Commutation in alternating-current machinery

As is now well known, the inductive reactive effect of a commuted 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 distribution of conductors on the stator is of much assistance in this connection. In a motor built 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.