Single Phase Induction Motors Questions and Answers

Answer: C

A centrifugal device short-circuits all the commutator segments together at speed, turning the rotor into a cage, so the machine gets a high starting torque and a good running characteristic. Its complexity is what made the capacitor machine displace it.

Answer: D

Correct answer: clocks and timing devices needing an exact speed

A synchronous machine runs at a speed fixed by the supply frequency and the pole number, with no slip at all. Since the mains frequency is held very accurately over time, such a motor keeps time to the same accuracy - which is what a clock requires and what no induction machine can offer.

Answer: A

Correct answer: capacitor start type

A compressor must start against the pressure remaining in the system, so a high starting torque is essential, and the capacitor start arrangement gives the largest starting torque of the common single-phase types. The motor is sealed inside the compressor housing, which is why its reliability matters so much.

Answer: B

A fan needs almost no starting torque and runs continuously, so a single capacitor left in circuit gives quiet running and a decent power factor with nothing to wear out. There is no switch to fail, which matters in a fitting nobody services.

Answer: C

Correct answer: capacitor start type

A pump may have to start against a head, so a good starting torque is needed. Once running the load is steady and modest, so the starting capacitor can be switched out and the machine runs on its main winding alone - exactly the duty the capacitor start type is designed for.

Answer: D

Correct answer: universal type

A vacuum cleaner needs a great deal of power from a very small and light machine, and the universal motor delivers it by running at 10,000 revolutions a minute or more. Its brushes wear and it is noisy, which is acceptable in an appliance used intermittently for short periods.

Answer: A

Correct answer: reluctance synchronous type

A clock needs an exact speed rather than a strong torque, and its load is minute. A small reluctance synchronous machine locks to the supply frequency and keeps time as accurately as the mains does. Any induction machine would slip, and the clock would lose time steadily.

Answer: B

The first two are the two defining consequences of a pulsating rather than a revolving field. Efficiency is lower rather than higher, which together with supply unbalance is why such machines are confined to small ratings.

Answer: C

Correct answer: 1-Q, 2-R, 3-P

Three ways of producing the phase displacement a single-phase machine needs to start. The split-phase machine uses a high-resistance auxiliary winding, whose current lags less than the main winding’s. The capacitor start machine puts a capacitor in the auxiliary circuit for a larger displacement and a much greater starting torque. And the shaded pole machine uses a short-circuited copper ring on part of each pole, which delays the flux in that portion.

Answer: D

Correct answer: 1-R, 2-Q, 3-P

Each type suits a different duty. The permanent split capacitor machine is quiet, efficient and easily speed-controlled but has a modest starting torque - ideal for a ceiling fan. The capacitor start machine gives the high starting torque a refrigerator compressor needs. And the universal motor gives the speed and power density a hand drill requires.