Synchronous Motor Working | Construction

Synchronous motor construction is very similar to that of a synchronous generator. As shown in the figure, essential parts of synchronous motor are same as those of the synchronous generator i.e. a stator and a rotor.
synchronous motor construction

Synchronous Motor Construction

The stator houses 3-phase armature winding in the slots of its laminated core which are distributed over the entire yoke. This winding is similar to that used in synchronous generators and induction motors. It is connected to 3-phase AC supply.
The field winding is placed in the rotor slots and it is connected to the DC supply via two slip-rings. Due to this DC supply (known as excitation), rotor poles become alternatively N and S poles. The number of rotor poles is made equal to the number of stator poles. Modern synchronous motors often use brushless excitation. The rotor can be a salient-projected type or cylindrical type. Normally the salient pole rotor is preferred.
A simple synchronous motor is basically not self- starting. However, it can be made self-starting with the help of damper windings carried by the rotor poles. This motor is called synchronous motor because its rotor runs in synchronism with the synchronously rotating stator field.

Working Principle of Synchronous Motor

The working principle of a synchronous motor is based on the principle of magnetic locking between stator and rotor poles.
To understand this, let the stator of the synchronous motor be wound for 2 poles. Let the rotor also produces two poles when they are excited by the external DC source.

  • As three-phase AC supply is connected to the stator winding, a rotating magnetic field (RMF) is produced.
  • This field rotates at the synchronous speed Ns. The two poles produced are N1 and S1 as shown in the figure.
  • synchronous motor working

  • These stator poles (N1 and S1) rotate in the air gap between stator and rotor at synchronous speed in the clockwise direction.
  • The rotor is then excited by external DC source. It produces two poles N2 and S2 as shown in the figure.
  • The rotor is accelerated, to rotate in the clockwise direction by some external engine. This is because the synchronous motor is not self-starting.

  • If unlike poles N1 – S2 and S1 – N2 come close to each other, then due to the strong force of attraction, magnetic locking takes place between them.

  • Once the stator and rotor poles are locked magnetically with each other, the rotor will continue to rotate at synchronous speed along with the rotating magnetic field.
  • Then external engine coupled to the rotor can be decoupled.
  • The rotor will rotate at Ns as long as the magnetic lock between the stator and rotor continues to exist.


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