Difference between Induction Motor and Synchronous Motor

Constructional Difference :

Synchronous Motor :

     We know that every electric motor gives mechanical energy ( rotation of shaft ) as the output when an electrical potential ( electrical energy ) is applied as the input ( to the input terminals ). Similarly, a synchronous motor is also an electric motor whose mechanical energy ( rotation of shaft ) developed will remains unchanged, i.e., the speed of rotation is constant and irrespective of the load. 

     The speed at which a synchronous motor rotates is known as synchronous speed. In a synchronous motor, armature winding is excited by a three-phase supply and a separate d.c. supply is given to the field winding. Hence it is also known as a doubly excited motor.

Induction Motor ( asynchronous motor ) :

       An asynchronous motor is also an electric motor that doesn't rotate as the synchronous motor. i.e., there will be a variation in speed with respect to load on the motor. There are various types of induction motors depends upon the construction. An induction motor is a singly excited machine. It does not require any dc excitation as seen in a synchronous motor. 

     Induction motors can be of single-phase and three-phase. Here the supply is given to the field winding ( stator ), and the rotor ( armature ) rotates in magnetic flux produced by the field poles, by the principle of induction. Hence it is known as an "Induction Motor". 

Difference Between Induction Motor and Synchronous Motor :

S.no.	Synchronous Motor	Induction Motor
1.	It is not self-starting.	It is self-starting.
2.	Construction is complicated.	Construction is simple, particularly in case of cage motor.
3.	It runs at constant speed i.e., synchronous speed irrespective of the load.	It can not run at synchronous speed. The speed decreases as the load increases.
4.	Speed control is not possible.	Sped control is possible.
5.	It is doubly excited machine and requires dc excitation.	It is singly excited machine and does not requires any dc excitation.
6.	It can operate with wide range of power factors both lagging and leading.	It always operates at lagging power factor.
7.	It is used for supplying mechanical loads as well as for p.f. improvement.	It is used for supplying mechanical loads only.
8.	It torque is less sensitive to change in supply voltage.	It torque is more sensitive to change in supply voltage.
9.	Hunting starts as loads changes suddenly.	Phenomenon of hunting is absent.
10.	It is very costiler and requires the maintenance.	It is cheaper and almost maintenance free particularly is case of cage motors.

Difference by Application :

Synchronous Motor :

i. An over-excited ( back emf E_b > supply voltage V ) synchronous motor is used to improve the power factor of a system that runs at lagging power factor.

ii. Due to its constant speed characteristic, they can be used in clocks, paper mills, conveyor belts, cement mills, etc.

iii. A synchronous motor can also be used as a frequency changer at the generating stations.

Induction Motor :

     The three-phase induction motor is the most widely used motor in industry and workshops. The squirrel cage motor is generally built in a small size ( up to a few kW ). For bigger sizes, wound-rotor is preferred. 

i. Squirrel Cage Motors : 

     This type of construction has moderate starting torque and constant speed characteristic preferred for driving fans, blowers, water pumps, grinders, printing machines, drilling machines, lathe machines, etc.

ii. Slip-ring Motors : 

     Slip-ring type rotors have high starting torque. Hence, these are preferred for lifts, hoists, elevators, cranes, compressors, etc.