Three Phase Induction Motors Questions and Answers

Answer: A

Setting the slip at maximum torque to unity requires the two to be equal, which places the peak of the characteristic at standstill. As the machine runs up, resistance is cut out to keep the operating point near the peak.

Answer: B

Rotor resistance determines where the peak occurs but not how high it is, whereas voltage and reactance both determine its height. That is precisely why added rotor resistance improves starting with no penalty in overload capability.

Answer: C

Correct answer: the square of the voltage

Torque depends on the product of flux and rotor current, and both are proportional to the applied voltage, so torque varies as its square. The consequence is severe: a ten per cent voltage drop removes nearly twenty per cent of the torque, which is why supply voltage is the first thing to check when a machine fails to start.

Answer: D

Because the equivalent circuit is a constant voltage across a series impedance whose resistive part varies, the current traces a circle in the complex plane. Torque, output, power factor and efficiency can all be read off it graphically.

Answer: A

They fix the two extreme points of the locus, the no-load point at nearly zero slip and the blocked-rotor point at unity slip. The circle through them, with its centre on a line determined by the geometry, completes the construction.

Answer: B

With the rotor turning almost at synchronous speed, its current is negligible and the input is nearly all core and mechanical loss. Repeating the test at reduced voltage lets the two be separated by extrapolating to zero voltage.

Answer: C

Core loss falls roughly with the square of the voltage while mechanical loss depends on speed alone, so a plot against voltage squared extrapolates to the mechanical loss at zero voltage. It is the standard construction.

Answer: D

Correct answer: rated voltage would give a destructive current

With the rotor held still the slip is unity, the machine is effectively a short-circuited transformer, and rated voltage would drive five or six times rated current through the windings. The test is therefore made at reduced voltage and the results scaled, which is legitimate because the series impedance is very nearly constant.

Answer: A

A direct current measurement avoids any reactive component, and the result is corrected to the working temperature and, for a star connection, halved to give the per-phase value. It is needed to split the blocked-rotor resistance between the two windings.

Answer: B

At standstill the high rotor frequency confines the current to the outer high-resistance cage, and at running slip it spreads into the inner low-resistance one. The machine therefore behaves as if its rotor resistance varied with speed.