These motors are relatively costly, and are used where exact speed (assuming an exact-frequency AC source) as well as rotation with a very small amount of fast variations in speed (called 'flutter" in audio recordings) is essential. Applications included tape recorder capstan drives (the motor shaft could be the capstan). Their distinguishing feature is their rotor, which is a smooth cylinder of a magnetic alloy that stays magnetized, but can be demagnetized fairly easily as well as re-magnetized with poles in a new location. Hysteresis refers to how the magnetic flux in the metal lags behind the external magnetizing force; for instance, to demagnetize such a material, one could apply a magnetizing field of opposite polarity to that which originally magnetized the material.
These motors have a stator like those of capacitor-run squirrel-cage induction motors. On startup, when slip decreases sufficiently, the rotor becomes magnetized by the stator's field, and the poles stay in place. The motor then runs at synchronous speed as if the rotor were a permanent magnet. When stopped and re-started, the poles are likely to form at different locations.
For a given design, torque at synchronous speed is only relatively modest, and the motor can run at below synchronous speed.
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