What type of field does a single-phase supply produce in the stator?

A pulsating field on a fixed axis

What is the starting torque and why?

Zero, as Tf and Tb are equal and opposite at s = 1

Zero, because the rotor current is zero

What is required to start the motor?

A rotating field at start, from an auxiliary winding with phase shift, a capacitor, or shaded poles

2071 Bhadra · 4 marks

Reason: single phase motor not self-starting

Short answer

Single-phase supply produces a pulsating field, which by double field revolving theory resolves into two equal counter-rotating fluxes. At standstill, forward and backward torques cancel, giving zero net torque, so the motor cannot start.

Give reason: Single phase induction motors are not self starting.

A single-phase induction motor is not self-starting because a single-phase supply gives only a pulsating magnetic field and no rotating field.

The stator winding carries an alternating current, so the flux $\phi = \phi_m\cos\omega t$ only alternates in magnitude along a fixed axis; its axis does not rotate.
This flux can be treated as two equal rotating fluxes of $\phi_m/2$ , rotating at $N_s$ in opposite directions (double field revolving theory).
At standstill the slip is 1 for both fields. The forward and backward fields induce equal rotor currents and produce equal and opposite torques:

T_{start} = T_f - T_b = 0

With zero net torque the rotor cannot start. If it is given a push in either direction, the torque in that direction becomes greater than the opposing torque, and the motor accelerates and keeps running.

So a starting arrangement (auxiliary winding with phase shift, capacitor, or shaded poles) is needed to produce a rotating field at start.

✓ Answer checked by Claude Opus 5.5 (high effort)

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