Are You Smarter Than Your Electric Motors?

2.

In “Principles of Large AC Motors” by the Electrical Apparatus Service Association (EASA) it is recommended to rotate the shaft monthly.

True
False

Correct Answer

A. True

Explanation

According to the Electrical Apparatus Service Association (EASA) in their publication "Principles of Large AC Motors," it is recommended to rotate the shaft of large AC motors on a monthly basis. This regular rotation helps prevent the shaft from becoming stuck or seized, ensuring that the motor remains in good working condition.

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3.

According to NEMA MG-1 the motor should not be run if phase-to-phase voltage imbalance is greater than 5%.

True
False

Correct Answer

A. True

Explanation

According to NEMA MG-1, it is stated that a motor should not be run if the phase-to-phase voltage imbalance is greater than 5%. This means that if the voltage difference between any two phases of the motor exceeds 5%, it is not recommended to operate the motor. This is because a significant voltage imbalance can cause various issues such as increased motor heating, reduced motor efficiency, and potential damage to the motor windings. Therefore, it is important to ensure that the phase-to-phase voltage imbalance is within the specified limit to maintain the motor's optimal performance and longevity.

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4.

The General System Limit for voltage Total Harmonic Distortion (THD) is 5%.

True
False

Correct Answer

A. True

Explanation

The General System Limit for voltage Total Harmonic Distortion (THD) being 5% means that the total harmonic distortion in the voltage should not exceed 5%. This limit is set to ensure that the quality of the voltage waveform remains within acceptable limits and does not cause any significant issues or disturbances in the electrical system. Therefore, the statement "True" indicates that the given limit of 5% for voltage THD is correct.

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5.

As stated in the EASA literature Understanding Energy Efficient Motors, the number one misconception about energy efficient motors is that “An oversized motor is less efficient.”

True
False

Correct Answer

A. True

Explanation

A motor runs most efficiently between 60-85% full load thus a 30HP load would be more efficiently supplied by a partially loaded 50HP motor than a fully loaded 30HP motor.

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6.

A Polarization Index test must be temperature corrected.

True
False

Correct Answer

A. False

Explanation

A Polarization Index (PI) test does not need to be temperature corrected. The PI test is a method used to assess the insulation condition of electrical equipment, such as motors and transformers. It measures the ratio of the insulation resistance at 10 minutes to the insulation resistance at 1 minute. This ratio provides an indication of the insulation's ability to withstand voltage stress. The PI test does not involve temperature correction because it is primarily focused on assessing the condition of the insulation, rather than the effects of temperature.

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7.

A variable frequency drive, connected to a 60 Hz distribution system, is supplying a 37 Hz output frequency to a squirrel cage induction motor. When performing current signature analysis for rotor evaluation of the squirrel cage induction motor, what would be the peak frequency of the current spectrum?

41 Hz
60 Hz
50 Hz
37 Hz

Correct Answer

A. 37 Hz

Explanation

When performing current signature analysis for rotor evaluation of the squirrel cage induction motor, the peak frequency of the current spectrum would be the same as the output frequency of the variable frequency drive, which is 37 Hz. This is because the current signature analysis is used to evaluate the performance of the motor at the specific frequency it is operating at. Therefore, the peak frequency of the current spectrum would match the output frequency of the variable frequency drive.

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8.

While viewing the current spectrum of a fully loaded squirrel cage induction motor you notice that the pole-pass frequency (Fp) sideband amplitude is 42 db. What fault zone would you be concerned with?

Rotor
Stator
Air Gap
None of the Above

Correct Answer

A. Rotor

Explanation

A high amplitude of the pole-pass frequency (Fp) sideband in the spectrum of a fully loaded squirrel cage induction motor indicates a fault in the rotor. This could be due to issues such as rotor bar cracks, broken rotor bars, or uneven air gap. The rotor is the rotating part of the motor and any faults in it can cause imbalance, vibration, and reduced motor performance. Therefore, a high amplitude of the Fp sideband in the spectrum suggests a fault zone in the rotor.

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9.

Cracked or broken rotor bars at the end ring of a squirrel cage induction motor will cause a reduction in reflected impedance resulting in a lower average inductance as measured from the stator windings.

True
False

Correct Answer

A. False

Explanation

Cracked or broken rotor bars at the end ring of a squirrel cage induction motor will cause an increase in reflected impedance resulting in a higher average inductance measured from the stator windings.

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10.

A fully loaded squirrel cage induction motor exhibits a 10% impedance imbalance and a 10% current imbalance, which fault zone is most likely the problem?

Rotor Fault Zone
Stator Fault Zone
Insulation Fault Zone
Power Circuit Fault Zone

Correct Answer

A. Power Circuit Fault Zone

Explanation

A fully loaded squirrel cage induction motor with a 10% impedance imbalance and a 10% current imbalance suggests a fault in the power circuit. The power circuit includes the electrical supply, wiring, and connections that deliver power to the motor. The imbalance in impedance and current indicates a problem in the circuit itself rather than in the motor's rotor, stator, or insulation. Therefore, the most likely problem is in the power circuit fault zone.

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Are You Smarter Than Your Electric Motors?

Reactive Maintenance is characterized by run to failure practices. Preventive maintenance is characterized by periodic or time based practices. Predictive maintenance is characterized by practices based on equipment condition.

Quiz Preview

In “Principles of Large AC Motors” by the Electrical Apparatus Service Association (EASA) it is recommended to rotate the shaft monthly.

According to NEMA MG-1 the motor should not be run if phase-to-phase voltage imbalance is greater than 5%.

The General System Limit for voltage Total Harmonic Distortion (THD) is 5%.

As stated in the EASA literature Understanding Energy Efficient Motors, the number one misconception about energy efficient motors is that “An oversized motor is less efficient.”

A Polarization Index test must be temperature corrected.

While viewing the current spectrum of a fully loaded squirrel cage induction motor you notice that the pole-pass frequency (Fp) sideband amplitude is 42 db. What fault zone would you be concerned with?

Cracked or broken rotor bars at the end ring of a squirrel cage induction motor will cause a reduction in reflected impedance resulting in a lower average inductance as measured from the stator windings.

A fully loaded squirrel cage induction motor exhibits a 10% impedance imbalance and a 10% current imbalance, which fault zone is most likely the problem?