Synchronous motor

Synchronous motor

A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the alternating current and resulting magnetic field which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip in order to produce torque.


Sometimes a synchronous motor is used, not to drive a load, but to improve the power factor on the local grid it's connected to. It does this by providing reactive power to, or consuming reactive power from the grid. In this case the synchronous motor is called a Synchronous condenser.

Electrical power plants almost always use synchronous generators because it's very important to keep the frequency constant at which the generator is connected.

Low power applications include positioning machines, where high precision is required, and robot actuators.

Mains synchronous motors are used for electric clocks.


Synchronous motors have the following advantages over non-synchronous motors:

  • Speed is independent of the load, provided an adequate field current is applied.
  • Accurate control in speed and position using open loop controls, eg. stepper motors.
  • They will hold their position when a DC current is applied to both the stator and the rotor windings.
  • Their power factor can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation.
  • Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus).


  • Brushless permanent magnet DC motor.
  • Stepper motor.
  • Slow speed AC synchronous motor.
  • Switched reluctance motor.

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