A motor in general is an electrical machine that converts electrical energy into mechanical energy. Electric motors can either be DC Motors or AC Motors, depending on the type of power supply that is supplied as its input.
AC motors are further classified into two types−
A synchronous motor always runs at synchronous speed, while an induction motor runs at a speed less than the synchronous speed.
Read through this article to find out more about synchronous motors and induction motors and how they are different from each other.
A synchronous motor is a type of AC motor in which the rotor rotates at a synchronous speed at all loads, provided that the load on the motor does not exceed the limiting value. If the load on the shaft of a synchronous motor exceeds the limiting value, then the motor comes to rest and the average torque developed by it becomes zero.
A synchronous motor has a stator and a rotor. The stator consists of a 3-phase armature winding and receives power from a balanced 3- phase supply, while the rotor has a set of salient poles excited by direct current to produce alternate N and S poles. In case of a synchronous motor, the stator is wound for the same number of poles as the rotor poles.
An induction motor is a type of AC motor in which the rotor rotates at a speed slightly less than the synchronous speed. Let's see how an induction motor works.
An induction motor consists of a stator and a rotor. The stator carries a 3-phase winding, while the rotor carries a short-circuited winding.
When the stator winding is energized from a 3-phase supply, a rotating magnetic field is produced which rotates around the stator.
This rotating magnetic field passes through the air-gap and cuts the rotor conductors, which are stationary. Therefore, due to relative speed between the rotating magnetic field and the stationary rotor conductors, an EMF is induced in the rotor conductors.
As the rotor circuit is short-circuited, currents start flowing in the rotor conductors.
The current carrying rotor conductor are placed in the magnetic field produced by the stator winding. As a result, a torque develops on the rotor conductors and the sum of the torque on all the rotor conductors, rotates the rotor in the same direction as the rotating magnetic field but at a speed slightly less than the synchronous speed.
An induction motor cannot run at synchronous speed. If it runs at synchronous speed, there would be no cutting of the flux by the rotor conductors and there would be no induced EMF, no current and no torque. Therefore, the rotor of the induction motor rotates at a speed slightly less than the synchronous speed. For this reason, an induction motor is also known as an asynchronous motor.
The following table highlights the major points that differentiate a Synchronous Motor from an Induction Motor −
KeySynchronous MotorInduction MotorType of MachineA synchronous motor is a doubly excitation machine, i.e., its armature winding is connected to an AC source and its field winding is excited from a DC source.An induction motor is a singly excited machine, that is, its stator winding is energized from an AC source.SpeedIts speed is independent of the load.Its speed decreases with the increase in load.StartingIt is not self-starting. It requires external means for starting.Induction motor has self-starting torque.EfficiencyA synchronous motor is more efficient than induction motor of the same rating.The efficiency of an induction motor is lesser than that of a synchronous motor of same rating.Power FactorA synchronous motor can operate under a wide range of power factors, both lagging and leading.The most important point that you should note here is that the rotors of a Synchronous motor rotates at a synchronous speed, while the rotors in an Asynchronous motor rotates at a speed slightly less than the synchronous speed.
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AC motors can be divided into two main categories - (i)and (ii). An asynchronous motor is popularly called as Induction motor. Both the types are quite different from each other. Major differences between a synchronous motor and an induction motor are discussed below.