Multispeed induction motor fed from a switched single-phase supply

A relatively simple naturally-commutated switching system is described which enables a polyphase a.c. motor to be operated as a speed-changing single-phase motor. A single-phase supply, of fixed frequency, is converted to a polyphase supply consisting mainly of a complementary pair of adjustable component frequencies. When supplied to a polyphase winding, an alternating m.m.f. is produced, the speed of which can be correspondingly adjusted. Natural commutation permits efficient operation at discrete speeds only, but several speeds below and above the normal synchronous speed are possible. The method of supply can, in principle, be applied to all types of polyphase a.c. machines, but special attention is given to the conventional squirrel-cage induction motor. Theoretical results obtained froni an idealised analytical treatment are compared with test results on a 5-speed experimental system.