Synchronous AC motor, principle, construction

The synchronous motor is an AC motor that works at a fixed speed in a particular frequency system. This motor requires direct current (DC) for power generation and has a low initial torque.

Therefore synchronous motors are suitable for initial use with low loads, such as air compressors, frequency changes and motor generators.

Synchronous motors are capable of improving system power factors, so they are often used in systems that use a lot of electricity.

Component of synchronous motor:

Image result for synchronous motor


The main difference between synchronous motors and induction motors is that synchronous engine rotors run at the same speed as the magnetic field rotation.

This is possible because the rotor magnetic field is no longer induced. Rotor has a permanent magnet or DC-excited current, which is forced to lock in a certain position when faced with other magnetic fields.


The stator produces a rotating magnetic field that is proportional to the frequency supplied.

Working principle of synchronous AC motors:

Synchronous motors are similar to induction motors where both have a stator winding which produces a rotating field. Unlike induction motors, synchronous motors are excited by a dc voltage source outside the engine and therefore require a slip ring and brush (brush) to provide current to the rotor.

In synchronous motors, the rotor is locked with a rotating field and rotates at synchronous speed. If the synchronous motor is loaded to the point where the rotor is pulled out of its simultaneity with the rotating field, no torque is produced, and the motor will stop.

Synchronous motors are not self-starting motors because the torque will only appear when the motor is working at synchronous speed; hence the motor requires equipment to bring it to synchronous speeds.

Synchronous AC Motor Construction:

As discussed above, that an important component of synchronous motors is the stator and rotor, which are the general or basic components of a motor.

The synchronous motor is an ac motor that has a constant speed, but the speed can be adjusted because the speed is directly proportional to the frequency. Synchronous motors, in particular, are very good for low speed.

Advantages and Disadvantages of synchronous motor:

  • The advantages of this synchronous motor, among others, can be operated on lagging and leading power factors, there is no slip that can result in power losses so that this motor has high efficiency.
  • While the disadvantage of synchronous motors is that they do not have a torque, so starting is needed in certain ways. If the starting method can be developed later, then this motorbike will be superior to other electric motors.


Instrumentation Engineer

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