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 a squirrel cage to have always the same number of poles as the stator winding. Conventional permanent magnet (PM) machines have 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 the paper the principles of operation of a pole-changing memory motor are described, and its measured performance is analyzed and discussed.