Single Phase Induction Motors Questions and Answers

Answer: A

Losing the run capacitor leaves the machine with a badly elliptical field and much reduced running torque, which is enough for no load but not for the duty. Measuring the capacitor is the first check on such a symptom.

Answer: B

Correct answer: a series machine that runs on either alternating or direct current

In a series machine the field and armature currents reverse together, so the torque keeps its direction whichever way the current flows - which is why the same machine works on alternating and direct supplies. Its very high speed and high torque per unit mass make it the machine of choice for hand tools and vacuum cleaners.

Answer: C

A solid field circuit would suffer heavy eddy current loss under alternating flux, so the whole magnetic circuit is built of laminations unlike an ordinary dc series machine. That is the principal constructional difference between the two.

Answer: D

Speeds of ten or twenty thousand revolutions a minute from a machine that fits in the hand are what drills and vacuum cleaners need. Brush wear and acoustic noise are the price, and brushless machines have displaced it in newer products.

Answer: A

Correct answer: phase-angle control of the supply voltage with a triac

A triac fired part way through each half cycle passes only the later portion of the waveform, so the mean voltage and hence the speed can be varied continuously with a single inexpensive device. It is the standard drill and food mixer control, and it works because a series machine’s speed depends strongly on applied voltage.

Answer: B

The rotor carries a commutator whose brushes are short-circuited at an angle to the stator axis, and the induced currents produce a torque of repulsion. It gives an excellent starting torque and was widely used before capacitors became cheap.

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.