BEE Electrical Machines 2

Question 1

The power factor of a three-phase induction motor operating at no load is:

Accepted Answer

Very low and lagging (0.1–0.2)

Answer

Unity (1.0)

Answer

Close to unity (0.9 lagging)

Answer

Very low and leading

Question 2

In a synchronous generator supplying a lagging power factor load, the effect of armature reaction on the air-gap flux is:

Accepted Answer

Demagnetizing (reduces flux)

Answer

Magnetizing (increases flux)

Answer

Cross-magnetizing (distorts flux)

Answer

No effect on flux

Question 3

The rotor construction of a squirrel cage induction motor consists of:

Accepted Answer

Short-circuited aluminum or copper bars embedded in slots

Answer

Copper windings connected to slip rings

Answer

Wound rotor with external resistance connections

Answer

Permanent magnets mounted on the rotor surface

Question 4

Voltage regulation of a transformer is correctly expressed as:

Accepted Answer

(VNL - VFL) / VFL × 100%

Answer

(VNL - VFL) / VNL × 100%

Answer

(VFL - VNL) / VFL × 100%

Answer

VNL / VFL × 100%

Question 5

Speed control of a DC shunt motor can be achieved by:

Accepted Answer

Both armature resistance variation and field current variation

Answer

Varying armature resistance only

Answer

Varying field current only

Answer

Varying the supply frequency only

Question 6

In a three-phase induction motor running at slip s, the frequency of the rotor currents is:

Accepted Answer

s × f

Answer

f (supply frequency)

Answer

f / s

Answer

(1 - s) × f

Question 7

An over-excited synchronous motor (field current greater than normal) operates at:

Accepted Answer

Leading power factor

Answer

Unity power factor

Answer

Lagging power factor

Answer

Zero power factor