Construction , Working And Features Of Synchronous Motor

In this post we will study and understand about the various aspects of Synchronous Motor.

Synchronous motor is a type of an AC motor in which the speed of the motor is independent of load and it is synchronized with the frequency of the supply current.

If the frequency of the supply current is kept constant, then it will run at constant speed which is also called synchronous speed.

Synchronous speed is given by:

N = (120*f)/P

where, N = Synchronous speed.

              f = frequency of supply current.

              P = Total number of poles.

CONSTRUCTION OF SYNCHRONOUS MOTOR

Like all motors , it has two main parts called Stator and Rotor.

Stator of the motor carries the 3-phase winding while the rotor is supplied DC.

Stator core is made of thin lamination's to avoid Eddy Current losses.

There are two types of rotor construction

1. Cylindrical type rotor.

2. Salient pole rotor.

WORKING OF SYNCHRONOUS MOTOR

Synchronous motor is doubly excited motor. 3-phase supply is provided to stator while DC is provided to rotor winding.

3-phase supply in stator winding produces a rotating magnetic field while the DC supply produces a constant flux.

Since the rotating magnetic field rotates depending on the frequency. If the frequency is 60 Hz , it means that magnetic field is rotating at 3600 revolutions per minute. At any instant, the rotor poles and the stator poles can achieve same polarity like N-N or S-S , which will result in the repulsion between the poles and at the next instant the poles can achieve same polarity which will result in the attraction between poles.

But due to the inertia of the rotor and rapid attraction and repulsion between the poles, the rotor remains standstill and doesn't moves. That's why synchronous motor are not self-starting.

It is started by some external means and the motor starts to run in the same direction. This causes the magnetic locking between the both fields and the motor continues to rotate even after removing the external means. Since then, due to magnetic locking, motor continues to rotate at constant speed called synchronous speed.

MAIN FEATURES OF SYNCHRONOUS MOTOR

Main features of synchronous motor are ;

1. It runs either at synchronous speed or does not rotate at all.

2. It can work on large range of power factor both lagging and leading, therefore finds its application in power factor improvement.

3. It is not inherently self-starting. It is to be started by some external means.