Rotor windings for induction motors with arc-shaped stators

Squirrel-cage induction motors in which the stator winding is limited to a fraction of the total periphery have performance inferior to conventional machines as the result of transient rotor currents set up by the edges of the energised arc. In particular, motors whose active zones contain only a few poles exhibit low efficiency, power factor and power/weight ratio. The paper describes a method of replacing the squirrel cage with a wound rotor which is short-circuited but which is so arranged that the undesirable transient effects at the two edges are made to cancel each other. The system is most favourable to cases in which the pole pitch of the machine is such that it divides evenly into the complete periphery. Results on an experimental machine indicate that the efficiency of a small machine, having only two poles in an energised arc of 120°, can be raised from just over 40% to over 80% by this technique, with correspondingly large increases in power output. The paper concludes with a brief discussion of the possible extension of the system to continuously variable-speed induction machines.