Effects of subharmonics and winding asymmetry on the performance of slip-ring machines

The acceptable level of subharmonics in the m.m.f. waveform of primary windings for slip-ring induction motors has been investigated, and certain maximum tolerable levels are here determined which, if exceeded, seriously affect the performance of slip-ring machines. It has already been established that surprisingly large subharmonics in the m.m.f. waveform of a squirrel-cage induction motor can be accepted without perceptible effects on the machine performance. In large single-speed machines, with relatively large numbers of slots per pole, there is little diffculty in minimising subharmonic m.m.f.s. The design of change-speed pole-amplitude-modulated (p.a.m.) windings is more difficult in this respect; but, with the aid of recently developed techniques, remarkably harmonic-fee windings can be obtained, given a sufficient number of slots per pole. It is in smaller units that the design problem is most severe, and that large subharmonics might inhibit the development of change-speed slip-ring motors. There are, however, certain unorthodox windings, two of which are described, which may be used successfully as secondary windings, in conjunction with primary windings which might otherwise be unacceptable.