Phase modulated 3-phase pole changing windings

The principle of phase modulation is described and methods of design are developed which enable 3-phase induction motors to be changed in speed using simple 6-lead switching of a single winding. The method of switching relies on special 3-parallel path connections which are derived by a novel `self paralleling¿ process. Three elemental windings are combined in such a way that the three parallel paths can be formed within a resultant single winding. The flexibility of design is more limited by comparison with the longer established method of pole-amplitude modulation, but good designs are possible for two speeds in close ratio particularly for constant-torque and constant-power duties. Examples are included together with results of tests on a trial 8/10-pole motor.