Synchronous Motor

Synchronous Motor

What do you mean by Synchronous Motor?

The synchronous motor is a constant speed motor. The rotor is normally rotating at the same speed as the rotating magnetic field on the motor. The stator is similar to an induction machine consisting of a cylindrical iron frame with windings, usually located in three-stage slots around the inner periphery. These motors are commonly used in constant speed applications where a constant and precise speed is required. These are widely used in robot actuators. Synchronous motors are also used in ball mills, record players, clocks, and turntables.

What is the Construction of Synchronous Motor?

The Synchronous motor is a constant speed motor. If we use more load then the speed of the motor is not changed. Wherever constant speed is required, such motors are used there. Synchronous motors have two main parts

  • Stator
  • Rotor

The stator is a fixed part that carries the armor winding of the motor. The armature winding is the main winding due to the EMF (Electromotive Force) inducing the motor. The rotating field carries the air molecules of the field. The flow of the main field motivates the rotor. The rotor is designed in two ways, namely the main pole rotor and the non-silent pole rotor.   

The salient pole rotor is used in the synchronous motor. The word salient poles mean the rotor poles are expected towards the armature winding. The rotor is usually made of steel lamination. The steel laminations or layers reduce the eddy current loss occurring in the air of the machine. The main pole or salient pole rotor is mostly used for medium and low-speed motor design. A cylindrical rotor is used in this motor to get high speed.

What is the working principle of Synchronous Motor?

The stator and rotor are the two major parts of a synchronous motor. The stator is the stationary part of this motor, and the rotor is the rotating part of the motor. A three-phase AC supply is supplied to the motor stator. Both the stator and the rotor are excited separately. Excitation is the process of inducing a magnetic field in the parts of this motor with the help of an electric current.

When a three-phase supply is supplied to the stator then, the rotating magnetic field between the stator and the rotor gap develops. The field of the synchronous motor has moving polarity. It is known as the rotating magnetic field. The rotating magnetic field only develops in the polyphase system. Due to the rotating magnetic field, the north and south poles of the motor develop at the stator.

The rotor is excited by providing a DC supply. The DC supply rotator induces EMF in the north and south poles. As the DC supply remains uninterrupted, or constant, the flux on the rotor remains the same. Thus, the magnetic flux has a fixed polarity. The North Pole of the motor develops at one end of the rotor and the South Pole develops at the other end of the motor.

In the AC sinusoidal, the positivity of the wave changes in each half-cycle. The wave remains positive in the first half cycle and it will be negative in the second half cycle. The positive and negative semicircles of the wave develop the north and south poles of the stator, respectively.


When both the rotor and the stator are on the same pole side, they push and repeal with each other. If the stator and rotor have opposite poles, they attract each other. The supplier pulls the rotor towards the pole of the stator for the first half cycle and leaves the second half cycle. So the rotor only vibrates in one place.

What are the main features of synchronous motor?

  • The speed of this motor does not depend on the load.
  • This motor is not a self-starting motor. The DC excitation or prime mover is used for rotating the motor.
  • This motor can operate on both leading and lagging power factors.

Note that: The synchronous or constant speed motor can also be easily started with the help of the damper windings.

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