NETA Motor and Generator Testing 4 — Questions and Answers
Question 1: When performing a surge comparison test on a motor winding, what condition is indicated when the two surge waveforms do NOT match?
- Turn-to-turn insulation fault within the winding (Correct answer)
- Open circuit in the supply conductors
- Excessive bearing wear
- Rotor eccentricity
Correct answer: Turn-to-turn insulation fault within the winding
Mismatched surge waveforms indicate a turn-to-turn insulation fault because the fault alters the inductance of that coil group, causing a different impedance response.
Question 2: According to NETA MTS standards, what is the minimum recommended polarization index (PI) value for a motor winding rated above 1 kV to be considered acceptable for service?
- 1.0
- 2.0 (Correct answer)
- 4.0
- 6.0
Correct answer: 2.0
NETA MTS specifies a minimum PI of 2.0 for windings rated above 1 kV; values below this indicate moisture or contamination that poses a dielectric risk.
Question 3: A motor's no-load current is measured at 45% of full-load current. What does this elevated no-load current most likely indicate?
- Normal operation for most induction motors
- Shorted turns in the stator winding (Correct answer)
- High bearing friction or mechanical overload
- Rotor bars with high resistance
Correct answer: Shorted turns in the stator winding
No-load current exceeding approximately 30–35% of full-load current for standard induction motors often signals shorted stator turns, which increase magnetizing current demand.
Question 4: During vibration analysis of a generator, a prominent frequency component at exactly 2× line frequency (2× 60 Hz = 120 Hz) is detected. What is the most probable cause?
- Rotor unbalance
- Misalignment
- Loose stator core laminations or electromagnetic excitation (Correct answer)
- Bearing defect at the outer race
Correct answer: Loose stator core laminations or electromagnetic excitation
Electromagnetic forces in AC machines produce twice-line-frequency (120 Hz) vibration, commonly caused by loose or uneven stator core laminations responding to alternating flux.
Question 5: What does a high AC resistance (compared to DC resistance) ratio in a motor winding suggest?
- Open rotor bars
- Significant skin effect or proximity effect losses in the conductors (Correct answer)
- Low insulation resistance to ground
- Excessive shaft voltage
Correct answer: Significant skin effect or proximity effect losses in the conductors
A high AC-to-DC resistance ratio indicates elevated skin and proximity effects, which increase effective conductor resistance at power frequencies and contribute to winding losses.
Question 6: When testing a DC motor, a high resistance reading between adjacent commutator bars using a milliohmmeter most likely indicates:
- Normal mica undercut condition
- An open armature coil or broken riser connection (Correct answer)
- Excessive brush spring pressure
- Field winding ground fault
Correct answer: An open armature coil or broken riser connection
High resistance between adjacent commutator bars points to an open armature coil or a broken riser where the coil connects to the commutator segment.
Question 7: A generator's dielectric absorption ratio (DAR) is calculated as the ratio of insulation resistance at:
- 10 minutes to 1 minute
- 60 seconds to 30 seconds (Correct answer)
- 5 minutes to 30 seconds
- 30 seconds to 15 seconds
Correct answer: 60 seconds to 30 seconds
The DAR is the ratio of the 60-second insulation resistance reading to the 30-second reading, providing a quick indicator of insulation condition.
NETA Motor and Generator Testing 4
When performing a surge comparison test on a motor winding, what condition is indicated when the two surge waveforms do NOT match?