Working Principle of a Synchronous Motor

What is a synchronous motor?


The definition of the synchronous motor states that, An AC motor in which at steady state, rotation of the shaft is in sync with the frequency of the applied current. The synchronous motor works as AC motor but here the total number of rotation made by the shaft is equal to the interference multiple of the frequency of the applied current.

Working principle of synchronous motor


  • The synchronous motor will work on the basic principle of magnetic locking.
  • When two unlike strong unlike magnets poles are brought together in the motor. There exists a tremendous force of extraction between those two poles. In such a condition, the two magnets said to be magnetically locked. 
  • The stator and the rotor are two main parts of the synchronous motor. The stator is the stationary part of the motor while the rotor is their rotating part. The stator excited by the three-phase supply, and the rotor is excited by the DC supply. 
  • The term excitation means the magnetic field induces in the stator and rotor both of the motor. The main aim of the excitation is to convert the stator and rotor both into an electromagnet.

The three-phase supply induces both north and south pole on the stator. The three-phase supply is sinusoidal. The polarity of their wave changes after every half cycle and because of this reason the north and south pole also varies. Thus the rotating magnetic filed develop on the stator.

The magnetic field develops on the rotor because of we have to apply DC supply. The polarity of the DC supply becomes fixed and no change in polarity, and thus the stationary magnetic field develops on the rotor. The term stationary means their north and south pole remain fixed.

 Synchronous Motor
 Synchronous Motor

The speed at which the rotating magnetic field that rotates is known as the synchronous speed. The synchronous speed of the motor  N depends on the frequency of f the supply and the number of poles of P  the motor. 

Nₛ =  120f/P

f =  The supply frequency in Hz
P = The number of poles
N = synchronous speed in RPM

When the opposite pole of the stator and rotor face with each other, the force of attraction occurs between them. The attraction force develops the torque in the clockwise direction. The torque is the kind of force that moves the object in the rotation. Thus the poles of rotor dragged towards the pole of the stator.

After every half cycle, the pole on the stator is reversed direction. The position of the rotor will remain the same because of some inertia. The inertia is the tendency of an object to remain fixed in one direction. When the pole of the stator and rotary face each other, the force of repulsion occurs between them and the torque develops in the clockwise direction.


Application of synchronous motor:


  • Power factor correction 
  • Reciprocating pump
  • rolling mills
  • Voltage regulation
  • Constant speed
  • Constant load drives.

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