Three Phase Induction Motors Questions and Answers

Answer: C

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

At synchronous speed the rotor and the field move together, nothing is cut, no emf is induced, no current flows and no torque is developed. The machine must therefore always run a little slower when motoring - the slip is not an imperfection but the condition of operation, which is why the machine is called asynchronous.

Answer: D

Correct answer: False, since the cage adapts to whatever pole number the stator produces

The cage has no fixed winding pattern: the currents induced in its bars arrange themselves into whatever pole number the stator field presents. This is what makes pole-changing possible - a stator reconnected from four poles to eight drives the same cage rotor at half the speed, with no change to the rotor at all.

Answer: A

Correct answer: the real component of current grows while the magnetising component does not

The magnetising current is set by the flux, which is set by the applied voltage, and is therefore nearly constant from no load to full. The load current adds in phase with the voltage, so as it grows the ratio of real to total current improves. At light load the almost purely reactive magnetising current dominates and the power factor is poor.

Answer: B

Correct answer: one minus the slip

Of the power crossing the air gap, the fraction \( s \) is lost in the rotor and \( 1-s \) becomes mechanical, so \( 1-s \) is an upper bound on efficiency before the stator, iron and mechanical losses are subtracted. At four per cent slip nothing better than 96 per cent is available, and the actual figure will be several points below it.

Answer: C

Correct answer: higher rotor loss and lower efficiency

Rotor loss is the slip fraction of the air gap power, so a design running at eight per cent slip dissipates twice as much in its rotor as one running at four. High-slip designs are chosen deliberately where a soft torque-speed characteristic is wanted - for punch presses and hoists - and the efficiency is knowingly given up for it.

Answer: D

A cage rotor has nothing to wear but its bearings and nothing to connect, which makes it cheap to build and almost indestructible. Its historic weakness was speed control, and the variable-frequency drive removed that.

Answer: A

Correct answer: a synchronous machine of the same rating and pole number

Both machines need a stator that produces a revolving field, and both do it the same way: a laminated core with a distributed three-phase winding in slots. The difference between the two lies entirely in the rotor - one excited and locked to the field, the other induced and slipping behind it.

Answer: B

At standstill it is exactly a short-circuited transformer, and the whole equivalent circuit is the transformer's with the rotor resistance divided by slip. That correspondence is the quickest route into understanding it.

Answer: C

Correct answer: unity

With the rotor stationary, \( N = 0 \) and \[ s = \frac{N_s - 0}{N_s} = 1. \] Unity slip is the starting condition, at which the rotor emf and frequency are at their highest and the machine draws its locked-rotor current of five or six times rated.

Answer: D

Correct answer: standstill

The rotor emf is proportional to the slip, and the slip is greatest - equal to one - at standstill. At that instant the rotor is being cut at the full synchronous speed. As the machine accelerates the relative motion falls and the rotor emf and its frequency fall in proportion.