Question

Explain double field revolving theory with the help of neat diagrams

Answer 1

The basic idea behind the double field revolving theory is that any alternating quantity can be resolved into 2 equal components rotating in opposite direction.

thus the resultant will give an alternating magnetic field. So, in case in a 1ph IM, these rotating fluxes get cut by the rotor conductors to induce emf in it, which in turn circulates current and thus producing rotor flux. This when interacts with the rotating stator fluxes to generate torque. The interaction with two rotating fluxes produces torque in opposite direction, leading net torque to zero. Thus making 1ph IM non-self starting.

Auxiliary coils are used to phase shift one of the fluxes, to create a rotating magnetic field instead of an alternating field and thus making the IM self starting.

reference: Rotating Magnetic Field & Synchronous Speed | Single Phase Motors

Whereas in case of 3ph IM, the conductors are placed at 120 degree apart and 3ph current with 120 degree phase difference flows through the respective conductors. This arrangement produces a magnetic field of uniform magnitude but with varying orientation, a rotating magnetic field(rmf). This is the cause for the motor action.