EGSA Electrical Systems Troubleshooting 5 — Questions and Answers
Question 1: During a voltage dip test, a generator's voltage recovers but takes longer than the manufacturer specifies. What is the most likely cause?
- Sluggish AVR response due to incorrect gain setting (Correct answer)
- Low engine coolant temperature
- Incorrect fuel filter
- Misaligned flexible coupling
Correct answer: Sluggish AVR response due to incorrect gain setting
The AVR gain setting controls response speed; too-low gain causes slow voltage recovery after transient load application, exceeding the specified recovery time.
Question 2: What is the significance of a high kW but low kVA reading on a generator under load?
- The power factor is near unity, indicating mostly resistive load (Correct answer)
- The generator is overloaded
- The generator is under-excited
- The stator windings are shorted
Correct answer: The power factor is near unity, indicating mostly resistive load
When kW approaches kVA, the power factor is near 1.0 (purely resistive load), meaning little reactive power is being consumed, which is an efficient operating condition.
Question 3: A generator produces correct voltage at no-load but the output collapses when connected to a motor starting load. What should be evaluated?
- Generator's transient reactance and short-circuit capability (Correct answer)
- Fuel tank capacity
- Engine block heater function
- Transfer switch bypass setting
Correct answer: Generator's transient reactance and short-circuit capability
Motor starting draws large inrush current; if the generator's transient reactance is too high or it is undersized, its terminal voltage will sag excessively or collapse during motor acceleration.
Question 4: Neutral-to-ground voltage above 2V in a solidly grounded 120/208V system typically indicates:
- Excessive neutral current due to unbalanced or non-linear loads (Correct answer)
- Normal system operation
- Overvoltage from the utility
- Correct bonding of the neutral and ground
Correct answer: Excessive neutral current due to unbalanced or non-linear loads
High neutral current from load imbalance or harmonic-generating equipment causes a measurable voltage drop along the neutral conductor, raising neutral-to-ground voltage above acceptable levels.
Question 5: When a generator fails to build voltage after a field flash attempt, the next step is to:
- Test the rotating rectifier diodes for open or short circuit (Correct answer)
- Recheck the engine speed
- Inspect the transfer switch contacts
- Replace the governor actuator
Correct answer: Test the rotating rectifier diodes for open or short circuit
If field flashing doesn't build voltage, the rotating rectifier diodes that supply DC to the main rotor field should be tested, as a failed diode prevents field current flow.
Question 6: Which protective relay function trips a generator when it detects reverse power flow from the bus into the machine?
- ANSI Device 32 (reverse power relay) (Correct answer)
- ANSI Device 27 (undervoltage relay)
- ANSI Device 81 (frequency relay)
- ANSI Device 87 (differential relay)
Correct answer: ANSI Device 32 (reverse power relay)
The ANSI 32 reverse power relay detects motoring condition where power flows into the generator from the bus, protecting the prime mover from damage.
Question 7: A technician notices the generator output voltage varies with changes in engine speed even though the AVR is functional. What is the most probable cause?
- The AVR voltage sensing input has a loose connection (Correct answer)
- The generator is over-fueled
- The brushes are too long
- The coolant thermostat is stuck open
Correct answer: The AVR voltage sensing input has a loose connection
A loose sensing input connection causes the AVR to receive an intermittent or incorrect voltage signal, preventing it from maintaining regulation when engine speed changes.
EGSA Electrical Systems Troubleshooting 5
During a voltage dip test, a generator's voltage recovers but takes longer than the manufacturer specifies.
What is the most likely cause?