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PAMS \ PDMA
PDMA – Motor Circuit Analysis (MCA) Testing
The goal of MCA is to ascertain the health of the motor. This assessment is accomplished through the detection of electrical imbalances in the motor and the detection of insulation degradation. The unbalances create stray, circulating currents through the motor. These circulating currents create excessive heat and lead to accelerated insulation degradation, inefficient operation and ineffective control techniques (in some motor types). Insulation degradation leads to shortened motor life and can lead to unsafe operating conditions.
MCA offline is most famous for the resistance-to-ground measurement. But other measurements make motor circuit defects easy to find. Measuring electrical characteristics like impedance, inductance and capacitance tell the analyst plenty about the condition of the windings. Inductance is a great indicator of turn-to-turn shorts. Capacitance to ground measures the amount of winding contamination (water, dirt, dust, etc.). Changes in each of these affect impedance (total resistance of an AC circuit). These characteristics are measured phase-to-phase and phase-to-ground and compared to each other and to percent change from baseline to identify motor circuit defects.
Some of the tests can serve as one-time go/no-go types of inspection. Some must be trended over time to understand the defect progression. The best strategy is the testing of motors on a set schedule. This allows you to properly trend these characteristics and gives the reliability program the best conditional probability of finding motor circuit defects.
All of the failure modes listed are very real and create unplanned downtime. A comprehensive failure modes-driven maintenance strategy for electric motors incorporates all of these test methods.
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