Having a motor test instrument does not mean that you will be able to diagnose problems.  Motor test instruments provide data that correlates to the interaction of the motor components and the characteristics of the circuit being measured. We will be exploring the measured parameters associated with de-energized electric motor testing. In this article, we will look at what can be determined by correlating resistance, inductance, and impedance data.

Most motor test instruments provide multiple data sets that can be utilized for fault localization. Referring to the table below we can correlate data to determine if a rotor, stator, or circuit fault exists.

Normal: Resistance, inductance, and impedance unbalance values are low and within acceptance tolerance.

High Resistance connection: A moderate to high resistance and impedance unbalance with normal or low inductance, indicates a connection problem in the motor or motor circuit. Split the circuit to determine if it is in the starter, interconnections, or motor.

Rotor Anomaly: Low resistance unbalance with moderate inductance or impedance unbalance indicates that the rotor is the source of the unbalance.  A stator fault would also affect resistance.

Stator Fault: A moderate unbalance in resistance, inductance and impedance indicates a winding anomaly in the stator.

Defective Power Factor Correction Capacitor: Power factor correction capacitors for individual load correction are installed in parallel with the motor leads usually on the load side of the starter. If one of the capacitors opens a severe reactance unbalance occurs. Resistance unbalance is low or normal with very high inductance and impedance unbalance. The motor will run normally, but- may cause a current imbalance on a common bus if several defective capacitor banks are present.

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