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

375 V

Answer

250 V

Answer

1500 V

Question 2

Hunting in synchronous machines refers to:

Accepted Answer

Oscillation of the rotor about the synchronous speed position

Answer

The machine searching for the optimal excitation level

Answer

A speed increase beyond the synchronous speed

Answer

High starting torque behavior

Question 3

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

Accepted Answer

Laminated silicon steel sheets insulated from each other

Answer

Solid cast iron

Answer

Solid carbon steel

Answer

Air-core (no magnetic core)

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

Double the original speed

Answer

Remain the same

Answer

Half the original speed

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

It uses a completely different core material

Answer

It operates at higher frequencies

Answer

It eliminates all core (iron) losses

Question 6

A three-phase synchronous generator (alternator) operates at 3000 RPM and produces 50 Hz output. The number of poles it has is:

Accepted Answer

2

Answer

4

Answer

6

Answer

8

Question 7

The purpose of a damper winding (amortisseur winding) in a synchronous machine is to:

Accepted Answer

Suppress hunting oscillations and assist in starting synchronous motors

Answer

Increase the output voltage

Answer

Provide additional excitation current

Answer

Improve the power factor to unity