Question

Compare the working of an Induction motor with a synchronous motor.

Answer 1

AC motors can be divided into two main categories - (i) Synchronous motor and (ii) Asynchronous motor. 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.

Constructional difference

• Synchronous motor: Stator has axial slots which consist stator winding wound for a specific number of poles. Generally a salient pole rotor is used on which rotor winding is mounted. Rotor winding is fed with a DC supply with the help of slip rings. A rotor with permanent magnets can also be used.
• Induction motor: Stator winding is similar to that of a synchronous motor. It is wound for a specific number of poles. A squirrel cage rotor or a wound rotor can be used. In squirrel cage rotor, the rotor bars are permanently short-circuited with end rings. In wound rotor, windings are also permanently short-circuited, hence no slip rings are required.

Difference in working

• Synchronous motor: Stator poles rotate at the synchronous speed (Ns) when fed with a three phase supply. The rotor is fed with a DC supply. The rotor needs to be rotated at a speed near to the synchronous speed during starting. If done so, the rotor poles get magnetically coupled with the rotating stator poles, and thus the rotor starts rotating at the synchronous speed.
  • Synchronous motor always runs at a speed equal to its synchronous speed.
    i.e. Actual speed = Synchronous speed
    or N = Ns = 120f/P

• Induction motor: When the stator is fed with two or three phase AC supply, a Rotating Magnetic Field (RMF) is produced. The relative speed between stator's rotating magnetic field and the rotor will cause an induced current in the rotor conductors. The rotor current gives rise to the rotor flux. According to Lenz's law, the direction of this induced current is such that it will tend to oppose the cause of its production, i.e. relative speed between stator's RMF and the rotor. Thus, the rotor will try to catch up with the RMF and reduce the relative speed.
  • Induction motor always runs at a speed which is less than the synchronous speed.
    i.e. N < Ns

Other differences

• Synchronous motors require an additional DC power source for energizing rotor winding. Induction motors do not require any additional power source.
• Slip rings and brushes are required in synchronous motors, but not in Induction motors (except wound type induction motor in which slip ring motors are used to add external resistance to the rotor winding).
• Synchronous motors require additional starting mechanism to initially rotate the rotor near to the synchronous speed. No starting mechanism is required in induction motors.
• The power factor of a synchronous motor can be adjusted to lagging, unity or leading by varying the excitation, whereas, an induction motor always runs at lagging power factor.
• Synchronous motors are generally more efficient than induction motors.
• Synchronous motors are costlier.
• A three phase Synchronous motor is a doubly excited machine, whereas an induction motor is a single excited machine.
• The armature winding of the Synchronous motor is energized from an AC source and its field winding from a DC source. The stator winding of Induction Motor is energized from an AC source.
• Synchronous Motor always runs at synchronous speed, and the speed of the motor is independent of load, but an induction motor always runs less than the synchronous speed. If the load increased the speed of the induction motor decreases.
• The induction motor has self-starting torque whereas the synchronous motor is not self starting. It has to be run up to synchronous speed by any means before it can be synchronized to AC supply.
• A synchronous motor can be operated with lagging and leading power by changing its excitation. An induction motor operates only at a lagging power factor. At high loads, the power factor of the induction motor becomes very poor.
• The Synchronous Motor can be used for power factor correction in addition to the supplying torque to drive mechanical loads whereas an induction motor is used for driving mechanical loads only.