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)

Shaded-pole motors are the original type of AC single-phase induction motor. A shaded-pole motor is basically a small squirrel-cage motor in which the auxiliary winding is composed of a copper ring or bar surrounding a portion of each pole.^[1] This auxiliary single-turn winding is called a shading coil. Currents induced in this coil by the magnetic field create a second electrical phase by delaying the phase of magnetic flux change for that pole (a shaded pole) enough to provide a 2-phase rotating magnetic field. The direction of rotation is from the unshaded side to the shaded (ring) side of the pole.^[1] Since the phase angle between the shaded and unshaded sections is small, shaded pole motors produce only a small starting torque relative to torque at full speed. Shaded-pole motors of the asymmetrical type shown are only reversible via disassembly and flipping over the stator, though some similar-looking motors have small, switch-shortable auxiliary windings of thin wire instead of thick copper bars and can reverse electrically. Another method of electrical reversing involves four coils (two pairs of identical coils).^[2]

The common, asymmetrical form of these motors (pictured) has only one winding, with no capacitor or starting windings/starting switch, making them economical and reliable. Larger and more modern types may have multiple physical windings, though electrically only one, and a capacitor may be used. Because their starting torque is low, they are best suited to driving fans or other loads that are easily started. They may have multiple taps near one electrical end of the winding, which provides variable speed and power via selection of one tap at a time, as in ceiling fans. 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 ¹⁄₄ horsepower (190 W) output. Above ¹⁄₃ horsepower (250 W), they are not common, and 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.

References

1. Wildi, Theodore (2006). Electrical machines, drives, and power systems. Upper Saddle River, NJ: Pearson Prentice Hall. ISBN 0-13-177691-6.
2. US 4017776, Fiegel, Josef, "Reversible Shaded-Pole Motor and Control Arrangement Therefor", published 11 December 1975, issued 12 April 1977