Single Phase Induction Motors Questions and Answers

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. At standstill the rotor slips equally against both components of the pulsating field, so their torques cancel exactly. Once turning, the slip against the forward field is small and against the backward field nearly two, so the forward torque greatly exceeds the backward one and the machine accelerates.

Answer: B

Electrolytic construction gives a large capacitance cheaply but cannot carry current for more than a second or two, and the auxiliary winding is equally short-rated. The reason is precisely why the centrifugal switch exists.

Answer: C

A capacitor between two of the terminals gives an approximation to a rotating field, and the machine will develop perhaps half its rated output. It is a makeshift rather than a proper arrangement and the machine runs hot and unbalanced.

Answer: D

Correct answer: True, because of the double-frequency torque pulsation

A single-phase machine’s instantaneous power pulsates at twice supply frequency, and so does its torque, since there is no second and third phase to fill the gaps. That pulsation shakes the machine and its mounting at 100 hertz on a 50 hertz supply. A three-phase machine has a constant instantaneous power and is correspondingly quieter.

Answer: A

The backward field's braking torque has to be overcome as well as the load, so the machine slips further to develop the necessary forward torque. Slips of five or six per cent are usual against two or three for a three-phase machine.

Answer: B

Correct answer: a cage of the same kind as in a three-phase machine

The rotor is an ordinary squirrel cage, identical in principle to that of a three-phase machine. Everything distinguishing the single-phase machine is in the stator - the auxiliary winding, the capacitor, the switch or the shading rings - which is why the rotor is the simplest part of it.

Answer: C

Correct answer: one alternating voltage between two conductors

A single-phase supply provides one alternating voltage, between a live conductor and neutral or between two lives. That single voltage is the whole difficulty: it produces a pulsating rather than a revolving field, so every single-phase motor needs some arrangement to start itself.

Answer: D

Correct answer: wound with heavier wire and occupies most of the slots

The main winding carries the load current continuously, so it is wound with heavier wire and given the greater part of the slot space. The auxiliary winding is fine-wired and higher in resistance, which both produces the phase displacement wanted and suits a winding that is energised only briefly.

Answer: A

Everything about its poorer efficiency, its hum and its need for an auxiliary winding follows from the pulsating rather than revolving field. Recognising that makes the whole family of starting arrangements make sense.

Answer: B

Correct answer: an oil-filled or metallised film type

A capacitor in circuit whenever the motor runs must carry alternating current continuously without heating, and film or oil-filled types are built for it. An electrolytic capacitor would overheat and fail within hours - which is why the two kinds must not be substituted for one another.