Brushless variable-speed induction motors using phase-shift control

The paper describes a method of `pole-stretching¿ for induction machines in which part of the stator windings are fed directly from the mains supply and part from phase-shifting transformers. Variation of the angle of phase-shift enables continuous speed control to be effected. An experimental machine is described, the test results from which demonstrate that speed control with constant efficiency can be obtained over a speed range of 1.5: 1. The limitations on the range of such machines imposed by the necessary condition that the stator be discontinuous are discussed, and a method of extending the speed range is then described. Machines of this type may be designed to run with a number of discrete synchronous speeds, in which case no phase-shifting transformers are necessary and speed change is effected by external switches only. The historical link between this type of machine and the spherical motor is outlined.