Electric motor

Synchronous Motor

Synchronous motor

Synchronous motors are a type of electric AC motor. Its speed of rotation is constant and depends on the frequency of the voltage of the electrical network to which it is connected and the number of pairs of poles of the motor, the speed is known as "synchronous speed".

The mathematical expression that relates the speed of the machine with the mentioned parameters is:

Synchronous motor


  • f: frequency of the network to which the machine is connected (Hz)
  • P: Number of pairs of poles the machine has
  • p: Number of poles the machine has
  • n: speed of synchronization of the machine (revolutions per minute)

For example, if you have a four-pole machine (2 pairs of poles) connected to a 50 Hz network, the machine will operate at 1,500 rpm

They work in a very similar way to an alternator. Within the family of synchronous motors we must distinguish:

  • Synchronous motors.
  • Synchronous asynchronous motors
  • Permanent magnet motors.

Synchronous motors are so named because the rotor speed and the stator speed of the magnet are the same. Synchronous motors are used in large machines that have a variable load and require constant speed.

Starting a three-phase synchronous motor

There are four types of different starters for this type of motor:

Braking of a three-phase synchronous motor

As a general rule, the desired speed of this type of motor is carried out by means of a rheostat. The synchronous motor when it reaches the critical torque will stop, this is not the most orthodox way to do it. The critical torque is reached when the load assigned to the motor exceeds the motor torque. Like I said, it's not the right way to stop the engine, we're going to abuse it, because it's reminiscent. The best way to do it is to vary the load until the absorbed intensity of the network is as little as possible, then disconnect the motor. Another way to do it, and more commonly, is to regulate the rheostat, so we vary the intensity and we can disconnect the motor without any risk.


Published: November 7, 2017
Last review: November 7, 2017