GATE 2022 EE Q.24: solution and answer

Official answer: DSource: Official answer key, GATE 2022 EE (IIT Kharagpur)

1. Running power factor of a single-phase induction motor is determined by whatever remains permanently in circuit in the auxiliary-winding path during steady-state running, not by the starting arrangement alone.

2. Split-phase (resistance-start) motor: the auxiliary (starting) winding is disconnected by a centrifugal switch once the motor reaches about 75% of synchronous speed. During running, phase-splitting is provided only by the natural resistance/reactance unbalance of the main winding, so the running power factor is relatively poor.

3. Shaded-pole motor: it has no auxiliary winding or capacitor circuit at all; a shading coil produces an auxiliary lagging flux only for starting torque. This construction is known for very low running power factor and low efficiency — the poorest of the four options.

4. Capacitor-start motor: like the split-phase motor, its (large, electrolytic) capacitor and auxiliary winding are switched OUT of circuit after starting by a centrifugal switch. During steady running it therefore behaves essentially as a single-winding motor, with no permanent power-factor correction — its running power factor is not materially better than split-phase.

5. Capacitor-start, capacitor-run motor: it uses two capacitors — a large starting capacitor that is disconnected after start, and a smaller running capacitor that stays permanently connected in series with the auxiliary winding during running.

6. This permanently connected running capacitor is sized to bring the auxiliary-winding current close to quadrature (90°) with the main-winding current at rated running load, closely approximating balanced two-phase operation.

7. Because balanced two-phase-like operation is the condition for near-unity/maximum power factor in such machines, the capacitor-start-capacitor-run motor has the best running power factor (and best running efficiency) among the four listed types.