Difference between induction motor and synchronous motor

AC motors are divided into two types, synchronous motor and induction motor also called an asynchronous motor. The biggest difference is whether the speed of the rotor is consistent with the speed of the rotating magnetic field in the stator. If the rotation speed of the rotor and the stator field are the same called a synchronous motor and if not then is called asynchronous motor. Let us have a deep insight into the difference between induction and synchronous motor. 

The key difference between the induction motor and the synchronous motor is given below.   
  • The synchronous motor is a doubly excited machine whereas an induction motor is a single excited machine.
  • The armature winding of the synchronous motor is energized from an AC source and its field winding from a DC source in the synchronous motor while the stator winding of induction motor is energized from an AC source in an induction motor. 
  • Synchronous motors require an additional DC power source for energizing rotor winding and induction motor does not require an additional power source.
  • The synchronous motor is more efficient than the same output and voltage rating of an induction motor. 
  • A synchronous motor is costlier than the same output and voltage rating of an induction motor.
  • The synchronous motor always runs at synchronous speed, and the speed of the motor is independent of load, but an induction motor always runs less than the synchronous speed. Induction motor speed is decreased if the load increased.
  • The induction motor has self-starting torque, on the other hand, the synchronous motor is not self-starting. It has to be run to synchronous speed by any means before it can be synchronized to AC supply.
  • Slip rings and brushes are required in synchronous motors but not in induction motors.
  • In addition to the supplied torque, the synchronous motor can be used to correct the power factor to drive mechanical load while an induction motor is only used to drive a mechanical load.
  • The synchronous motor can be operated with lagging and leading power by changing its excitation and the induction motor operates only at a lagging power factor. The power factor of the induction motor becomes very poor at light loads.

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