Here two capacitors are used one for starting and one for running.Theoretically optimum starting and running performance can both be obtained.However, any pulsating field can be resolved into two components, equal in magnitude but oppositely rotating, as shown in Fig. Each field component acts independently on the rotor and in a fashion similar to that of the rotating field in a three-phase motor.

Principle of operation of single phase capacitor start induction motor-10

The flux alternates back and forth but, unlike the flux in three-phase stator, no revolving field is produced.

The flux induces an ac voltage in the stationary rotor which, in turn, creates large ac rotor currents.

The single phase induction motor resembles, three-phase, squirrel-cage motor except that, at full speed, only a single winding in the stator is excited.

In a single-phase motor we have only a single field winding excited with alternating current; therefore, it does not have a revolving field like three-phase motors. Several methods have been devised to initiate rotation of the squirrel-cage rotor and the particular method employed to start the motor will designate the specific type.

The principle of operation of a single-phase induction motor can be explained from the cross-field theory.

As soon as the rotor begins to turn, a speed emf E is induced in the rotor conductors, as they cut the stator flux at synchronous speed.

The flux rotates counterclockwise in the same direction as the rotor and it rotates at synchronous speed irrespective of the actual speed of the rotor.

As the motor approaches synchronous speed, The single-phase supply given to the single-phase winding will produce pulsating field in the air gap. This method of analysis is commonly known as the double-revolving field theory.

In effect, the rotor behaves like the short-circuited secondary of a transformer; consequently, the motor has no tendency to start rotating by itself.

However, if we spin the rotor in one direction or the other, it will continue to rotate in the direction of spin and the rotor quickly accelerates until it reaches a speed slightly below synchronous speed.

To produce rotating magnetic field at starting an auxiliary winding is placed in the motor.