EGSA Generator and Alternator Principles 3

Question 1

What is the significance of the short-circuit ratio (SCR) in a synchronous generator?

Accepted Answer

Ratio of field current at rated voltage to field current at rated short-circuit current

Answer

The SCR is the ratio of field current required for rated open-circuit voltage to that required for rated armature current at short circuit, indicating machine stiffness.

Question 2

In a brushless excitation system, what component converts AC exciter output to DC for the main field?

Accepted Answer

Rotating rectifier diode assembly

Answer

A rotating rectifier assembly mounted on the rotor shaft converts the AC output of the rotating armature exciter to DC, which is fed directly to the main field winding.

Question 3

What causes voltage waveform distortion (harmonics) in the output of a generator?

Accepted Answer

Non-sinusoidal distribution of flux in the air gap and non-linear loads

Answer

Harmonics arise from non-sinusoidal flux distribution in the machine's air gap and are amplified by non-linear loads such as variable frequency drives and rectifiers.

Question 4

What is the effect of a leading power factor load on the terminal voltage of an isolated synchronous generator?

Accepted Answer

Terminal voltage rises above no-load voltage

Answer

A leading power factor (capacitive load) causes the armature reaction to magnetize (aid the field), resulting in terminal voltage rising above the no-load value.

Question 5

In a three-phase generator, what is the phase displacement between successive phase voltages?

Accepted Answer

120 electrical degrees

Answer

In a balanced three-phase generator, the three phase voltages are separated by 120 electrical degrees to produce symmetrical three-phase power.

(EGSA) Electrical Generating Systems Association Certified Practice Test

(EGSA) Electrical Generating Systems Association Certified Practice Test

EGSA Generator and Alternator Principles 3