NETA Motor and Generator Testing 1

Question 1

The polarization index (PI) test for motor winding insulation compares the insulation resistance at:

Accepted Answer

30 seconds and 10 minutes using the same test voltage

Answer

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.

Question 2

The no-load test on an induction motor primarily measures:

Accepted Answer

Core losses and friction and windage losses (no-load losses)

Answer

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.

Question 3

A surge comparison test (or surge test) on motor windings detects:

Accepted Answer

Turn-to-turn insulation faults and winding asymmetries that might not appear in standard insulation resistance tests

Answer

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.

Question 4

Motor winding resistance unbalance exceeding what percentage between phases typically indicates a problem?

Accepted Answer

2% unbalance between phases

Answer

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.

Question 5

The purpose of performing a hi-pot (high potential) test on a generator after a rewind is to:

Accepted Answer

Verify the integrity of the new groundwall insulation by stressing it above normal operating voltage

Answer

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.

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

NETA Motor and Generator Testing 1