Shaded Pole Synchronous Motor

All single-phase motors require a means of producing a rotating magnetic field for starting. In the shaded-pole type, a part of the face of each field pole carries a copper ring called a shading-coil. A shaded pole motor is a type of AC single phase induction motor.

Baldor Motors Inc - www.baldor.com

As in other induction motors the rotating part is a squirrel-cage rotor. Currents in this coil delay the phase of magnetic flux in that part of the pole enough to provide a rotating field. The effect produces only a low starting torque compared to other classes of single-phase motors.

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.

The first photo is of a common C-frame motor. With the shading coils positioned as shown, this motor will start in a clockwise direction as viewed from the long shaft end. The second photo shows detail of the shading coils.

Shaded-pole synchronous motors are a class of AC motor.

Like a shaded pole induction motor, they use field coils with additional copper shading coils (see the illustration) to produce a weakly rotating magnetic field. But unlike a shaded pole induction motor (which uses a squirrel cage rotor), the synchronous version of this motor uses a magnetized rotor. This rotor rotates exactly in synchronism with the rotating magnetic field.

Because of this, these motors are often used to drive electric clocks and, occasionally, phonograph turntables. In these applications, the speed of the motor is as accurate as the frequency of the mains power applied to the motor.

Even by the standards of shaded pole motors, the power output of these motors is usually very low. Because there is often no explicit starting mechanism, the rotor must be very light so that it is capable of reaching running speed within one cycle of the mains frequency. Alternatively, the rotor may be provided with a squirrel cage, so that the motor starts like an induction motor, once the rotor is pulled into synchronism with its magnet, the squirrel cage has no current induced in it and so plays no further part in the operation.

Electric Motors