The working of an AC machine is Primarily "one field following other field". In the case of Multi Phase induction motor, there will be a virtual Rotating Magnetic field. But considering the case of a Single Phase induction Motor! It's only a Pulsating field that is Produced and not a Rotating one. This can also be explain on the basis of "Double Revolving field theory" which goes as follows. Current through a conductor Produces a flux. The flux can be Resolved in to 2 components, each one Rotating in the opposite direction at the same (slip) speed. As a Result, the Nelt flux is zero. the induced current in the Rotor conductor is zero. Resulting in zero torque. So, a single Phase induction Motor is not self starting.