Induction-motor speed-changing by pole-amplitude modulation

An original method of changing the speed of squirrel-cage induction motors has recently been developed which enables efficient and economic operation to be obtained at either of two chosen speeds, using a single winding of standard type. The method has considerable generality, but is particularly applicable to speed ratios not very different from unity. The initial tests have therefore been performed on a motor for two speeds in the ratio 5:4, but a variety of ratios can be obtained by using the same principle. The control gear needed is extremely simple.