What is Synchronous Speed and Asynchronous Speed in Induction Motor?


Synchronous Speed and Asynchronous Speed in Induction Motor


What is Synchronous Speed?


The virtual rotational speed at which the rotating magnetic field is produced in the stator winding of the three-phase induction motor is called Synchronous Speed. The stator winding of the three-phase induction motor has three coils and they are placed and arranged in such a way that, when a three-phase power supply is applied to that coil a rotating magnetic field is produced which is further responsible for the rotation of the rotor of the induction motor. The synchronous speed of the induction motor is denoted by Ns.

The formula of the synchronous speed is,

synchronous speed formula


Here, the Ns - Synchronous speed
                f - frequency of the applied power supply
                p - no poles in the stator coil

So, you can understand the synchronous speed of the induction motor is depends upon the frequency of the power supply applied to the coil and the no of poles. If the frequency of the power supply is increased then the synchronous speed also be increased and if the frequency is decreased the synchronous speed also be decreased, so the synchronous speed is directly proportional to the frequency.

On the other hand, synchronous speed is inversely proportional to the no of poles in the stator coil. So, if the no of poles is increased the synchronous speed of the motor will be decreased and if the no of poles is decreased then the synchronous speed of the motor will be increased.

The motor rotation is completely depending upon the synchronous speed and the synchronous speed depends upon the poles and frequency. Practically, the increasing or decreasing of no of poles is not possible when the motor is running. The synchronous speed also does not depend upon the voltage, current, and any other electrical quantity. So, there is only one way to control the speed of the induction motor is frequency control. Variable frequency can vary the speed of the induction motor. VFD or Variable frequency drives are mostly used in modern days for the speed control of induction motors. They control the speed of the motor by varying the supply frequency.


What is Asynchronous Speed?


The speed at which the rotor of the induction motor is rotating is called Asynchronous Speed. The rotor of the induction motor never rotates at the same speed as the stator rotating magnetic field. The speed of the rotor is always less than the synchronous speed. The speed of the rotor depends upon the slip. The speed of the rotor is denoted by Nr.

The formula of the slip(S) is,

Slip in induction motor


The asynchronous speed of the induction motor depends upon so many factors such as load variation in the motor, air gap between stator and rotor, the internal resistance of the rotor coil. By varying the resistance of the rotor coil, the speed of the induction motor can be controlled very easily and very smoothly. The rotor resistance starter or rotor resistance controller helps to control the speed of the induction motor by varying or changing the resistance. Generally, they connect and disconnect external resistors to the rotor coil to increase or decrease the total resistance of the coil.


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What is Synchronous Speed and Asynchronous Speed in Induction Motor? What is Synchronous Speed and Asynchronous Speed in Induction Motor? Reviewed by Author on 11/14/2021 Rating: 5
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