The basic difference between induction motor and synchronous motor is that induction motor is an asynchronous machine whereas the other one, as the name suggests is a synchronous machine.
- Synchronous motors operate at synchronous speed (RPM=120f/p) while induction motors operate at less than synchronous speed (RPM=120f/p – slip). Slip is nearly zero at zero load torque and increases as load torque increases.
- Synchronous motors require a DC power source for the 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.
- Synchronous motors require rotor windings while induction motors are most often constructed with conduction bars in the rotor that are shorted together at the ends to form a “squirrel cage.”
- Synchronous motors require DC excitation to be supplied to the rotor windings; induction motors don’t.
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.
- The power factor of a synchronous motor can be adjusted to be lagging, unity or leading while induction motors must always operate with a lagging power factor.
- Synchronous motors are generally more efficient that induction 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.
- 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.