Synchronous motor belongs to AC class of motor which operates at a constant speed from no-load condition to full load condition. Due to its fixed speed characteristics, the synchronous motor is widely employed in industries. Besides of this induction motors are also used for the improvement of power factor in three-phase AC circuits. Given below are the top 10 Basic questions on Synchronous motors.

What do you mean by amortisseur winding

The amortisseur winding is a type of winding used in the rotor of the synchronous motor. It contains copper bars embedded in the cores of poles. The copper bars are welded to the end rings on both sides.

Explain the working principle of Synchronous Motors

A rotating magnetic field is developed by applying the rated three phase voltage to the stator winding. The field travels at synchronous speed. When a magnetic field cuts the winding of rotor, voltage and current are induced in the bars of the rotor winding. The resulting magnetic field interacts with the stator field and creates a torque that causes the rotor to turn. The rotation of rotor increases in speed till it reaches to a point that is slightly below the synchronous speed of the motor. After this, the dc current is supplied to field circuit. Application of DC current sets fixed magnetic poles in rotor field cores. Rotors magnetic poles are attracted to, unlike magnetic poles that are set by the magnetic field of the stator. Once field poles are locked, the rotor rotates at the same speed that is set up by the stator winding. The rotor speed now equals the synchronous speed.

On what factors does Synchronous speed depends

Synchronous speed depends on number of poles, operating frequency, and a constant factor 120.


Sync Speed = (120 * f) / P

Does the variation of load impacts the speed of motor

The speed of motor remains constant from no load to full load. In fact the motor always turns at same speed from no load to full load condition.

What are some methods that can be used to start a synchronous motor

Three methods can be used to start a motor:

  1. By using amortisseur windings
  2. By using an external prime mover
  3. By reducing the seed of stator magnetic field