Synchronous Motor


Synchronous  Motor

  • “When Synchronous motor rotating at constant speed equal to the synchronous speed, It is convert electrical energy to mechanical energy.”
  • These motor is also known as constant speed motor. its operates in synchronism with line frequency.
  • Synchronous motor is not self starting motor,hence external DC source is necessary.
  • Speed control of this motor is not possible, because it is constant speed motor.

Method to starting the synchronous motor:

Basically there are three methods that are used to start a synchronous motor:

  1. By reduce the speed of the rotating magnetic field of the stator.
  2. Auxiliary Motor Starting: 
  3. By use damper windings or amortisseurs windings.



synchronous motor construction

       Construction of three phase synchronous motor is same like three phase alternator. which is consist of main two parts stator and rotor. Stator is stationary part of motor and rotor is rotating part of motor. Stator consist of stator core and three phase star connected AC distributed winding and three phase 50Hz AC supply is given to the Stator winding. Rotor has DC field winding. which is exited by the direct current obtain from DC source. its known as Exciter. rotor of motor can be salient pole or cylindrical pole type of construction. if synchronous motor is salient pole type extra winding is require. which is less in the pole phase of the rotor that winding known as Damper Winding. The ends of copper bar are sort circuited by copper ring of damper winding. The function of damper winding is to provide starting torque and increase stability.

     Synchronous motor is doubly excited motor because two input are require. Stator winding is supplied by three phase AC supply and rotor is energize by DC source.


  •  The speed of synchronous motor is given by, synchronous speed(Ns),

Ns = 120f / P

synchronous motor working

In synchronism which rotating magnetic field produce by three phase supply which is given to the three phase stator winding. hence synchronous speed is produce. three phase AC voltage is given to stator winding and DC supply is given to rotor winding. these motor is not self starting. hence external DC source is require to rotate the rotor. Now consider synchronous motor having two rotor pole (Nr & Sr) and stator also having two poles (Ns & Ss).  Stator is wound for the same number of pole as the rotor pole.

Stator has Two pole rotating At synchonous speed in clockwise direction. Suppose it is at the position X respectively at particular instant with the rotor at these condition the pole Ns and Nr as well as Ss and Sr will repel each other. Therefore repulsive force act on the rotor and rotor start to rotate Anticlockwise. But after half cycle the position of stator pole Ns and Ss will inter change and Ns at Y and Ss at X. But polarity of pole remain same.  under these condition Ns attracts Sr and Ss attracts Nr. hence force is act on rotor and rotor will rotate in clockwise. Alternating torque is acting on  the rotor . and due to high inertia of rotor motor fail to start hence these motor is not self starting. Hence to obtain continuous torque the rotor pole are rotated by some external DC source. or By providing Damper Winding mounting on the rotor.

if any query about synchronous motor, comment below…………………. ?


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