Shaded-pole synchronous motor
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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. Frequently, the rotor and its associated reduction geartrain is encased in an aluminium, copper, 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!).
Further reading
- Institute of Electrical and Electronics Engineers, "Analysis and Control of subsynchronous resonance : presented at the IEEE Power Engineering Society 1976 winter meeting and Tesla symposium". Piscataway, N.J., IEEE, c1976. 76CH1066-O-PWR. LCCN 77153757 //r842