Two-speed salient-pole synchronous machines

A constant-speed drive is not, in its nature, what industry usually requires. On the contrary, the ability either to change speed uniformly over a declared range, or to obtain two or more alternative fixed speeds, is commonly very advantageous. Like the squirrel-cage induction motor, the salient-pole synchronous motor has secured wide use because of its many well-known virtues. This is in spite of its inherent fixed speed, which is only sometimes required and is often actually disadvantageous.