NCCER 26410 EXAM – QUESTIONS AND ANSWERS | VERIFIED AND WELL DETAILED ANSWERS | PLUS
RATIONALES | GUARANTEED PASS | LATEST EXAM UPDATE
Core Domains
Motor Operation Principles
Motor Maintenance and Lubrication
Insulation Resistance Testing (Megger Testing and Polarization Index)
Troubleshooting and Diagnostics
Motor Installation, Alignment, and Startup
Motor Protection and Control Circuits
NEMA Standards and Insulation Classes
Safety and Professional Best Practices
Introduction
This comprehensive assessment covers the key principles of motor operation and maintenance as outlined in the
NCCER curriculum. The exam is designed to evaluate your knowledge of proper motor care, from installation and
alignment to diagnostic testing and repair. Questions will focus on the practical skills required to clean, test, and
perform preventive maintenance on electric motors, along with basic troubleshooting procedures for common motor
failures. The format includes both foundational knowledge questions and scenario-based challenges that test your
ability to apply concepts, comply with NEMA standards, and make sound professional decisions in the field. Mastery of
,this material is essential for ensuring the reliability and longevity of electric motors in industrial and commercial
settings.
Question 1
What is the recommended minimum Polarization Index (PI) for an AC motor to be considered acceptable for service?
A. 1
B. 3
C. 4
D. 2
🟢 Correct Answer: D. 2
🔴 Explanation: The minimum acceptable Polarization Index is 2. A PI below 2 indicates the insulation may be
contaminated or deteriorating and requires further investigation or corrective action.
Question 2
According to NEMA, under usual service conditions, the allowable frequency variation at the motor terminals is not
more than __________ of the nameplate frequency.
A. 10%
B. 15%
C. 20%
D. 5%
🟢 Correct Answer: D. 5%
🔴 Explanation: NEMA standard MG-1 specifies that motors should be able to operate successfully with a frequency
variation of plus or minus 5% from the nameplate rating.
,Question 3
If a motor is suspected of having a bad bearing, you should look for a __________.
A. low current draw
B. hot bearing housing
C. low pull-in torque
D. sparking at the brushes
🟢 Correct Answer: B. hot bearing housing
🔴 Explanation: Excessive heat in the bearing housing is a classic and reliable indicator of bearing failure due to
friction, loss of lubrication, or wear.
Question 4
The first time a motor is started, it should be allowed to run __________ before being placed into service.
A. with load for 10 minutes
B. without load for 1 hour
C. with load for 1 hour
D. without load for 10 minutes
🟢 Correct Answer: B. without load for 1 hour
🔴 Explanation: Running a new or rebuilt motor without load for at least one hour allows it to "run in" and allows
the technician to monitor for vibration, excessive temperature, and abnormal noises before the load is applied.
Question 5
Single-phasing in a three-phase motor means that __________.
A. one of the legs of a three-phase power source is lost
B. all phases are overloaded
C. two of the legs are shorted
D. none of the legs are functioning
🟢 Correct Answer: A. one of the legs of a three-phase power source is lost
, 🔴 Explanation: Single-phasing occurs when one phase of the three-phase supply opens. The motor will continue to
run but will draw excessive current on the remaining phases, leading to overheating and failure if not shut down
quickly.
Question 6
The loss of a single phase in a running polyphase motor will most likely cause __________.
A. failure of the remaining phase windings
B. bearing damage
C. insulation damage
D. excessive noise and vibration
🟢 Correct Answer: A. failure of the remaining phase windings
🔴 Explanation: The motor continues to run on the remaining phases, drawing significantly higher current. This
excessive current generates heat that rapidly degrades and destroys the insulation on those windings.
Question 7
When a motor is powered down and off-line, unusual bearing noise can be detected by using a(n) __________.
A. megger
B. infrared thermometer
C. stethoscope
D. vibration tester
🟢 Correct Answer: C. stethoscope
🔴 Explanation: A mechanic's stethoscope is used to listen to the internal sounds of bearings while the motor is
decelerating or being manually rotated, allowing the detection of grinding or clicking noises indicative of wear.
Question 8
An autotransformer is used in a motor starter circuit primarily to __________.
A. increase startup voltage
B. increase startup current
RATIONALES | GUARANTEED PASS | LATEST EXAM UPDATE
Core Domains
Motor Operation Principles
Motor Maintenance and Lubrication
Insulation Resistance Testing (Megger Testing and Polarization Index)
Troubleshooting and Diagnostics
Motor Installation, Alignment, and Startup
Motor Protection and Control Circuits
NEMA Standards and Insulation Classes
Safety and Professional Best Practices
Introduction
This comprehensive assessment covers the key principles of motor operation and maintenance as outlined in the
NCCER curriculum. The exam is designed to evaluate your knowledge of proper motor care, from installation and
alignment to diagnostic testing and repair. Questions will focus on the practical skills required to clean, test, and
perform preventive maintenance on electric motors, along with basic troubleshooting procedures for common motor
failures. The format includes both foundational knowledge questions and scenario-based challenges that test your
ability to apply concepts, comply with NEMA standards, and make sound professional decisions in the field. Mastery of
,this material is essential for ensuring the reliability and longevity of electric motors in industrial and commercial
settings.
Question 1
What is the recommended minimum Polarization Index (PI) for an AC motor to be considered acceptable for service?
A. 1
B. 3
C. 4
D. 2
🟢 Correct Answer: D. 2
🔴 Explanation: The minimum acceptable Polarization Index is 2. A PI below 2 indicates the insulation may be
contaminated or deteriorating and requires further investigation or corrective action.
Question 2
According to NEMA, under usual service conditions, the allowable frequency variation at the motor terminals is not
more than __________ of the nameplate frequency.
A. 10%
B. 15%
C. 20%
D. 5%
🟢 Correct Answer: D. 5%
🔴 Explanation: NEMA standard MG-1 specifies that motors should be able to operate successfully with a frequency
variation of plus or minus 5% from the nameplate rating.
,Question 3
If a motor is suspected of having a bad bearing, you should look for a __________.
A. low current draw
B. hot bearing housing
C. low pull-in torque
D. sparking at the brushes
🟢 Correct Answer: B. hot bearing housing
🔴 Explanation: Excessive heat in the bearing housing is a classic and reliable indicator of bearing failure due to
friction, loss of lubrication, or wear.
Question 4
The first time a motor is started, it should be allowed to run __________ before being placed into service.
A. with load for 10 minutes
B. without load for 1 hour
C. with load for 1 hour
D. without load for 10 minutes
🟢 Correct Answer: B. without load for 1 hour
🔴 Explanation: Running a new or rebuilt motor without load for at least one hour allows it to "run in" and allows
the technician to monitor for vibration, excessive temperature, and abnormal noises before the load is applied.
Question 5
Single-phasing in a three-phase motor means that __________.
A. one of the legs of a three-phase power source is lost
B. all phases are overloaded
C. two of the legs are shorted
D. none of the legs are functioning
🟢 Correct Answer: A. one of the legs of a three-phase power source is lost
, 🔴 Explanation: Single-phasing occurs when one phase of the three-phase supply opens. The motor will continue to
run but will draw excessive current on the remaining phases, leading to overheating and failure if not shut down
quickly.
Question 6
The loss of a single phase in a running polyphase motor will most likely cause __________.
A. failure of the remaining phase windings
B. bearing damage
C. insulation damage
D. excessive noise and vibration
🟢 Correct Answer: A. failure of the remaining phase windings
🔴 Explanation: The motor continues to run on the remaining phases, drawing significantly higher current. This
excessive current generates heat that rapidly degrades and destroys the insulation on those windings.
Question 7
When a motor is powered down and off-line, unusual bearing noise can be detected by using a(n) __________.
A. megger
B. infrared thermometer
C. stethoscope
D. vibration tester
🟢 Correct Answer: C. stethoscope
🔴 Explanation: A mechanic's stethoscope is used to listen to the internal sounds of bearings while the motor is
decelerating or being manually rotated, allowing the detection of grinding or clicking noises indicative of wear.
Question 8
An autotransformer is used in a motor starter circuit primarily to __________.
A. increase startup voltage
B. increase startup current
