NETA Motor and Generator Testing 5

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?

Accepted Answer

This is normal, as no-load operation is dominated by magnetizing reactive current

Answer

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?

Accepted Answer

Bearing fluting damage from electrical discharge through the bearing lubricant

Answer

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?

Accepted Answer

Motor Current Signature Analysis (MCSA)

Answer

MCSA analyzes the stator current spectrum for sidebands at (1 ± 2s)×f₁ 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?

Accepted Answer

20%

Answer

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?

Accepted Answer

Investigate for a partial open winding or high-resistance connection in the high phase

Answer

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.

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

NETA Motor and Generator Testing 5