Commutation in Alternating-Current Machinery

Presenied at the Sixth Midwinter Convention of the American Institute of Electrical Engineers, New York, February 16, 1918. Copyright 1918. By A_ I. E. E. COMMUTATION IN ALTERNATING-CURRENT MACHINERY BY MARIUS C. A. LATOUR ABSTRACT OF PAPER As is now well known, the inductive reactive effect of a com- muted winding in a revolving magnetic field decreases directly with the increase in speed from standstill to synchronism, when its value becomes zero. As first pointed out by the author some years ago, it becomes negative at speeds above synchronism, under which condition the rotor of a motor operates as a capacity. The author introduces into the discussion of the commutating characteristics of alternating-current commutating motors, his theory that perfect commutation in a continuous-current motor depends substantially on the production of a mean resultant neutral field in the region where commutation is taking place, and shows that the production of a perfect revolving field in a polyphase commutator motor assists in insuring perfect commutation at exact synchronism. In a single-phase commutator motor a "polyphase" revolving field can be produced at synchronism by utilizing supplementary brushes, short-circuited upon themselves, displaced by 90 electrical space degrees from the main single-phase brushes on the commutator. As in the case of polyphase motors, the problem of securing perfect commutation at synchronism becomes that of producing!a perfect rotating field. It is shown by the author that the use of fractional-pitch windings on the rotor and a sinusoidal distri- bution of conductors on the stator is of much assistance in this connection. In a motor buLilt in accordance with the principles set forth, the commutator difficulties are not serious, the overload range is in excess of that of an induction motor, and the machine can act as a condenser on the system. THE writer has already had the honor of presenting two com- munications on the use of commutators with alternating currents before the American engineering public, the first being in June, 1903, in co-operation with Mr. A. S. Garfield (see The Com- pounding of Self-Excited Alternating Current Generators, for Vari- ation in Load-Factor, in A. I. E. E. TRANSACTIONS, VOl. XXI, pages 569-577, and discussion, pages 583-585); the other being a paper presented at the St. Louis Congress, 1904, ("Alternating Current Machines with Gramme Commutators,' published in Transactions of the International Electrical Congress of St. Louis, 1904, Vol. III, pages 149-154). 355 356 LATOUR: COMMUTATION [Feb. 16 The purpose of the present paper is to enter more deeply into the theoretical considerations which were outlined by the writer in his earlier publications, and which have since been verified by practical experience. A distinction will be made between the use of commutators for polyphase and for single-phase alternating currents. I-COMMUTATOR IN POLYPHASE MACHINERY Let us consider a direct-current bipolar armature, A, Fig. 1, placed in a uniform air gap inside the laminated stator, B, with regularly spaced slots and receiving sinusoidal currents of p phases, of frequency equal to w/2 7r through p brushes located at points situated 2 wr/p in angular distance from each other. Fig. 1 shows diagrammatically a smooth core armature, on which four brushes, a, b, c, d, make contacts through which two-phase (more properly four-phase) currents are supplied. The brushes, a, c, receive a current I sin w t, and the brushes b, d, receive a current I cos w t. Suppose the armature to be at rest and let Lw be its inductance per phase. If the armature is made to turn at the angular velocity co,, measured in the direc- tion of the rotating field developed by the polyphase currents, it will be found that the inductance of the armature becomes L (co- co,); consequently, it will vanish at synchronism and, above synchionism, the ar?angement will operate as a capacity (condenser). When this property was first made known by the writer, in his patents, in 1900-1901, its correctness was questioned by several prominent electricians. Among these, M. Maurice Leblanc devoted a long article (see Eclairage Electrique, October 26th, 1901, pages 113 et seq) to the refutation of the writer's conclusions. Many electricians have, since then, accepted as satisfactory the theory which introduces the relative velocity (- w1) of the revolving field developed by the armature with respect to the armature itself. Mr. Leblanc, however, main- tained, in his article, that since the current, i, in each turn of the armature winding, preserved its variable character, no matter what the velocity co, might be, at any instant i was subject to the di same rate of variation, dt , and consequently the e. m. f. of self-induction between the brushes should persist at all speeds As was shown by the writer, in a detailed article, published in November, 1901, (see Eclairage Electrigue, 1901, pages 294 1918] LA TOUR: COMMUTATION 357 et seq), it is quite true that the e. m. f. of self-induction of the sections included in the circuit between the brushes persists inde- pendently of the velocity col; but, in consequence of the phenome- non of commutation, there appears between the brushes an e. m. f. of opposite sign which is proportional to the velocity co, and which is due to the mutual induction between the sections which are short-circuited and those which are in the circuit. In this connection, let us consider Fig. 1. Let M be the co- efficient of mutual induction between the sections which are short-circuited by the brushes, b, d, and the sections that remain in circuit between the brushes a, c. Let n be the number of sections of the armature. With brushes covering the width of a commutator segment, the duration of commutation for the sec- FIG. 1 tions of winding passing under the brushes b, d, at the velocity wi, will be T-= t) n The e. m. f. EM developed by mutual induction between the short-circuited sections passing under the brushes b, d, (where the reversal of a current I I cos w t is taking place during the time T) and the sections which are in circuit between the brushes a, c, (through which the current I sin co I I - passes )will be T cos co t= 2 co, I cos cwt. We first ascertain that EM is opposed in polarity to the e. m. f. of self-induction EL = L w I cos w t, due to the self-induction of the armature; and we then can show that, in the case where 358 LA TOUR: COMMUTATION [Feb. 16 each section of the armature is supposed to produce a sinusoidal Mn flux at the armature periphery, we will have Mn = L and consequently EL - EM = L (o - cw1). It is to be noted that the same expression may be obtained for the e. m. f. (EM) by supposing it to be induced by the rotation of the armature in its own field produced by the current sent through brushes a, c; but this explanation of the appearance of the e. m. f. EM, though it may always be correct from the mathe- matical point of view, is not quite so near the physical reality, since, as a matter of fact, the field of the armature itself must follow the armature in its rotation. The fact that the armature in the arrangement shown in Fig. 1 can operate as a capacity (namely when co, exceeds c) was of particular interest to Mr. Leblanc, who, about that time (in 1902) was endeavoring to find a simple electrical system suscep- tible of being used in place of the condensers which he was placing in the rotors of induction motors, for the purpose of improving their power-factor. He had even, for that purpose, devised the combination of two single-phase machines provided with commutators. In Mr. Leblanc's arrangement an induction motor C was connected in cascade with a simple armature A, such as that shown in Fig. 1, running at a speed greatly in excess of synchronism with respect to the frequency of the rotor currents of machine C (see Fig. 2). This was an immediate indication of the value of the writer's article of November 23rd, 1901, as Mr. Leblanc himself has since expressly acknowledged, (see La Lumigre Electrique, July 12th, 1913, page 60, and the Electricien, July 25th, 1913, page 658).* The Swiss firm of Brown-Boveri, utilizing the methods patented by M. Leblanc for improving the power-factor of induction motors, has put on the market, in the last few years, under the name of phase-compensator, armatures with commutators rotating above synchronous speed. Fig. 3, made from a -photograph, shows one of these arrangements, designed for a *The writer had formally proposed that combination in a letter addressed to Industrie Electrique (in 1902) which was not published; but the writer, in any case, considers that M. Leblanc should be credited with the idea of placing condensers or any dynamic apparatus equivalent to them in the rotor of induction motors for the purpose of improving their power-factor. It is by mistake that the arrangement shown in Fig. 2 was, during a certain time, attributed to Mr. Scherbius. 1918] LA TOUR: COMMUTATION 359 500 h.p. motor, operated by a motor of 0.8 h.p., running at 1000 rev. per min. This outfit relieves the supply mains of the necessity of furnishing 180 kv-a. of reactive current.

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