`Sum¿ and `difference¿ winding modulation with special reference to the design of 4/6-pole p.a.m. windings

In several respects, the design and performance of 4/6-pole p.a.m. windings present features which are contrary to the earlier and more usual forms of close-ratio pole-amplitude modulation (p.a.m.), and also to certain orthodox concepts relating to the harmonic m.m.f. content of the waveform of an induction motor. A very careful examination has therefore been given, in general terms, to the special features of 4/6-pole windings and to the effects both of subharmonics and of higher-order harmonics on the operation of an induction motor. (This latter part of the paper is of importance in relation to single-speed machines also.) Taking a 4/6-pole winding in 36 slots as an important example, the paper uses it to consolidate earlier p.a.m. windings with a number of later p.a.m. design methods. The differences between `sum¿ and `difference¿ modulation are also very fully considered. The whole paper serves to emphasise that pole-amplitude modulation is a general philosophy for polyphase windings, and not a specific design method. In principle, there is no limit to the number of design methods which may be evolved for embodying this philosophy in particular windings.