BEE Electrical Machines 3

Question 1

The EMF equation of a DC generator is E = PφNZ / (60A). In a 4-pole lap-wound generator with 600 conductors, flux per pole 0.05 Wb, rotating at 1500 RPM, the generated EMF is:

Accepted Answer

750 V

Answer

For lap winding A = P = 4; E = 4 × 0.05 × 1500 × 600 / (60 × 4) = 180000 / 240 = 750 V.

Question 2

Hunting in synchronous machines refers to:

Accepted Answer

Oscillation of the rotor about the synchronous speed position

Answer

Hunting is the oscillation of the rotor angle (and speed) around the equilibrium synchronous position, caused by sudden load changes or disturbances.

Question 3

To minimize eddy current losses, transformer cores are constructed from:

Accepted Answer

Laminated silicon steel sheets insulated from each other

Answer

Thin laminations insulated from each other break up the eddy current path, drastically reducing eddy current losses which are proportional to the square of lamination thickness.

Question 4

In a DC series motor, if the load torque is doubled, the speed approximately changes to:

Accepted Answer

1/√2 of the original speed

Answer

In a series motor T ∝ Ia² and N ∝ 1/Ia; doubling torque requires √2 times the current, so speed drops to N/√2 of its original value.

Question 5

The primary advantage of an autotransformer over a two-winding transformer of the same rating is:

Accepted Answer

Part of the power is transferred conductively rather than magnetically, reducing copper requirement

Answer

In an autotransformer, power is transferred both conductively (through the common winding) and magnetically, so less copper is needed and efficiency is higher for the same apparent power rating.

BEE - Bachelor of Electrical Engineering Practice Test

BEE - Bachelor of Electrical Engineering Practice Test

BEE Electrical Machines 3