EGSA Electrical Systems Troubleshooting 4 — Questions and Answers
Question 1: Excessive third-harmonic distortion in a generator's output is most commonly caused by:
- Non-linear loads such as UPS systems and VFDs (Correct answer)
- Incorrect governor speed setting
- Worn brushes on the slip rings
- Low coolant temperature
Correct answer: Non-linear loads such as UPS systems and VFDs
Non-linear loads draw current in pulses rather than sinusoidal waves, generating third and other odd harmonics that distort the generator output waveform.
Question 2: When load testing a generator, what parameter confirms the generator is operating within its rated kVA capacity?
- Both kilowatts and kVAR readings within nameplate rating (Correct answer)
- Engine coolant temperature only
- Fuel consumption rate
- Battery voltage during cranking
Correct answer: Both kilowatts and kVAR readings within nameplate rating
kVA capacity is determined by both the real power (kW) and reactive power (kVAR) components, and both must be within nameplate limits to confirm proper operation.
Question 3: A parallel-connected generator is importing reactive power (absorbing VArs) instead of exporting. The likely cause is:
- Under-excitation of the generator field (Correct answer)
- Oversized fuel injectors
- Governor set point too high
- Tripped overcurrent relay on the bus
Correct answer: Under-excitation of the generator field
Under-excitation causes the generator to operate in a leading power factor mode, absorbing reactive power from the bus rather than supplying it.
Question 4: What is the most appropriate test to verify continuity of a generator's rotor winding without disassembly?
- Slip ring resistance measurement with an ohmmeter (Correct answer)
- Megohmmeter test at 5000V
- Capacitance test on the stator leads
- Power factor test on load bank
Correct answer: Slip ring resistance measurement with an ohmmeter
Measuring resistance across the slip rings with an ohmmeter checks rotor winding continuity and compares the reading against the manufacturer's specification.
Question 5: Which fault condition causes unequal voltage readings across each phase of a three-phase generator but normal frequency?
- Partial short in one phase stator winding (Correct answer)
- Incorrect governor calibration
- Low engine oil pressure
- Clogged air filter
Correct answer: Partial short in one phase stator winding
A partial short in one phase stator winding reduces the turns ratio and output impedance of that phase, lowering its voltage while the other phases remain normal.
Question 6: When a generator's voltage regulation is poor under varying loads, which component should be calibrated or replaced first?
- AVR (Correct answer)
- Governor actuator
- Transfer switch
- Main circuit breaker
Correct answer: AVR
The AVR continuously adjusts excitation to maintain constant output voltage; poor regulation under varying loads is the primary symptom of an AVR fault or misadjustment.
Question 7: A technician measures 0 ohms between two phase conductors in a de-energized three-phase system. This indicates:
- A phase-to-phase short circuit (Correct answer)
- Normal winding resistance
- An open circuit between the phases
- Correct diode bridge operation
Correct answer: A phase-to-phase short circuit
Zero resistance between two phase conductors in a de-energized system indicates a bolted or solid phase-to-phase short that must be located and cleared before re-energizing.
EGSA Electrical Systems Troubleshooting 4
Excessive third-harmonic distortion in a generator's output is most commonly caused by: