NETA Motor and Generator Testing 5 â Questions and Answers
Question 1: During a no-load test on an induction motor, the power factor is measured at approximately 0.1 lagging. What does this indicate about the motor?
- The motor has a shorted stator coil
- This is abnormally low and indicates bearing problems
- This is normal, as no-load operation is dominated by magnetizing reactive current (Correct answer)
- The motor is operating in generator mode
Correct answer: This is normal, as no-load operation is dominated by magnetizing reactive current
At no-load, an induction motor draws mostly magnetizing (reactive) current with very little real power, resulting in a naturally low power factor near 0.1 lagging.
Question 2: A shaft voltage measurement on a large motor reads 500 mV AC. What is the primary concern associated with elevated shaft voltages?
- Stator insulation breakdown from overvoltage
- Bearing fluting damage from electrical discharge through the bearing lubricant (Correct answer)
- Rotor overheating due to eddy currents
- Increased brush wear on slip rings
Correct answer: Bearing fluting damage from electrical discharge through the bearing lubricant
Elevated shaft voltages cause current to discharge through the bearing film, eroding bearing surfaces and lubricant, leading to characteristic fluting (washboard) patterns on the race.
Question 3: Which test method is most effective for detecting cracked or broken rotor bars in a squirrel-cage induction motor while the motor is running?
- Megohmmeter insulation resistance test
- Motor Current Signature Analysis (MCSA) (Correct answer)
- Hi-pot (high-potential) test
- Winding resistance balance test
Correct answer: Motor Current Signature Analysis (MCSA)
MCSA analyzes the stator current spectrum for sidebands at (1 ± 2s)Ăfâ around the fundamental frequency, which are characteristic signatures of broken rotor bars.
Question 4: When performing an overspeed test on a generator rotor, what is the typical overspeed percentage applied above rated speed per NETA and ANSI standards?
- 5%
- 10%
- 20% (Correct answer)
- 25%
Correct answer: 20%
Overspeed tests are typically conducted at 120% of rated speed to verify rotor mechanical integrity against centrifugal forces that could occur during governor failure.
Question 5: A three-phase motor shows unequal winding resistance readings of 2.1 Ω, 2.1 Ω, and 2.6 Ω across the three phases. What is the most appropriate action?
- Place the motor in service â the variation is within tolerance
- Investigate for a partial open winding or high-resistance connection in the high phase (Correct answer)
- Perform a hi-pot test to ground on each phase
- Replace the motor immediately without further testing
Correct answer: Investigate for a partial open winding or high-resistance connection in the high phase
A resistance imbalance of roughly 24% far exceeds the typical ±5% acceptance criterion and strongly suggests a partial open winding, bad splice, or high-resistance connection requiring investigation.
Question 6: When testing the insulation of a generator stator using a step voltage test, what does a sudden drop in insulation resistance at a specific voltage step indicate?
- The insulation is fully dry and in excellent condition
- A voltage-sensitive weak spot or partial discharge site that breaks down at that voltage (Correct answer)
- Normal capacitive charging of the winding
- The megohmmeter battery is depleted
Correct answer: A voltage-sensitive weak spot or partial discharge site that breaks down at that voltage
A sudden resistance drop at a specific voltage step reveals a weak spot in the insulation that undergoes partial or complete breakdown at that electric field stress level.
Question 7: What is the primary purpose of performing a load test on a motor after rewinding and before returning it to service?
- To verify that the motor develops rated torque and does not exceed rated temperature rise under full load (Correct answer)
- To check the insulation resistance after the varnish cures
- To balance the rotor dynamically at operating speed
- To verify proper alignment with the driven equipment
Correct answer: To verify that the motor develops rated torque and does not exceed rated temperature rise under full load
A post-rewind load test confirms that the motor achieves rated torque output and that temperature rise under full load remains within the insulation class rating.
NETA Motor and Generator Testing 5
During a no-load test on an induction motor, the power factor is measured at approximately 0.1 lagging.
What does this indicate about the motor?