SYNCHRONOUS MOTOR

alternating-current electric motor designed to run at a constant speed that varies only with the frequency of the power source. Many synchronous motors are unable to start unassisted and must have an auxiliary motor to bring them to synchronous speed. Others have an additional, built-in start-up winding. The constant speed is maintained by applying a constant magnetic field to the rotor. Such a field may be created by applying a direct current to the rotor windings or by constructing the rotor from permanently magnetic materials. The rotor's magnetic field tends to keep it in step, revolving as fast as the changing magnetic field produced by the alternating current that drives the motor. Synchronous motors are used in low-load applications in which constant speed is crucial, as in electric clocks, timers, phonographs, and tape recorders. See also electric motor.