Are You Smarter Than Your Electric Motors?

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.

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.

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.

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.

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.

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.

Explanation
Reactive Maintenance is characterized by waiting for equipment to fail before taking action, while Preventive Maintenance involves regularly scheduled inspections and maintenance tasks to prevent failures. Predictive Maintenance, on the other hand, uses data and analysis to predict when equipment failures are likely to occur and takes action accordingly. Therefore, the statement that Reactive Maintenance is characterized by run to failure practices, Preventive Maintenance is characterized by periodic or time-based practices, and Predictive Maintenance is characterized by practices based on equipment condition is true.

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.

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.

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.