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Difference Between Synchronous and Asynchronous Motor

Comparison between Synchronous and Asynchronous (Induction) Motor :

Synchronous Motor Asynchronous Motor
A synchronous motor is a electrical machine it convert electrical power to mechanical power & when an alternator may operate as a motor by connecting its armature winding to a 3-phase supply then it is called a synchronous motor.Asynchronous motor is a machine whose rotor rotates at the speed less than the synchronous speed
N < NS
Brushless motor, Variable Reluctance Motor, Switched Reluctance Motor and Hysteresis motor are the synchronous motor.AC Induction Motor is known as the Asynchronous Motor.
The synchronous motor can be operated over a wide range of power factors both lagging and leading but induction motor always run with a lagging power factor which may become very low at light loadAsynchronous motor is inherently not self starting
Does not have slip. The value of slip is zero.This types of motor’s value of slip is not equal to zero.
It requires DC excitation power source to initially rotate the rotor near to the synchronous speed.It does not require any additional starting source.
Slip ring and brushes are requiredSlip ring and brushes are not required.
Synchronous motor are usually more costly Less costly
Efficiency is greater than Asynchronous motor.It’s have Less efficient
By changing excitation the power factor can be adjusted accordingly as lagging, leading or unity.Asynchronous motor runs only at a lagging power factor.
Current is given to the rotor of the synchronous motorThe rotor of Asynchronous motor does not require any current.
The Speed of the motor does not depend on the variation in the load. It is constant.The Speed of the Asynchronous motor decreases with the increasing load.
Synchronous motor is not self startingIt is self starting
operate smoothly at low speed that is below 300 RPMAbove 600 rpm speed motor operation is excellent.

for a given frequency the synchronous motor runs at the constant average speed whatever the load while the speed of an induction motor false somewhat with increase in load.

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