AC Machines | Explain double field revolving theory for 1 phase induction motor.

Double Field Revolving Theory:

The sinusoidally alternating single phase supply given to the winding of the single phase motor produces an alternating magnetic field in the air gap around the rotor. But a sinusoidally alternating single phase field, having oscillating nature, can be expressed as the sum of two oppositely rotating fields (ɸf forward rotating field & ɸb backward rotating field) having the same angular speed as the alternating field but having constant magnitude of half the amplitude of the alternating field (Ferrari’s principle). 

The fields ɸf  and  ɸb are the forward and backward rotating components each of constant magnitude of ɸ1m/2. The speed of rotation is ‘ω’ radians per second. Hence the resultant of the addition of these two fields is given by taking and adding the components along the vertical and horizontal axis. The horizontal component sum is zero as they are equal and opposite in direction at all times.

The resultant is along the vertical axis always for the given configuration but varies sinusoisally as seen below. 

Thus representation of an alternating magnetic field in terms of two oppositely rotating fields is the concept of Double revolving field theory.

Both the rotating fields are cut by rotor conductors, emfs are induced, rotor currents flow and according to basic motor principle torques are produced on the rotor. However, since the fields are oppositely rotating, the torques produced  on the rotor are also opposite to each other. At start, (Fig. a) these two torques are equal in magnitude but opposite in direction. Each torque tries to rotate the rotor in its own direction. Thus the net torque experienced by the rotor is zero at start, hence the single phase induction motors are not self- starting.