Induction Motor Drives Questions and Answers

Answer: A

Correct answer: False, since no relative motion means no rotor current and no torque

At synchronous speed the rotor turns with the field, the conductors cut no flux, no emf is induced, no rotor current flows and no torque is produced. The machine cannot even overcome its own friction there, so it always settles at some slip. This dependence on relative motion is why the machine is called asynchronous. Only if the rotor is driven above \( N_s \) by an external prime mover does it pass through that point, and it is then generating.

Answer: B

Correct answer: False, since a self-ventilated machine cools poorly at low speed

A totally enclosed fan-cooled machine carries its fan on its own shaft, so the airflow falls off with the speed while the current, and with it the copper loss, may be undiminished. Running such a machine at 20 per cent speed on a V/f drive can overheat it at a torque it handles indefinitely at full speed. The usual remedies are a separately driven blower or derating the torque at low speed.

Answer: C

Correct answer: rugged, cheap and needs little maintenance

The cage rotor has no windings, no brushes, no slip rings and no commutator, so there is nothing to wear out and nothing to adjust, and it can be built to a low cost in very large numbers. Its efficiency is good but not the best available at every rating, and its torque at standstill is well below its maximum. It is variable-speed capable only with a drive, which is a recent advantage rather than an inherent one.

Answer: D

Short-circuited bars in slots need no brushes or slip rings, which makes the machine simple and reliable but leaves no way to reach the rotor circuit. Rotor resistance control and slip recovery both require the wound alternative.

Answer: A

Correct answer: poor, the magnetising current dominating

The magnetising current of an induction motor is set by the applied voltage and is nearly constant from no load to full load, typically 30 to 40 per cent of the rated current. At light load the in-phase, work-producing component is small while that magnetising component is undiminished, so the total current is largely reactive and the power factor may fall to 0.2 or 0.3. The machine can never draw a leading current from the supply, and matching the motor to its load is the standard remedy for a poor plant power factor.

Answer: B

A specially wound stator can be reconnected for different pole numbers, and the synchronous speed changes in inverse proportion. It is a discrete rather than a continuous method and is now largely displaced by converters.

Answer: C

Correct answer: efficient over the whole speed range

The older methods all threw away the energy they used to reduce the speed - rotor resistance dissipated the slip power as heat, and stator voltage control ran the machine at high slip to do it. The V/f drive changes the synchronous speed itself, so the machine runs at a few per cent slip whatever the speed and the efficiency is preserved. Above base speed it cannot hold rated torque, since the flux must be weakened once the voltage ceiling is reached.

Answer: D

Correct answer: inducing currents in its rotor from a rotating field

The three-phase stator winding produces a field rotating at synchronous speed. The rotor turns more slowly, so the field sweeps past it, induces currents in the shorted bars, and the interaction of those currents with the field produces torque. Everything follows from that: the rotor must lag the field, so slip can never be zero under load, and no electrical connection to the rotor is needed at all.

Answer: A

Spatial displacement of the windings combined with temporal displacement of the currents gives a field of constant magnitude revolving at synchronous speed. A single-phase winding gives a pulsating field instead, which is why such machines need starting help.

Answer: B

A pulsating field can be resolved into two counter-rotating fields of equal strength, whose torques cancel at standstill. A capacitor or auxiliary winding makes the field elliptical and gives a net torque to start with.