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

At no load the motor draws mainly reactive magnetizing current with very little active power, resulting in a very low lagging power factor.

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

A lagging power factor load causes the armature MMF to oppose the main field MMF, producing a demagnetizing armature reaction.

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

A squirrel cage rotor has conducting bars (usually aluminum) embedded in rotor slots and short-circuited at both ends by end rings, requiring no external connections.

Question 4

Voltage regulation of a transformer is correctly expressed as:

Accepted Answer

(VNL - VFL) / VFL × 100%

Answer

Voltage regulation = (VNL − VFL) / VFL × 100%, measuring the change in secondary voltage from full-load to no-load relative to rated (full-load) voltage.

Question 5

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

Accepted Answer

Both armature resistance variation and field current variation

Answer

DC shunt motor speed N ∝ V/(φ), so speed can be controlled below base speed by adding armature resistance and above base speed by weakening the field (reducing field current).

BEE - Bachelor of Electrical Engineering Practice Test

BEE - Bachelor of Electrical Engineering Practice Test

BEE Electrical Machines 2