Predictive Maintenance Techniques Flashcards

Motor Current Signature Analysis (MCSA) detects broken rotor bars primarily by identifying:

Answer: Sidebands at line frequency ± slip frequency × number of pole pairs around the fundamental

Broken rotor bars produce current sidebands at f_line ± n×s×f_line, where s is slip and n is the pole pairs, which appear as symmetric sidebands around the fundamental in the current spectrum.

Insulation resistance testing (Megger test) on a motor winding measures resistance in megohms; a reading that has trended downward over successive tests most likely indicates:

Answer: Moisture ingress or contamination degrading winding insulation

Declining insulation resistance trend is a key indicator of moisture absorption, contamination, or thermal aging breaking down the winding insulation, increasing the risk of ground fault.

Partial discharge (PD) testing is primarily applied to electrical assets rated above which voltage level, because PD is insignificant at lower voltages?

Answer: 2,300 V

Partial discharge becomes a significant insulation degradation mechanism at medium voltage (typically ≥2,300 V); at low voltage the electric field stress is insufficient to sustain PD activity.

In power quality analysis, which parameter quantifies the ratio of harmonic content to the fundamental frequency component in a waveform?

Answer: Total Harmonic Distortion (THD)

THD is calculated as the ratio of the root-mean-square of all harmonic components to the fundamental component, expressed as a percentage, quantifying waveform distortion.

Surge (high-potential impulse) testing on motor windings is specifically designed to detect which type of defect?

Answer: Turn-to-turn insulation failures within a coil

Surge testing applies a high-voltage impulse to compare waveform patterns between phases; differences indicate turn-to-turn insulation weaknesses that Megger testing cannot locate.

When analyzing motor current data, a significant increase in the amplitude of sidebands at frequencies corresponding to 'slot harmonics' around the supply frequency indicates:

Answer: Rotor eccentricity or air gap variation

Slot harmonic sidebands in the motor current spectrum are amplified by air gap variations caused by rotor eccentricity, making them useful for detecting dynamic or static eccentricity.

A 'Dielectric Absorption Ratio' (DAR) in insulation resistance testing is calculated as the ratio of the 60-second reading to the 30-second reading; a DAR of less than 1.0 is significant because it indicates:

Answer: Conductive contamination or moisture-saturated insulation

Healthy insulation shows resistance that increases over time as polarization currents diminish; a DAR < 1.0 means resistance dropped, indicating conductive contamination or wet insulation dominating leakage current.