Single Phase Induction Motors Questions and Answers

Answer: C

Correct answer: less phase displacement is needed once the machine is running

Starting needs the largest possible displacement and therefore a large capacitance, while running needs only enough to improve the power factor and smooth the torque. The run capacitor is correspondingly smaller - typically a tenth or less of the start value - and this is why a capacitor start capacitor run machine has both.

Answer: D

Correct answer: the capacitor the machine requires in its auxiliary circuit

The nameplate states the capacitance and voltage rating of the capacitor to be fitted, because the machine was designed around that value and will not perform correctly with another. It is a specification for a replacement part rather than a measurement of any property of the machine itself.

Answer: A

Correct answer: overheat the auxiliary winding

The auxiliary winding is wound with fine wire and sized for the current the correct capacitor passes. A larger capacitance passes more current through it continuously, and the winding overheats and eventually fails. Fitting the nameplate value is a requirement rather than a starting point.

Answer: B

Correct answer: the temperature of the winding, opening the circuit

A thermal protector is buried in or against the winding and opens when the winding gets too hot, from whatever cause - a stalled rotor, an overload, blocked ventilation or a failed capacitor. Sensing temperature rather than current means it protects against all of them, and it usually resets itself once the machine has cooled.

Answer: C

Locked rotor current for a few seconds heats the winding quickly, so repeated tripping points to a starting failure or a mechanical obstruction. Checking the capacitor and turning the shaft by hand are the first two tests.

Answer: D

A magnet rotor with electronic commutation avoids the backward field, the auxiliary winding and the centrifugal switch, and it can be run at whatever speed the appliance needs. Mass production has made the controller cheap.

Answer: A

Correct answer: that it runs directly from a domestic supply with no electronics

Its whole justification is availability: it works from the supply that exists in every house, with nothing between it and the socket. On every other count it is inferior to a three-phase machine - less efficient, larger for its output, with pulsating torque and a poorer power factor.

Answer: B

Correct answer: The load commutated inverter machine

A load commutated inverter drive is a converter arrangement for large synchronous machines and has nothing to do with single-phase induction motors. The other three are all standard types, distinguished by how they generate the phase displacement needed to start.

Answer: C

Correct answer: The shaded pole machine

The shaded pole machine has only its main winding; the phase displacement comes from a short-circuited copper ring around part of each pole, in which induced current delays the flux locally. This makes it the simplest and cheapest of all, at the cost of very low starting torque, poor efficiency and no easy means of reversal.

Answer: D

Correct answer: only a single-phase supply is available

That is the whole of the case. Where three phases are available the three-phase machine is better in every respect - more efficient, smaller for its rating, self-starting, smooth in torque and better in power factor. The single-phase machine exists because domestic and many small commercial premises have only one phase.