Single Phase Induction Motors Questions and Answers

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.

Answer: A

A large single-phase load unbalances the supply and the machine's own efficiency and power factor are poor, so beyond a few kilowatts a three-phase machine is used instead. Domestic and small commercial duties are where they belong.

Answer: B

Correct answer: the run capacitor supplies part of the reactive current needed

The capacitor stays in circuit while the machine runs, and the leading current it draws offsets part of the lagging magnetising current the windings take from the supply. The net current is therefore more nearly in phase with the voltage. A split-phase machine disconnects its auxiliary winding entirely and has no such compensation.

Answer: C

The auxiliary winding and the starting capacitor are both rated for a second or two of duty, and leaving them in circuit destroys them within minutes. A burnt auxiliary winding with a good main winding is the classic symptom of that fault.

Answer: D

Correct answer: hum but fail to start, and burn out if left energised

Without the capacitor there is no phase displacement, so the field merely pulsates and the two component torques cancel at standstill. The machine sits still drawing locked-rotor current, which is five or six times rated, and the windings overheat within a minute or two unless the thermal protector opens.