Working Principle of Synchronous Motor & Applications

Synchronous motor :

     A device which converts an electrical energy into a mechanical energy running at synchronous speed ( or constant speed ) is called as Synchronous Motor. Its speed is constant irrespective of load. It is doubly excited machine because its field winding is excited from a separate dc source.

Magnetic Locking :

     Synchronous motor works on the principle of the magnetic inter locking. It means that when we place two unlike poles ( N and S ) of two magnets they will attract each other, with a force of attraction towards each other. 

     Suppose when we rotate any one of the magnet. Simultaneously the other magnet which is attracted toward it also rotates with it. Then this two magnets are said to be magnetically locked or coupled and this phenomenon is known as Magnetic Locking.

Working Principle :

     Let us see the working principle of synchronous motor. When a 3-phase supply is given to stator winding of the motor. It setups a magnetic poles of  N1 and S1. Due to the effect of this magnetic poles a magnetic field rotating at constant speed or at synchronous speed, Ns is produced by the stator winding.

Case - 1

     During the first half cycle of the supply. Let A and B are two positions of the stator magnetic poles N1 and S1. Also N2 and S2 are the positions of the rotor magnetic poles as shown in below figure.

Working Principle of Synchronous Motor & Applications

     At this position, as we know that unlike poles will attract each other, The stator pole N1 will try attract with S2. At the same time the two unlike poles and will repel each other. This force of attraction causes the rotor generating a torque on it. Hence rotor will rotate in anti-clock wise direction. 

Case - 2

     Now after the completion of the half cycle. The stator poles N1 and Swill interchange, as the next half cycle of the supply will be in the opposite direction. Now , here the condition will reversed i.e., the stator pole S1 will attract rotor pole N2 and the poles S1 and S2 will repel each other.This causes the rotor to rotate in opposite direction ( clock-wise direction ). 

Working Principle of Synchronous Motor & Applications

     This process will go on continue as the supply changes for every half cycle. Due to this rapid change in direction of rotor, the rotor tends to remain at stationary position and the motor starts to fails. Hence a synchronous motor is not an self-starting motor.

Advantages of Synchronous Motor :

1. The power factor can be controlled easily. The motor can work at lagging, unity and leading power factors.

2. An over excited synchronous motor works as synchronous condenser when connected in parallel with a load having a lagging power factor, improves the power factor of the combined load.

3. The speed is constant and independent of the load.

Disadvantages of Synchronous Motor :

1. It is not self starting and needs some arrangement for starting and synchronising the motor. It requires separate dc excitation.

2. Initial cost is very high.

3. It can not be used for variable speed jobs.

4. If the motor is overload, it may fall out of synchronism and stop.

6. It has a tendency to hunt.

Applications :

   Synchronous motors are most commonly used for

Power Factor Correction :

     Over excited synchronous motors works as a synchronous condensers are used for improvement of power factor of industrial loads having lagging power factors.

Constant Speed Applications :

     Synchronous motor providing constant speed at a high efficiency, so they can be used for constant speed applications such as centrifugal pumps, blowers, line shafts. motor generator sets, synchronous clocks. ammonia and air-compressors, textile mills, paper mills, cement mills etc.

Improving Voltage Regulation of Long Transmission Lines :

      Synchronous motor with a field regulator can be used to control the voltage at the end of long transmission lines by varying its excitation

Frequency Changer :

     Synchronous motor can be used to drive another alternator to generate supply at different frequency due to its constant speed nature. In this case it is called as Frequency Changer.

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