Synchronous motor

A synchronous electric motor is an AC electric motor in which, at steady state,^[1] the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integer number of AC cycles. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.^[2]

Synchronous and induction motors are the most widely used AC motors. Synchronous motors rotate at a rate locked to the line frequency since they do not rely on induction to produce the rotor's magnetic field. Induction motors require slip: the rotor must rotate at a frequency slightly slower than the AC alternations in order to induce current in the rotor.

Small synchronous motors are used in timing applications such as in synchronous clocks, timers in appliances, tape recorders and precision servomechanisms in which the motor must operate at a precise speed; accuracy depends on the power line frequency, which is carefully controlled in large interconnected grid systems.

Synchronous motors are available in self-excited, fractional^[3] to industrial sizes.^[1] In the fractional horsepower range, most synchronous motors are used to provide precise constant speed. These machines are commonly used in analog electric clocks, timers and related devices.

In typical industrial sizes, the synchronous motor provides an efficient means of converting AC energy to work (electrical efficiency above 95% is normal for larger sizes)^[4] and it can operate at leading or unity power factor and thereby provide power-factor correction.^{[citation needed]}