AC induction motors, also known as asynchronous motors, use a rotating magnetic field to produce torque. Three-phase motors are widely used because they are reliable and economical. The rotating magnetic field is easily achieved in three-phase asynchronous motors because the phase angle offset between the individual phases is 120 degrees. However, single-phase AC motors require external circuitry which creates the phase angle offset in order to produce a rotating magnetic field. This circuitry can be realized using advanced power electronics, or more simply using a motor capacitor.
AC induction motors usually use two or more coils to generate a rotating magnetic field, which produces torque on the rotor. When a single coil is used, it will generate a pulsating magnetic field, which is enough to sustain rotation, but not sufficient to start the motor from a standstill. Motors with a single coil have to be started by using an external force, and can rotate in either direction. The direction of the rotation depends on the external force. If the motor was started in a clockwise direction, it will continue to rotate and build up speed in the clockwise direction, until it reaches a maximum speed which is defined by the power source frequency. Similarly, it will continue rotating counter-clockwise if the initial rotation was counter-clockwise. These motors are not practical due to their inability to reliably start rotation on their own.