What makes an alternating-current electric motor rotate in a preferred direction?
AC motors need more than one phase in order to start up in a particular direction.

With 3-phase motors, the direction of rotation is determined by the order of the phase connections.

With single-phase motors, there are a number of ways to create a second phase:


shaded pole - (used primarily in small motors) part of each stator pole is wrapped with a ring of copper (a shorted turn) that causes the phase of the magnetic field to be shifted.
capacitor-start - a second winding is energized through a capacitor (which shifts the phase), but is disabled once the motor is up to speed by a centrifugal switch. Once the motor is running, a single phase will keep it going by itself.
capacitor run - a second winding is energized at all times through a capacitor, which shifts the phase. Some motors of this type are symmetrical and can be reversed by swapping the direct-connected and capacitor-connected windings.