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Shaded-pole synchronous motors are a class of AC motors.
Like a shaded pole induction motor, they use field coils with additional copper shading coils to produce a weakly rotating magnetic field. But unlike a shaded pole induction motor , the synchronous version of this motor uses a magnetized rotor, e.g. a permanent magnet. This rotor rotates synchronously with the rotating magnetic field: if the rotor begins to lag behind the rotating field, driving torque increases and the rotor speeds up slightly until the rotor's position within the rotating field is a point where torque = drag; similarly, if the rotation of the field slows down, the rotor will advance relative to the field, torque will decline, or even become negative, slowing the speed of the rotor until it again reaches a position relative to the field where torque = drag.
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. These motors are also used in shavers.
Frequently, the rotor and its associated reduction geartrain are 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 revolutions per hour.