Synchronous motor

A synchronous motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current. It uses permanent magnets or electromagnets for the rotor and electromagnets for the stator: the stator creates a magnetic field that rotates in time with the current oscillations, and the rotor turns in step with it at the same rate, giving a second synchronized rotating magnetic field. Synchronous and induction motors are the two most widely used types of AC motor. Because a synchronous motor does not rely on induction to produce the rotor's magnetic field, it rotates at a rate locked to the line frequency, whereas an induction motor needs slip. Small synchronous motors serve timing applications such as clocks, appliance timers, tape recorders and precision servomechanisms where speed must be exact, and permanent magnet types have been used as gearless elevator motors since 2000.