Synchronous motors operate at a constant speed in absolute synchronism with the line frequency. Synchronous motors are classified according to their rotor’s design, construction, materials and operation into the four basic groups:

• electromagnetically-excited motors • PM motors • reluctance motors • hysteresis motors In electromagnetically excited and PM motors a cage winding is frequently mounted on salient-pole rotors to provide asynchronous starting and to damp oscillations under transient conditions, so-called damper .