EGSA Generator and Alternator Principles 3 — Questions and Answers
Question 1: What is the significance of the short-circuit ratio (SCR) in a synchronous generator?
- Ratio of field current at rated voltage to field current at rated short-circuit current (Correct answer)
- Maximum allowable short-circuit current before damage
- Ratio of stator turns to rotor turns
- Percentage voltage drop under rated load
Correct answer: Ratio of field current at rated voltage to field current at rated short-circuit current
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?
- Carbon brush assembly
- Rotating rectifier diode assembly (Correct answer)
- Voltage regulator module
- Permanent magnet pilot exciter
Correct answer: Rotating rectifier diode assembly
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?
- Imbalanced three-phase loading only
- Non-sinusoidal distribution of flux in the air gap and non-linear loads (Correct answer)
- Excessive field excitation current
- Bearing misalignment
Correct answer: Non-sinusoidal distribution of flux in the air gap and non-linear loads
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?
- Terminal voltage decreases significantly
- Terminal voltage rises above no-load voltage (Correct answer)
- Terminal voltage remains constant
- Terminal voltage becomes unstable
Correct answer: Terminal voltage rises above no-load voltage
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?
- 90 electrical degrees
- 60 electrical degrees
- 120 electrical degrees (Correct answer)
- 180 electrical degrees
Correct answer: 120 electrical degrees
In a balanced three-phase generator, the three phase voltages are separated by 120 electrical degrees to produce symmetrical three-phase power.
Question 6: What is 'voltage regulation' of a synchronous generator expressed as?
- (VFL - VNL) / VFL × 100%
- (VNL - VFL) / VFL × 100% (Correct answer)
- (VFL - VNL) / VNL × 100%
- VFL / VNL × 100%
Correct answer: (VNL - VFL) / VFL × 100%
Voltage regulation = (VNL - VFL) / VFL × 100%, where VNL is no-load terminal voltage and VFL is full-load terminal voltage at rated conditions.
Question 7: Why do salient pole rotors exhibit reluctance torque that cylindrical rotors do not?
- Salient poles have higher resistance windings
- The d-axis and q-axis reactances are unequal due to the non-uniform air gap (Correct answer)
- Salient poles produce stronger magnetic fields
- Cylindrical rotors have no field winding
Correct answer: The d-axis and q-axis reactances are unequal due to the non-uniform air gap
The non-uniform air gap in salient pole machines creates different reluctances along the d-axis and q-axis (Xd ≠ Xq), producing reluctance torque in addition to excitation torque.
EGSA Generator and Alternator Principles 3
What is the significance of the short-circuit ratio (SCR) in a synchronous generator?