Double Revolving Field Theory - 1Φ Induction Motor Theory

We have already discussed in my previous article Basics of Induction Machines principle of working of Induction motor and its classifications. There are two main theories that can be used for the production of rotating magnetic field in Single Phase Induction machines. These are :

• Double Revolving Field Theory
• Cross Field Revolving Theory

In my current article I'll discuss Double Revolving Field Theory. Double Revolving Field Theory : -
This theory is based on the factor that any pulsating quantity whether magnetic or electric quantity can be resolved into two anti parallel components. Say we have a magnetic flux having magnitude of Φ_m then this flux can be broken into two components say each having magnitude of  Φ_m/2. Now the flux have two opposite direction and equal magnitude so the resultant flux i.e. stator flux will be of alternating nature and will be perpendicular to these flux components.
Initially when the single phase induction motor is started, these two fluxes are equal in magnitude but opposite in direction and hence these will produce torque that will also be equal in magnitude and opposite in direction, so each torque tries to rotate the rotor in its own direction. Thus net torque experienced by the rotor is zero at starting of the 1 Φ Induction motor and it explains the fact that single phase induction motors are not self starting.

Torque Speed Curve of Single Phase Induction Motor