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Difference Between Synchronous Motor And Induction Motor On the Basis Of Different Factor,

Synchronous Speed

Synchronous Motor: Operate at RPM=120f/p

Induction Motor: Operate at less than synchronous speed (RPM=120f/p – slip)

DC Excitation

Synchronous motors require DC excitation to be supplied to the rotor windings

Induction motors don’t require DC excitation to be supplied to the rotor windings

DC Power Source

Synchronous motors require a DC power source for the rotor excitation.

Induction motors don’t require a DC power source for the rotor excitation.

Rotor Excitation

Synchronous motors require slip rings and brushes to supply rotor excitation.

induction motors don’t require slip rings, but some induction motors have them for soft starting or speed control.

Rotor Winding

Synchronous motors require rotor windings

Induction motors are most often constructed with conduction bars in the rotor that are shorted together at the ends to form a “squirrel cage.”

Starting Mechanism

Synchronous motors require a starting mechanism in addition to the mode of operation that is in effect once they reach synchronous speed.

Three phase induction motors can start by simply applying power, but single phase motors require an additional starting circuit.

Power factor

The power factor of a synchronous motor can be adjusted to be lagging, unity or leading

Induction motors must always operate with a lagging power factor.


Synchronous motors are generally more efficient than induction motors.

Induction motors are generally less efficient than synchronous motors.


Synchronous motors can be constructed with permanent magnets in the rotor eliminating the slip rings, rotor windings, DC excitation system and power factor adjustability.

Induction motors contain slip.


Synchronous motors are usually built only is sizes larger than about 1000 Hp (750 kW) because of their cost and complexity. However, permanent magnet synchronous motors and electronically controlled permanent synchronous motors called brushless DC motors are available in smaller sizes.

Smaller induction motors are built and commonly in household applications.


Synchronous motor: Timing applications such as in synchronous clocks, timers in appliances, tape recorders, and precision servomechanisms

induction motor: Three-phase squirrel-cage induction motors are widely used in industrial drives because they are rugged, reliable and economical. Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans.
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