Single Phase Induction Motors Questions and Answers

Answer: C

The large electrolytic unit gives the starting torque and is switched out, while a small oil-filled unit stays in and improves the running torque, power factor and efficiency. It is the best-performing and dearest of the family.

Answer: D

With nothing to switch, it is simple and quiet and runs with a nearly circular field, but its starting torque is low because the capacitor is sized for running. Fans and blowers, which need little to start with, are its market.

Answer: A

The ring delays the flux in the part of the pole it encircles, which gives a weak sweeping motion of the field across the pole face. Efficiency and starting torque are both poor, but the machine is extremely cheap and is used in fractional-horsepower duties.

Answer: B

Current circulates in the rings whenever the machine is energised, and the field it produces is useful only for starting. Efficiencies of twenty per cent or less are normal, which is acceptable only in very small machines.

Answer: C

Starting torque follows how nearly circular the starting field can be made, and the capacitor start arrangement comes closest to quadrature. Cost and complexity run in the same order, which is why all four types survive in the market.

Answer: D

Correct answer: interchanging the connections to the auxiliary winding

The direction is decided by whether the auxiliary current leads or lags the main current, so reversing the auxiliary connections reverses the phase relation and with it the rotation. Swapping the supply leads reverses both windings together and changes nothing, which is why a single-phase machine does not reverse the way a three-phase one does.

Answer: A

Each half-amplitude field gets its own rotor branch, one at slip \( s \) and one at slip \( 2 - s \), and the two are placed in series with the stator impedance. The difference between the two torques is the useful output.

Answer: B

Correct answer: half the value it would have for the whole machine

The double revolving field model splits the pulsating field into two components each of half the amplitude, and represents each by its own half of the equivalent circuit. Since the magnetising branch carries half the flux, its reactance is half the whole-machine value, and the two halves are placed in series.

Answer: C

The backward component drives current in the rotor without contributing useful torque, and the elliptical field gives a poorer utilisation of the iron and copper. Single-phase machines are therefore confined to small ratings.

Answer: D

The interaction produces a torque component at double frequency that the shaft and mounting have to absorb, which is why such machines hum audibly. Three-phase machines have no such component, their field being of constant magnitude.