Single Phase Induction Motors Questions and Answers

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.

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.