Correct Answer - Option 3 : The machine produces a rotating MMF at synchronous speed.
Single-Phase Induction Motor:
A single-phase induction motor consists of a single-phase winding on the stator and a cage winding on the rotor. When a 1 phase supply is connected to the stator winding, a pulsating magnetic field is produced. In the pulsating field, the rotor does not rotate due to inertia.
A single-phase induction motor is inherently not a self-starting motor.
Working:
Consider a single-phase induction motor whose rotor is at rest. Let a single-phase a.c source be connected to the stator winding. Let the stator be wound for two poles.
When the power supply for the stator is switched on, an alternating current flows through the stator winding. This sets up an alternating flux. This flux crosses the air gap and links with the rotor conductors.
By electromagnetic induction, e.m.f’s are induced in the rotor conductors. Since the rotor forms a closed circuit, currents are induced in the rotor bars.
Due to the interaction between the rotor-induced currents and the stator flux, a torque is produced.
It is readily seen that if all rotor conductors in the upper half come under a stator N pole, all rotor conductors in the lower half come under a stator S pole. Hence the upper half of the rotor is subjected to a torque which tends to rotate it in one direction and the lower half of the rotor is acted upon by an equal torque which tends to rotate it in the opposite direction. The two equal and opposite torques cancel out, with the result that the net driving torque is zero. Hence the rotor remains stationary. Thus the single-phase motor fails to develop starting torque.
