Common Mode Voltage based Stator Insulation Testing for Inverter fed Motors with High Resistance Grounding Systems

The failure rate of stator insulation is higher for inverter-fed motors than that of mains-fed motors due to higher electrical and thermal operating stresses. Considering that commercial insulation tests in the field rely on off-line testing at the motor terminals, remote and automated testing is desirable as inverter-fed motors become more prevalent. A novel approach for testing the condition of the ground insulation for inverter-fed motors by leveraging high resistance grounding (HRG) systems is proposed in this paper. HRG systems are becoming more common for drive systems, as they provide safe operation for the equipment and maintenance personnel, while providing service continuity under fault conditions. The main idea of the proposed method is to 1) apply common mode voltage to the ground insulation using the inverter whenever the motor is de-energized, and 2) measure the leakage current through the grounding resistor for performing the capacitance (C) and dissipation factor (DF) tests. C and DF measurements can be trended for identifying thermal aging and contamination in the insulation before ground faults occur. Experimental testing on induction motors shows that the change in C and DF due to insulation defects can be detected for providing remote and automated inverter-embedded insulation testing.