Single-phase current alone does not create a rotating field that can turn the rotor. A single-phase motor has an auxiliary winding alongside the main winding; the capacitor shifts the phase of the current in this winding and lets the motor start. Some models also have a second, starting capacitor that is switched in at start and disconnected once the motor speeds up; this arrangement is preferred for duties that start under load.

When replacing a capacitor

  • The new one must have the same capacitance (µF) and voltage as written on the old one; a different value weakens starting or heats the winding.
  • If the motor hums and will not start, but runs when turned by hand, the capacitor is the first part to check.
  • A capacitor may hold a charge after the power is cut; discharge it before touching.

Reversing the direction

In a single-phase motor, direction changes when the connection of the auxiliary winding relative to the main winding is reversed; this is done with the link arrangement in the terminal box. The correct connection is shown in the diagram inside the terminal box cover; swapping leads without the diagram can damage the winding. Unlike a three-phase motor, swapping the two supply leads does not reverse a single-phase motor. If direction will be changed often, state this need when ordering.