hundred that he knows nothing about the really fundamental conditions in practical de- sign. The chances are that he doesn't even know the real starting point in making up a commercial design. Even worse, if he has taken such training seriously, he may' have to "unlearn" iriany of his ideas, if the use of this term is allowable. The mental' train- ing and the aid in grasping principles which he may have obtained through his school design is, of course, worth something, but in many cases the same time expended in other channels may produce larger results. Teaching of design shoiild, therefore, be for the purpose of exemplifying principles rather than practice. There are, of course, some Hnes of specialization in colleges which lead directly to practical results afterwards. Research work is one of these. However, it is probable that if a large part of the time given to research work by the student in college were^x^ended in ac- TECHNICAL TRAINING FOR ENGINEERS 763 guiring a broader foundation in fundamental principles the results^vTOuiar^e" better in the etid. As referred to before, there has been one serious defect, in our systems ot technical training today,, nam^ely, it holds, back the leaders and j)ushes the laggards, thus tending toward mediocrity as the general result. There should be, sqme system in colleges for weeding out the "negatives" in any given line of endeavor. Many, of these are simply "misapplications," to use a manufactur- ing company term. In some other lines they may be highly success- ful. / ' ' . ' In an ideal engineering course each student should be pushed to the utmost of his capabilities. One solution of this problem "would be for each teacher to assign a certain amount of work to his students indivi iually, and they should report to him.! individually on such work, explaining to him fully what they have accomplished. Each man thus could be pushed along independently of his fellows. The weaknesses of the individual men would' soon appear. If, for example, it develops that certain of the students are behind in the necessary mathematics, then steps could be taken to correct this defect. Each student would 'have to think more for himself and would be put more or less upon his own resources. His various characteristics could be studied and developed. He should be made to work out and apply fundamental principles. He would thus practice using his own mind. As soon as it develops that he has no mind of his own, then he could be dropped. In such a course of teaching the advancement of each man would be dependent upon himself, to a large extent. At this point a principle of mechanics can be applied rather aptly. In machines a force does work in overcoming resistance. In man the same principle holds true. No matter how much force a man may have,' if no resistance is presented, no result is accomplished. And if the force is small, then the result is also Hable to be small. But a smaller force over- coming a larger resistance may result in greater accomplishment than a larger force with but little resistance. An unusually bril- liant boy with too small a task set for him may accomplish little. His task must be enlarged to suit his abilities; for, as in machines, to obtain the greatest result the resistance, or task, must be com- mensurate with the force acting. Unfortunately, many good men of great capabilities accomplish practically nothing, through too little resistance, due to life being made too easy for them. 754 ELECTRICAL ENGINEERING PAPERS Such a course of "forcing," as indicated above, might be diffi- cult to apply in many of the schools as constituted today. But the writer's personal experience indicates that the better class of men will develop rapidly imder such treatment, while the laggards are eliminated more quickly. He has tried this system in general on many graduates from the technical schools and imusually satis- factory results have been obtained. All of the foregoing points to the fact that the mere accumula- tion of knowledge is not a training, nor an education. The old saying that "knowledge is power" is not technically correct any more than is the statement that torque (or force) is power, to use an engineering comparison. Torque, or force, is not power, but torque in motion is power and, to continue this comparison, knowl- edge in motion, or in action, is power. Activity in some form is one of the essential factors. To sirni up, the colleges should aim to develop the student's characteristics, as far as practicable. They should aim to develop analytical ability, imaginative faculty, ability to do independent thinking. They should teach fundamental principles, and the course of teaching should be such as to give the individual student a real grasp of such principles. A broad general training is most desirable for the man who has the ability to do something in the world. ENGINEERING BY ANALYSIS 755 ENGINEERING BY ANALYSIS FOREWORD — In tlie latter part of 1916, the engineering students at the Ohio State University decided upon the publication o£ a college engineering paper, and the author was asked to prepare something for the first issue. From various notes at hand, he prepared a short paper under the title, "The Electrical Engineer of Today," which was submitted in answer to' this request. This paper ap- peared in the first issue of the Ohio State Engineer in January, 1918. Upon request of a member of the Engineering Faculty at the University for several hundred copies of this paper for his students, it was reprinted, and is here re- produced in practically the same form as the reprint, except the title. — (Ed.) THE early engineering in any field is usually of the "cut-and- try" kind, followed later by the refinements of more highly trained specialists. A comparatively recent development in indus- trial and manufacturing engineering is the analytical engineer. By this is meant the engineer who translates facts into relationships, formulae and figures, and eventually retranslates them into other facts. The analytical engineer in this sense does not mean the mere user of figures and formulae. He starts with fundamental principles and laws from which he then draws his conclusions, the applications of which are made directly to the final product with- out intermediate experimentation. The analytical engineer has led the way to new and more difficult fields of endeavor and many of our most rapid advances have been made under his guidance. Electrical engineering, is one of the yoimgest of the en- gineering lines of endeavor, but its "cut-and-try" period was of comparatively short duration. The coming of the analytical en- gineer was almost coincident with the rise of electrical engineering as a business. This branch of engineering deals with more or less ob- scure phenomena, of which there are only indirect evidences in many cases. Many of the laws primarily are only mathematical relationships. Many of them can only be grasped or handled by those who have considerable analytical and mathematical ability. In consequence, even comparatively early in the work, the highly technical engineer was a necessity. Probably in no other branch of engineering, since its first development, has there been as large percentage of men, having high technical training, engaged in the work; and as a consequence, in no other lines of engineering has there been as rapid growth as in the electrical. Coincidentally with the growth of electrical engineering, there have been rapid advances in the older and better established lines 756 ELECTRICAL ENGINEERING PAPERS of engineering, especially in those which have been rather inti- mately associated with the electrical industry. The steam tur- bine which now dominates the field of steam prtiiie movers, re- ceived its greatest impetus in connection with electrical work, and its present high development may be said to be the product of the analytical engineer. Water-wheel development has also made great advances under much the same conditions. One characteristic of the analytical engineer of the present time, especially in electrical work, is that he is very often working far ahead of his available data. He is obliged to plot his existing data and experience and then exterpolate for the new points which he finds necessary in his work. He is thus working in the un- known to a greater or less extent, but his ability to analyze and correlate very often leads him to be fairly certain of his results. It is this ability to work with confidence in comparatively un- known fields, which has produced such astonishing results in electrical engineering. The analytical engineer of today, whether electrical or other- wise, must forsee, through his analysis of data and practice, what the trend of future practice Avill be. If his analysis shows him that certain lines of development are scientifically more consistent than other Unes, he will naturally tend to work along what he considers to be the correct direction. If he sees that certain practices are fundamentally wrong and represent only makeshift conditions, or jnerely commercial expediency, he will naturally feel that such practices eventually will be replaced. He must weight both theoretical and practical conditions in determining which direction to work. With the true analytical engineer there will be no standardiza- tion of practice unless such practice has good fundamental reasons back of it. His tendency is rather toward standardization ac- cording to certain scientific principles and limitations than by practices which have insufficient basis. The latest standardiza- tion rules of the American Institute of Electrical Engineers repre- sent an attempt along this line, and it is a pretty safe prediction that the basic features of these new rules will be retained for many years to come. Analytical engineering, of a very advanced kind is represented by the modern research and testing departments and laboratories of the big engineering concerns who do electrical and other manu- facttiring. Much of the technical data, which the designing. ENGINEERING BY ANALYSIS^ \ 757 developing and manvifactiiring departments require, is a direct product of such departments. No progressive industrial estab- lishment of the present time can get along without extensive research departments. Recently Congress has approved of a large Naval Laboratory for research and experimental work, in Une with other engineering and industrial organizations. A good example of modem electrical design work of a highly analytical character, is the present turbo generator. The present huge capacity high speed machines are almost beyond the dreams of ten years ago. These machines are almost entirely the product of the analytical designing engineer. In these machines nearly all pre- vious developments and experience in other lines of apparatus have counted for little. New methods, new materials, new practices and new limitations have been established in these machines, and for these reasons, the turbo generator engineer has been compelled to work ahead of his data and experience much of the time. For ex- ample: the twenty thousand kilowatt, 1800 r.p.m., 60-cycle, turbo generator was undertaken when the ten thousand kilowatt ma- chine of the same speed and frequency was the nearest size from which to obtain data, and this smaller size unit had already been carried up to what were considered as the permissible limits, in many ways. In such case the designer had to overstep his data and limits, and depend largely upon analysis. Another good example of analytical engineering is the in- duction motor. While such motors possibly could have been developed by cut and try methods, at great expense and with many failures, yet the present advanced status of this type of apparatus can be considered only as the product of the analyst. The production of cage-wound induction motors with good start- ing torque, suitable for general purposes, was the result of analysis, not experiment. In the electrical manufacturing industry the analysts, as repre- sented by the designing engineers, hold an important place. The term is here used broadly to include the designers of systems, applications, methods, etc., as well as apparatus. They form a very necessary part of the organization, especially so in connection with those departments where cut-and-try methods have been largely eliminated. ' Many of the largest engineering under- takings are on customers' orders, covering apparatus which has never been built before. In most cases, by the time any tests of the completed apparatus are obtainable, the work as a whole 768 ELECTRICAL ENGINEERING PAPERS has progressed beyond the point where any important changes can be made. Even such preliminary tests as are obtainable in the shop are Uable not to teU the whole tale, for the real test or proof of the adequacy of the design comes from duration tests furnished by actual service. The real troubles may not show up until six months or a year after the apparatus has been put in service. Here is one of the difficulties that the designing engineer encounters; and, the more progressive he is, the more liable he is to run into this very difficulty, simply because he is pushing ftirther into unknown grotind. A serious difficulty possibly de- velops a year or so after the apparatus has been put in service. Then he is criticised both for not having forseen and for not having immediately corrected it. Such criticism might be con- sidered, in one sense, as complimentary, for it is an assumption that he knows much more than he really does. However, most engineers are not particularly pleased over such criticism, for they usually find it hard enough to cure an imknown and unforseen trouble, without being told that they were careless and did not use proper foresight. A true engineer has pride in his work, and a defect or failure, in itself, usually hurts him even more than criticism. He also feels that when a man has done the best he can and has attempted something never accomplished before, he should have sjonpathy in his trouble, or at least constructive criticism. It may be added here that in addition to ability to undertake and carry through a given design, it is important that the engineer be able to "let go" of it at the proper time. Each new develop- ment or test shows the way to still further improvements or de- velopments, and if each of these is to be incorporated in the de- sign, then it will never reach completion until absolute perfection is attained or the designer has reached the ultimate limit of his ability. Neither of these conditions is practicable in a live manufacturing business, and, therefore, the engineer shottld be able to let go of his design when a sufficiently good practical result is obtained. Some engineers seem to know just when to stop. This is to some extent dependent upon a proper appreciation of commercial requirements. To be a successful electrical engineer does not mean one is fitted to be a manufacturing engineer; further, one may be a very good electrical manufacturing engineer and yet not be fitted for elec- trical design, for this latter is a branch of the industry which re- ENGINEERING BY ANAL YSIS 759 quires rather special characteristics. Experience shows that the designing engineer must have a special aptitude for such work regardless of his education or general abilities, if he is to be thor- oughly successful. In design work, experience has also shown that combinations of the requisite natural aptitude and the necessary- technical training are comparatively rare, and the really successful men in this line of work are but very few in number. If certain aptitudes and characteristics are essential for the designing engineer it might be asked— what are these essentials? However, it is almost impossible to pick out any characteristic which could be considered as the one essential in the electrical designing engineer, except, possibly, good common sense; but as this is at the bottom of all true success, it shoiild not be considered as peculiarly characteristic of the engineering profession. As the competent electrical designing engineer must necessarily be an analyst, obviously analytical ability, in the broad sense, must be one of his foremost characteristics. He should also have a certain amount of mathematical ability and training. In general, skiU in the ordinary mathematics, such as in algebra and analytical trigonometry is of more use than a mere working knowledge of the higher mathematics. There are certain lines of work in which the higher mathematics are, of course, very valuable and necessary. These, however, represent a relatively small per cent of the total field. The young engineer should not become tinduly impressed with the idea that ability to use extremely complicated mathe- matics is the prime requisite. He shoiild, however, recognize that without mathematical aptitude of any sort, he is very greatly handicapped. The "handy man" with mathematics appears to have a decided advantage over others, in practical work. The engineer who can develop a mental pictiu-e or a "physical conception" of what is going on in a machine, in distinction from a purely mathematical conception, appears to have a very consider- able advantage over his fellows. The man with both the physical conception and with good mathematical ability will probably go further in analysis than any of the others. Let us return to one of the conditions which is very necessary in all engineering, namely — a good knowledge of fundamental principles. The engineer should Imqw the_d£riya±ion_of _his var- ious methods "and formulaeT^Many of these which are now used by rapid workers'are'really short cuts or empirical methods which are primarily based upon correct but more complex methods. 760 ELECTRICAL ENGINEERING PAPERS Their use, without a proper knowledge of their derivations and, therefore, their limitations, is dangerous and not infrequently leads to serious trouble. Above all the electrical designing engi- neer should have a broad conception of certaiti fundamental rela- tionships or laws entirely apart from the mathematics of the case. With a clear understanding of fundamental principles there is much less liability of waste of time and effort from following out impracticable schemes. There was a time, and not so many years agoy when an elec- trical engineer could cover almost the entire field. At that- time a fairly complete trainiiig in the various branches of electrical engin- eering was possible, but with the widening of the field, it has become too great for the single individual to cover, and the problems have become too difficult for any one man to handle all of them. There- fore, it has become necessary for individual engineers. to devote themselves to some special field of endeavor and to leave the broad field to be covered by the co-operation of many specialists. Con- sequently, the engineering of today is sub-divided into many groups, each more or less distinct in itself, but ■ each' overlapping and interrelated with many other groups. The engineer of today is, therefore, always some kind of a specialist, for it is impossible to be otherwise if he is to lead in anything. It is on account of this specialization that it is so' important that the young engineer of today obtain a broad knowledge of the fundamentals of his chosen line of engineering. The same fiin- damentals tmderlie the whole electrical field, so that a' knowledge of them is about aS near as he can come to a broad kno^lrledge of the whole. Such should be obtained as early as possible in his career, for, after specialization begins, his own particular field of endeavor is liable to absorb all of his efforts. It is now' being recognized by the ablest engineers that much specialization in the schools .s not an advantage to the student. If the colleges ' could confine themselves' to a broad teaching of fundamental principles they would turn out vastly more effective men than at present. Analytical ability (not necessarily mathe- matical) is one of the crjdng needs of the electrical industry of today, as regards its young men. And this need exists in spite of the fact that this industry doubtless gets its full share of the analytical men turned out by the schools. An analytical man per se is one who thinks for himself and, therefore, the problem really narrows down to the thinking man. If the schools cotdd turn out ENGINEERING BY ANALYSIS 761 a much higher percentage ,of thinking men, the engineering pro- fession would be vastly benefited. There is another quality or characteristic which, while possibly not as valuable as analytical ability, goes a long way toward suc- cess, namely — persistency. A brilliant mind with but little per- sistency back of it, will usually accomplish less than a much less brilliant mind backed by great persistency. This latter charac- teristic has turned many an apparent failure into positive success. A brilliant man without persistency is liable to pass from scheme to scheme and perfect none of them. However, persistency alone usually accomplishes no more than brilliancy alone. Men have expended years of patient effort along lines which a little common sense analysis woiild quickly have shown to be impracticable. Here is persistency gone to waste. The eiriphasis placed upon the above mentioned characteristics is not intended to belittle other very important ones, such as initiative, originality, resourcefulness, etc. These qualities might be classed even higher than analytical ability and persistency by some persons, and possibly rightly in some lines of effort. But in the higher electrical work the conditions may be otherwise. Here one may have strong initiative, but be utterly- unable to make any great progress due to lack of analytical ability; he may have great originality, but, lacking the fundamentals, be unable to touch on the higher work; he may be exceedingly resourceful, but be limited only to lesser things due to lack of knowledge of basic principles, and, thus, inability to hahdle advanced work. However, a leader must have all of these qualities to a certain extent. Now and then a man is found who has all of them to a fairly high degree, com- bined with unusual analytical ability and perseverance. Such a man eventually is liable to become known as a genius, but it should be remembered that genius is of two kinds, — ^creative, in the sense of being able to think in new fields, and constructive, in, the sense of being able to use present known facts and principles to bring about successful results. Then there is another feature which may be referred to, name- ly, the commercial side of engineering. An electrical manufacturing business lives by the goods, not the engineering, which it sells. The successful designer of such goods must, therefore, have con- siderable knowledge of commercial conditions or he catmot design adequate or competitive apparatus. This is a feature of the busi- ness about which the young engineer, fresh from school, knows 762 ELECTRICAL ENGINEERING PAPERS nothing. This appears to be a very difficitlt thing for some en- gineers to acquire, while certain of them never really do so. On the other hand, it has been said of some very good engineers that they ought to have been salesmen, because they grasped so readily the customer's conditions and requirements. The broad gauge elec- trical designer is usually quite successful in aiding the salesman, because he sees the commercial bearing of his engineering work. This relation of the engineer to the commercial side of the busi- ness brings up another point, namely, his ability to talk clearly and logically in private and in_public. It was once~siipposed that an engineer never had to talk in public and that all he had to do was to go off in a comer, by himself, and use a slide-rule. But that day is long past, for now the man who knows most about the appa- ratus must be able to tell others what he knows. Presumably in all large concerns there are men who are seldom or never sent out- side on account of their inability to make a good presentation of a subject. Assuming equal ability otherwise such men are of less value than those who can make a good presentation of any given matter. In general, a good logical thinlcer can develop into a fairly good logical speaker through practice. The foregoing has had most to do with electrical designing en- gineers, but while they are a very important part of the industry, yet they are not the only engineers in the electrical manufacturing business. In fact, the electrical industry today is managed almost entirely by men who should be classed as engineers. A large per- centage of the electrical salesmen of today have had a very good engineering training of one kind or another. In fact, in many lines they must have such training in order to be successful. In the manufacturing part of the business, many of the leading men are also good engineers. Also many of the high executives in the industry are trained engineers of high grade. ' In conclusion it may be said that this is an age of engineering construction. It is, or rather it forshadows, the golden age of the engineer. His successes and attainments have led him to view hopefully hitherto totally unattainable things, and in conse- quence his problems are becoming increasingly difficult. At no time has such boldness been shown in attacking the problems of nature for the benefit of mankind, and it is the engineer in one guise or another who is behind the attack, and his aim almost invariably is something which is ultimately for the advancement of humanity. Construction, not destruction, is his preference. ENGINEERING BY ANALYSIS 763 He is an optimist and not a pessimist. In research work he is delving into the unknown in search for properties, principles and laws of nature and of material. He is making vast strides in the conservation of natural resources, by the economical generation and utilization of power. In transportation he is bringing the whole world together. He is making steel and concrete the rule in constructions, doing away with more perishable materials. Engineering shovdd be considered of highest rank among the professions. No engineer need apologize for his calling. He should feel the greatest pride in it, for it may be said that it is the very heart and soul of material progress.