Note on Operating Principle and Starting Methods of Three Phase Synchronous Motor

  • Note
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Construction of synchronous motor is same as that of the synchronous generator.They convert electrical energy to mechanical energy.Synchronous motors are AC motor which always rotates at a constant speed equal to synchronous speed. It can run either at synchronous speed or not at all i.e. while running it maintains a constant speed equal to the synchronous speed. It is not self-starting.Some auxiliary means has to be used to start the motor.The motor can be operated at a wide range of power factors both lagging and leading.

Q. Why the synchronous motor is not self-starting?

Ans: When the supply is switched on to the stator of the synchronous motor a rotating field at synchronous speed is created.Since the rotor field is stationary, the oscillating torque with zero average is developed.As a result, rotor remains stationary.

fig:Action of synchronous motor
fig:Action of the synchronous motor

Operating Principle:

When the stator winding are supplied by 3 – ø voltage, rotating magnetic field will produce. At starting, the rotor field windings are unexcited and the rotor is driven at synchronous speed by some auxiliary means. Then, the rotor field windings are supplied with DC current and the auxiliary means axis disconnected, the rotor pole and stator pole will get engaged with a strong force and the rotor continuously rotates with synchronous speed.

fig:Synchronous motor
fig:Synchronous motor

Starting methods:

Since Synchronous motor is not self-starting, Various methods can be used to start the motor.

Use of DC motor coupled to the shaft of synchronous motor:

The unexcited rotors are rotated by means of a DC motor coupled to the shaft of the synchronous motor. The speed of the DC motor is adjusted by its field regulator. As the speed reach near to synchronous speed, the field winding of the synchronous motor is excited by the DC current and the DC motor is switched off. Then the motor continuously rotates with synchronous speed.

Using the field exciter of synchronous motor as DC motor:

It is similar to the previous method except that the rotor of the synchronous motor is initially operated as DC motor and when the speed reaches close to the synchronous speed, three phase supply is given to armature and DC voltage from exciter.

Use of small induction motor of at least one pair of poles less than synchronous motor:

Using an auxiliary induction motor with at least one pair of pole-less involves the same synchronizing process as that of the first method.

Let us take an example for synchronous motor P=8,

Then rated speed(Ns)= (120f/p)


For induction motor,




But we have,

N= Ns (1 – s)

Say s=0.2


So,the speed of induction motor may vary from 0 to 800 rpm.We need to keep the speed of induction motor as that of Ns of the synchronous motor which is 750rpm.So, the motor starts and run at synchronous speed.

Using damper winding as a squirrel cage induction motor:

To make a synchronous motor self-starting, an additional winding is known as the damper winding (the induction winding) is provided in the pole faces of the motor. The damper winding also called a squirrel- cage winding, is a short-circuit winding. For small machines, a squirrel-cage winding requires the placement of the rotor laminations in a mold and then forcing of the molten conducting material into the slots. The mold has cavities on either side of the rotor, which is filled with the molten conducting material at the same time. The conducting material from one end of the slot to the other forms a conducting bar. The conducting bars are shorted by the end rings. The entire one-piece construction looks like a squirrel cage. For large machines, the squirrel-cage winding may be formed by driving metal bars into the slots one at a time and then shorting them with annular conducting strips on both ends.

fig:Staring as squirrel cage induction motor
fig:Staring as squirrel cage induction motor

fig: Showing rotor bars and end connections
fig: Showing rotor bars and end connections

In any case, the damper winding forms a closed loop. The uniform revolving field induces an electromotive force (emf) in the damper winding which, in turn, results in an induced current in it. The damper winding not only enables a synchronous motor to develop the high starting torque but also serves another useful purpose. It tends to minimize motor hunting.

Using Slip ring induction motor:

To providing high starting torque ,this technique is used.Synchronous motor with phase wound damper winding will have a rotor with five slip rings.The motor is started with full external resistance per phase and dc field circuit open.As the motor approaches synchronous speed,starting resistance will reduce & when field voltage is applied the motor pulls to synchronism.

fig:starting with slip ring induction motor
fig:starting with slip ring induction motor

  • The synchronous motor is not self-starting.

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