NETA Motor and Generator Testing 1 — Questions and Answers

Question 1: The polarization index (PI) test for motor winding insulation compares the insulation resistance at:

500V and 1000V test voltages
30 seconds and 10 minutes using the same test voltage (Correct answer)
60 Hz and 120 Hz test frequencies
Ambient temperature and elevated temperature (100 degrees C)

Correct 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 Polarization Index (PI) measures the change in insulation resistance over time under a sustained DC voltage. Clean, dry insulation shows a significant increase over time (high PI); contaminated or wet insulation reaches its resistive limit quickly with little change (low PI). IEEE 43 recommends PI greater than 2.0 for acceptable insulation on AC machines. Values below 1.0 indicate a short-circuit fault path. The test is performed with Megger test sets at voltages per IEEE 43 recommendations.

Question 2: The no-load test on an induction motor primarily measures:

Full-load copper losses in the stator windings
Core losses and friction and windage losses (no-load losses) (Correct answer)
Locked rotor impedance and starting current
Winding resistance of rotor and stator

Correct 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.

The no-load test applies rated voltage to the motor with no mechanical load. The measured parameters are: no-load current (Inl), no-load real power consisting of core loss plus friction and windage, and no-load reactive power for magnetizing reactance. Since load is zero, slip is minimal and rotor copper loss is negligible. No-load current is typically 25 to 50% of rated current. Comparing no-load current and losses to nameplate and historical data identifies core problems (shorted laminations increase core loss) and bearing condition issues.

Question 3: A surge comparison test (or surge test) on motor windings detects:

Turn-to-turn insulation faults and winding asymmetries that might not appear in standard insulation resistance tests (Correct answer)
Insulation resistance between winding and ground
Unbalanced three-phase voltages during operation
The breakdown voltage of the phase-to-phase insulation

Correct 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.

The surge comparison test (also called impulse test) applies repetitive high-voltage impulse pulses to motor windings, generating a damped oscillating waveform. When two identical windings are compared, their waveforms should be identical. A turn-to-turn short reduces the effective inductance of the affected winding, changing its resonant frequency and waveform shape. Even a single shorted turn (which may not be detectable by insulation resistance or resistance tests due to very small resistance change) is reliably detected by surge comparison. This is a critical motor acceptance and maintenance test, as incipient turn-to-turn shorts are the leading cause of stator winding failure.

Question 4: Motor winding resistance unbalance exceeding what percentage between phases typically indicates a problem?

0.5% unbalance between phases
2% unbalance between phases (Correct answer)
10% unbalance between phases
25% unbalance between phases

Correct 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.

For a three-phase motor with symmetrical windings, all phase winding resistances should be essentially equal. Unbalance is calculated as: [(max R - min R) / average R] x 100%. NETA acceptance criteria specify that resistance unbalance should not exceed 2% between phases. Values above 2% indicate high-resistance connections, partial conductor breakage, incorrect manufacturing, or different temperatures between windings. Winding resistance measurements must be corrected to a common reference temperature before comparison.

Question 5: The purpose of performing a hi-pot (high potential) test on a generator after a rewind is to:

Measure the generator's output voltage capability
Verify the integrity of the new groundwall insulation by stressing it above normal operating voltage (Correct answer)
Test the generator's response to high-load conditions
Calibrate the generator's voltage regulator

Correct 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.

After a generator stator rewind, the new coil groundwall insulation must be tested to verify quality and correct installation. IEEE 95 and IEEE 434 specify AC or DC hi-pot test procedures and voltages (typically 2 times rated voltage plus 1000V AC for acceptance testing of new windings, reduced for in-service testing). The test applies the high voltage for 1 to 5 minutes while monitoring leakage current. A sudden increase in current or flashover indicates an insulation defect from moisture contamination, mechanical damage, or manufacturing defect.

Question 6: Partial discharge (PD) testing of high-voltage motor insulation detects:

Complete insulation failures that cause immediate shutdown
Localized electrical discharges within voids in the insulation that indicate deterioration before complete failure (Correct answer)
The level of electrical noise generated by the motor during operation
Commutation sparking in DC motor brushes

Correct 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.

Partial discharge (PD) is a localized dielectric breakdown in a void or weak point within the insulation that does not completely bridge the insulation between conductors. In high-voltage motor stator coils, PD occurs in air voids within the groundwall insulation causing erosion, delaminations between insulation layers, at conductor-insulation interfaces, and at end-arm stress grading interfaces. PD measurements are made by capacitively coupling PD sensors to the winding and measuring high-frequency current pulses. PD testing per IEC 60034-27 provides early warning of insulation degradation. PD levels are trended over time; rapid increases indicate accelerating deterioration.

NETA Motor and Generator Testing 1

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