EGSA Generator and Alternator Principles 5

Question 1

What is the effect of increasing the air gap length in a synchronous generator on its synchronous reactance?

Accepted Answer

Synchronous reactance decreases

Answer

A larger air gap increases the magnetic reluctance, reducing the flux per ampere of field current and thereby decreasing the synchronous reactance of the machine.

Question 2

When paralleling two generators, what condition must be met just before closing the synchronizing breaker?

Accepted Answer

Incoming generator voltage, frequency, phase sequence, and phase angle must match the bus

Answer

Successful paralleling requires matching voltage magnitude, frequency, phase sequence, and phase angle (synchronism) between the incoming generator and the live bus.

Question 3

What is 'hunting' in a synchronous generator and what typically causes it?

Accepted Answer

Oscillation of rotor speed about synchronous speed caused by sudden load changes or governor instability

Answer

Hunting is oscillation of the rotor about its synchronous speed, typically triggered by sudden load changes, governor droop mismatch, or weak damper windings.

Question 4

A generator nameplate shows a power factor of 0.8 lagging. What does this mean for its kVA and kW ratings?

Accepted Answer

kW rating is 80% of the kVA rating

Answer

At 0.8 power factor, real power (kW) = kVA × 0.8, so the kW rating is 80% of the apparent power kVA rating.

Question 5

What distinguishes a 'permanent magnet generator (PMG)' pilot exciter from a conventional pilot exciter?

Accepted Answer

PMG provides initial excitation independent of external power, enabling self-excited startup

Answer

A PMG pilot exciter uses permanent magnets on its rotor, providing a power source for the AVR that is completely independent of the main generator output, ensuring reliable startup and fault ride-through.

(EGSA) Electrical Generating Systems Association Certified Practice Test

(EGSA) Electrical Generating Systems Association Certified Practice Test

EGSA Generator and Alternator Principles 5