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

Synchronous reactance increases significantly

Answer

Synchronous reactance is unaffected by air gap length

Answer

Synchronous reactance becomes equal to armature resistance

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

Both generators must have identical kVA ratings

Answer

The incoming generator must be running at slightly higher load than the bus generator

Answer

The field current of both generators must be identical

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

Periodic voltage spikes caused by AVR over-correction

Answer

Thermal cycling of the stator windings under variable load

Answer

Intermittent contact of brush gear on slip rings

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

kVA rating equals kW rating

Answer

kVA rating is 80% of the kW rating

Answer

The generator can only supply resistive loads

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

PMG uses carbon brushes for more reliable contact

Answer

PMG only works above 1000 RPM

Answer

PMG requires a separate battery supply to magnetize its rotor

Question 6

In a generator stator winding, what is the purpose of using a fractional-pitch (short-pitch) coil design?

Accepted Answer

Reduce specific harmonic voltages in the output waveform

Answer

Increase copper usage to reduce cost

Answer

Increase the synchronous reactance for better voltage regulation

Answer

Allow easier rewinding in the field

Question 7

What is the primary reason high-capacity generators use hydrogen gas cooling instead of air cooling?

Accepted Answer

Hydrogen has approximately 7 times lower density than air, greatly reducing windage losses and improving cooling efficiency

Answer

Hydrogen is non-flammable, making it safer than air

Answer

Hydrogen prevents corona discharge better than air

Answer

Hydrogen has lower thermal conductivity than air, acting as an insulator