Pole-changing permanent-magnet machines

Electric drives in which discrete speed control is required are today equipped exclusively with squirrel-cage induction motors, due to the property of the squirrel cage to always have the same number of poles as the stator winding. Conventional permanent-magnet (PM) machines have a constant number of poles and can be operated from a constant frequency source only at one speed. If a PM machine is built after the principles of memory motors, one can change its number of poles as simply as in a squirrel-cage machine. In this paper, the principles of. operation of a pole-changing memory motor are described, and its measured performance is presented and discussed.