EGSA Electrical Systems Troubleshooting 3 — Questions and Answers
Question 1: When a generator's output voltage collapses immediately upon applying load, the most likely cause is:
- Shorted exciter diodes (Correct answer)
- High coolant temperature
- Incorrect fuel timing
- Loose ground connection on the load panel
Correct answer: Shorted exciter diodes
Shorted exciter diodes prevent the field from building up sufficient current under load, causing voltage collapse when current demand increases.
Question 2: What is the purpose of performing a polarization index (PI) test on generator windings?
- To assess insulation condition over time by comparing resistance at 1 and 10 minutes (Correct answer)
- To measure phase-to-phase voltage balance
- To determine rotor winding resistance
- To calculate generator efficiency
Correct answer: To assess insulation condition over time by comparing resistance at 1 and 10 minutes
The PI is the ratio of 10-minute to 1-minute insulation resistance readings; values above 2.0 generally indicate good insulation condition.
Question 3: A ground fault indicator lamp glows dimly on one phase of a 480V ungrounded delta system. This indicates:
- A partial ground fault on that phase (Correct answer)
- Normal system operation
- An open circuit on that phase
- Overvoltage on the indicated phase
Correct answer: A partial ground fault on that phase
In an ungrounded delta, a dim lamp indicates a partial or high-resistance ground fault; a solid ground fault would cause the lamp to go dark with the other two lamps glowing brighter.
Question 4: Which condition would cause a generator to produce voltage but fail to sustain it when a load is applied?
- Weak residual magnetism in the rotor (Correct answer)
- Excessive belt tension on the drive pulley
- Overspeed condition
- High ambient humidity
Correct answer: Weak residual magnetism in the rotor
Insufficient residual magnetism in the rotor prevents the field from self-exciting enough to sustain output voltage once load current is drawn.
Question 5: When troubleshooting a circuit with intermittent power loss, which technique is most effective for identifying a loose connection?
- Thermal imaging under load (Correct answer)
- DC hipot test
- Megohmmeter test
- Clamp-on current measurement
Correct answer: Thermal imaging under load
Thermal imaging detects heat generated by elevated resistance at loose connections under load, making intermittent faults visible before they cause failures.
Question 6: A generator's AVR sensing voltage input is measuring correctly, but the output voltage is too low. What should be checked next?
- Exciter field winding continuity (Correct answer)
- Engine oil pressure
- Battery charging circuit
- Transfer switch contacts
Correct answer: Exciter field winding continuity
If the AVR senses correct voltage but output remains low, the exciter field winding may be open or have high resistance, limiting field current to the main rotor.
Question 7: What happens to neutral current in a 3-phase 4-wire system when all phase currents are balanced and contain no harmonic distortion?
- Neutral current is zero (Correct answer)
- Neutral current equals phase current
- Neutral current is three times phase current
- Neutral current is unpredictable
Correct answer: Neutral current is zero
In a perfectly balanced 3-phase system with sinusoidal currents, the phasor sum of the three phase currents cancels to zero in the neutral conductor.
EGSA Electrical Systems Troubleshooting 3
When a generator's output voltage collapses immediately upon applying load, the most likely cause is: