Motor and Generator Testing Flashcards
6 cards from real NETA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Motor and Generator Testing flashcards as text
The polarization index (PI) test for motor winding insulation compares the insulation resistance at:
Answer: 30 seconds and 10 minutes using the same test voltage
The Polarization Index (PI) = IR at 10 minutes divided by IR at 1 minute (same DC voltage). A PI above 2.0 generally indicates acceptable insulation; below 1.5 indicates deteriorated or moisture-contaminated insulation.
The no-load test on an induction motor primarily measures:
Answer: Core losses and friction and windage losses (no-load losses)
With the motor running unloaded at rated voltage and frequency, the input power equals core losses (hysteresis plus eddy current in stator iron) plus friction and windage losses, since copper losses are minimal at no-load current.
A surge comparison test (or surge test) on motor windings detects:
Answer: Turn-to-turn insulation faults and winding asymmetries that might not appear in standard insulation resistance tests
The surge test applies high-voltage impulses to motor windings and compares the resulting oscillating waveforms between phases. Differences between phases indicate turn-to-turn shorts, coil-to-coil faults, or groundwall insulation weaknesses.
Motor winding resistance unbalance exceeding what percentage between phases typically indicates a problem?
Answer: 2% unbalance between phases
NEMA MG-1 and NETA standards indicate that winding resistance unbalance exceeding 2% between phases is abnormal and warrants investigation for loose connections, broken conductors, or unequal turns.
The purpose of performing a hi-pot (high potential) test on a generator after a rewind is to:
Answer: Verify the integrity of the new groundwall insulation by stressing it above normal operating voltage
After rewinding, the new groundwall insulation (coil-to-core insulation) is hi-pot tested to verify it was installed correctly and has no defects, by applying a voltage significantly above rated voltage for a specified period.
Partial discharge (PD) testing of high-voltage motor insulation detects:
Answer: Localized electrical discharges within voids in the insulation that indicate deterioration before complete failure
Partial discharge occurs in air voids, delaminations, or at conductor-insulation interfaces within the groundwall insulation. PD activity indicates insulation deterioration and, if unchecked, leads to progressive insulation failure.