Synchronous Motors

 Published On: 30, May, 2022
 Posted by: Mechtex
Different Methods to start a synchronous motor

Synchronous  Motors

A synchronous motor is an electric motor that will run at a synchronous speed, to convert the supplied electrical energy into rotational (mechanical) energy, or it will not run at all.


Construction of Synchronous Motor

A synchronous motor consists of the following parts :

1. Stator

It is a static part made of silicon-steel stampings having internal slots.

2. Stator winding

It is a 3-phase star or delta connected super enamelled copper winding inserted in the stator slots.

3. Rotor

The cylindrical part with poles on the surface. It is also made of silicon steel stampings.

4. Rotor winding

Enamelled copper winding is put on the poles and excited by the DC supply received from the exciter.

5. Exciter

It is a small capacity DC shunt generator generally mounted on the same shaft of the rotor and when moving generates DC supply to be fed to the rotor winding.

6. Slip-rings and brushes

There are two slip-rings made of phosphor bronze fitted on the shaft. Two carbon brushes are kept touching the slip-ring. Via this arrangement, the rotor is fed by DC obtained from the rotating exciter. 


When a 3-phase supply is connected to the stator winding, a rotating magnetic field is produced by the stator. It rotates at a synchronous speed. Exciter produces DC voltage which is supplied through brushes and slip-rings to the field winding of the rotor. Thus rotor poles are formed.

Similar poles try to repel each other and hence rotor poles are repelled by stator poles and the rotor tries to rotate in an anti-clockwise direction. Now, as the stator poles are also changing their position (rotating field), after the half revolution, the upper part of the stator will form an S pole and the N pole will be at the bottom. Meanwhile, the rotor’s N pole reaches up to the bottom. Since the stator N pole is formed at the bottom, there is a force of repulsion and the rotor tries to rotate in a clockwise direction. Again after the half revolution, the stator poles will change their position and the rotor will try to rotate in an anticlockwise direction. Due to the inertia of the rotor, the rotor will not rotate at all. There is no unidirectional torque and hence the synchronous motor is not self-starting.

Different Methods to start a synchronous motor

 1. Providing Damper Winding

In the pole shoe of the rotor, circular bars called dampers are placed. This makes the construction just as similar to a squirrel cage. Hence, the rotor behaves like a squirrel cage motor’s rotor and when the 3-phase supply is switched on the motor starts as a 3-phase induction motor. When it almost reaches the synchronous speed, the DC excitation is turned on. The motor is then pulled into synchronism and it works as a synchronous motor. This motor is called an induction start synchronous run motor.

2. Use of Poney Motor

It is a small-sized induction motor that can be geared to a synchronous motor. This induction motor drives the synchronous motor and when the speed has nearly reached the synchronous speed the DC excitation is made onto the rotor. The induction motor is switched off when the motor pulls into synchronism.

3. Exciter used as a DC motor

The exciter can be run as a DC motor if supplied with a DC supply which may be separately available. This will drive the synchronous motor upto the required synchronous speed.

4. DC generator, as a load of synchronous motor, can be used as DC motor

When a synchronous motor is used to drive a DC generator, in that case, the DC machine is supplied with a DC source and runs as a DC motor to drive the synchronous motor. The motor is speeded up to the synchronous speed. Then the motor is synchronised. Afterwards, the field of the DC machine is increased so that it will act as a DC generator and be driven by the running synchronous motor.

Maintenance of Synchronous Motor

1. The air gap should be checked by a filler gauge.

2. Bearings should be greased/oiled/lubricated periodically.

3. Insulation resistance of the windings should be checked at frequent intervals.

4. Compressed air at 6.5 kg/cm pressure from the blower should be used for cleaning the dust.

5. Brush tension/Brush shape/Slip-ring surfaces should be cleaned and checked.

6. Current taken by the motor should be checked and recorded frequently.

7. Terminal connections should be tightened frequently.

8. Controlling equipment should be periodically checked, worn out parts should be replaced and contacts should be cleaned.

9. The exciting circuit should be checked.

10. Foundation rigidness or vibrations should be checked.

11. The cooling system should be thoroughly checked.

12. The coupling arrangement should be checked.


1. It can be operated at any desired power factor (unity, leading or lagging) by changing excitation.

2. It can be used for power factor correction.

3. It can be constructed with air gaps that are wider than that in induction motors. This makes it more stable mechanically.

4. Electromagnetic power varies linearly with the voltage in such motors.

5. It offers higher efficiency, usually more than 90%, in the case of low speed and unity power factor applications. 


1. Since there is no unidirectional torque, it is not self-starting.

2. It requires frequent maintenance.

3. An external DC source is required to provide excitation.

4. It required additional damper windings.

5. Hunting (surging or phase swinging) occurs if there is a sudden change in load. 


1. Single-phase fractional h.p. synchronous motors are rarely used. They are used for driving stroboscopes at a constant speed or used for electric clocks to drive the wheels at a constant speed.

2. Polyphase synchronous motors are used for :

a. Driving DC generators

b. Driving air compressors  

c. Rubber mills, textile mills, pumps, pulp-grinders, where constant speed is required.

d. Ship propulsion eg. Compressors 

3. It can be used as a synchronous condenser to improve the power factor when running light.

4. It is used in industries along with induction motors for power factor improvement as well as supplying mechanical power.

5. It can be used as a voltage regulator machine for long-transmission lines at the receiving end side.

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