Shaded-pole motor

Small C-frame shaded-pole squirrel-cage motor. With the poles shown, the rotor will rotate in the clockwise direction.

Shading coils (copper bars)

A shaded-pole motor is a type of AC single-phase induction motor. It is basically a small squirrel-cage motor in which the auxiliary winding is composed of a copper ring surrounding a portion of each pole. This auxiliary single-turn winding is called a shading coil. Currents induced in this coil by the magnetic field delay the phase of magnetic flux for that pole (a shaded pole) enough to provide a rotating magnetic field. The direction of rotation is from the unshaded side to the shaded (ring) side of the pole. Since the phase angle between the shaded and unshaded sections is small, 2-phase shaded poles produce only a low torque, so they are only used to start the rotor turning in non-self-starting motors. Shaded-pole motors are not normally reversible.

These motors have only one winding, no capacitor nor starting switch, making them economical and reliable. Because their starting torque is low, they are best suited to driving fans or other loads that are easily started. Moreover, they are compatible with TRIAC-based variable-speed controls, which often are used with fans. They are built in power sizes up to about 1/6 hp or 125 watts output. For larger motors, other designs offer better characteristics.

Types

• Squirrel-cage induction motor: The most common type of shaded-pole motor in fractional horsepower use is the squirrel-cage induction motor. This has a rotor that consists of a laminated steel cylinder with conductive copper or aluminum bars embedded lengthwise in its surface, connected at the ends. The rotating magnetic field of the stator induces current in the bars that creates a magnetic field in the rotor that causes it to rotate.