Synchronous Motor

A synchronous motor is an AC electric motor whose shaft rotates in exact step with the frequency of its supply current, using permanent magnets or electromagnets on the rotor together with electromagnets on the stator, so the rotor turns at a fixed rate locked to line frequency rather than slipping the way an induction motor rotor does. Because it does not need induced rotor currents to create its magnetic field, a synchronous motor can also run at a leading or unity power factor, letting large installations use synchronous machines for power factor correction on the grid as well as for driving loads. Small synchronous motors have long served in timing applications such as synchronous clocks, appliance timers and tape recorders, while industrial sized machines commonly reach electrical efficiencies above 95 percent. Synchronous and induction motors together are the two most widely used families of AC motor, and the synchronous motor is also the same basic machine, run in reverse, as the synchronous generator that supplies most of the world electricity grid.