SYNCHRONOUS MOTOR - CONSTRUCTION OF SYNCHRONOUS MOTOR

Synchronous Machine

Construction of Synchronous Machines:

Stator:

The stator is similar in construction that of a induction motor.

It is made up of laminated sheet steel having slot on its inner periphery.

Three phase winding is placed in the slot on stator and it serves as armature winding.

Rotor two types Salient:

• for low speed Non-Salient (cylindrical rotor)
• for high speed  Slip rings:

Two slip ring at rotor shaft to provide supply to field winding

Brushed: Two brushes Shaft:

Bearings:

Principle of operation:

Generator: Field produced on rotor by dc current through slip rings

Rotor field is turned at synchronous speed (Ns) by a prime mover.

EMFs induced in stator coils with frequency given by Motor:

The stator is wound for the similar number of poles as that of rotor, and fed with three phase AC supply. 

The 3 phase AC supply produces rotating magnetic field (RMF) in stator.

The rotor winding is fed with DC supply which magnetizes the rotor.

The rotor gets locked to the RMF and rotates unlike induction motor at synchronous speed under all load condition

Why Synchronous motor is not self starting?

How to make it self starting?

Consider a two pole synchronous machine as shown in figure below

Why Synchronous motor is not self starting?

Now, the stator poles are revolving with synchronous speed (lets say clockwise). If the rotor position is such that, N pole of the rotor is near the N pole of the stator (as shown in first schematic of above figure), then the poles of the stator and rotor will repel each other, and the torque produced will be anticlockwise.

The stator poles are rotating with synchronous speed, and they rotate around very fast and interchange their position. But at this very soon, rotor can not rotate with the same angle (due to inertia), and the next position will be likely the second schematic in above figure. In this case, poles of the stator will attract the poles of rotor, and the torque produced will be clockwise.

Hence, the rotor will undergo to a rapidly reversing torque, and the motor will not start.

How to make it self starting?

But, if the rotor is rotated upto the synchronous speed of the stator by means of an external force (in the direction of revolving field of the stator), and the rotor field is excited near the synchronous speed, the poles of stator will keep attracting the opposite poles of the rotor (as the rotor is also, now, rotating with it and the position of the poles will be similar throughout the cycle). Now, the rotor will undergo unidirectional torque. The opposite poles of the stator and rotor will get locked with each other, and the rotor will rotate at the synchronous speed.

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