 shading-coils within the magnetic circuit of the field coil 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. Frequently, the rotor and its associated reduction geartrain is encased in a metal or plastic enclosure; the enclosed rotor is driven magnetically through the enclosure. Such geared motors are commonly available with the final output shaft or gear rotating from 600 RPM down to as low as 1/168 Revolution Per Hour (1 Revolution Per Week!).
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