Synchronous Motor

SYNCHRONOUS MOTOR

Synchronous motor runs only at synchronous speed, Ns = 120f / P on no-load and loaded conditions. It maintains speed while running. The only way to change the speed is by varying the supply fréquency. It is not a self starting motor. This motor has to run upto its synchronous speed by some method, and then it can be synchronised.

1. Construction

The construction of synchronous motor is same as that of 3-phase ac generator. A synchronous machine may be run as synchronous motor and as synchronous generator.

2. Working Principle of Operation

When a 3-phase balanced winding is supplied with a balanced 3-phase supply, a rotating magnetic field of constant magnitude and synchronous speed is produced. Let the motor stator have 2-poles marked as Ns and Ss as shown in fig. 2.44.

The stator poles are rotating at synchronous speed. Let us assume that it moves in clockwise direction. The rotor is positioned as shown in fig. 2.45 initially with reference to the position AB marked. As like poles repel, the rotor will tend to move in anticlockwise direction. At the end of first half cyclce of supply, the rotor poles occupy the position as shown in fig. 2.45 with respect to the reference plane AB.

Now, because of attractive force between the unlike poles, the rotor will tend to move in the opposite, i.e., clockwise direction. So, with rapid movement of the stator poles, the rotor is subjected to a torque which is also rapidly reversing. But because of the inertia of the rotor, it will not respond to such a rapid reversing torque. So, the rotor remains stationary, which shows that it is not a starting motor.

Now consider the position of the rotor as shown in fig. 2.45. The stator and rotor poles attract each other. Let the rotor also be rotating in clockwise direction at synchronous speed as that of stator field. So, the stator and rotor poles attract each other, at any instant, which result in unidirectional torque production, in clockwise direction in this case.

3. Synchronous Motor Starting Method

The rotor with unexcited magnetic poles may be made to run at synchronous speed by a dc motor which is mechanically coupled to the rotor of synchronous motor. The 3-phase ac supply is given to the stator winding of 3-phase generator. The dc supply to magnetic poles is switched on. The rotor magnetic poles and the stator magnetic poles are magnetically locked and will run at synchronous speed. Now dc motor can be switched off. The synchronous motor will drive the dc motor as generator.

The methods for starting synchronous motor are :

(i) A dc motor coupled to the synchronous motor shaft.

(ii) Using the damper windings and a squirrel cage induction motor.

(iii) Using the field excited generator as a dc motor.

4. Synchronous Motor-application

(i) It is used in power house and in major substations to improve the power factor. 

(ii) Used in textile mills, rubber mills, mining and other big industries for power factor improvement purpose and to maintain the facility of system voltage.

(iii) It is used to drive continuously operating and constant speed equipments like fans, blowers, centrifugal pumps, air comparators, etc.