A synchronous motor is an AC motor that runs at a speed fixed to a fraction of the power supply frequency, and an asynchronous motor is an AC motor, usually an induction motor, whose speed slows with increasing torque to slightly less than synchronous speed. An asynchronous motor requires slip - relative movement between the magnetic field (generated by the stator) and a winding set (the rotor) to induce current in the rotor by mutual inductance. The most ubiquitous example of asynchronous motors is the common AC induction motor which must slip to generate torque. In the synchronous types, induction (or slip) is not a requisite for magnetic field or current production (e.g. permanent magnet motors, synchronous brush-less wound-rotor doubly fed electric machine).
