Remote and Automated Insulation Testing for VFD-Driven Motor Systems With High Resistance Grounding

The failure rate of motor insulation in variable frequency drive (VFD) motors is higher compared to that of mains-fed motors due to the increased electrical stresses inflicted by the VFD. At present, motor insulation testing in industry relies on offline testing at the motor terminals, which requires physical access to the de-energized motor. Considering the increased likelihood of failure and difficulty of off-line testing, remote and automated testing is desirable, as VFD motors become prevalent. A new concept for testing the ground insulation of VFD motors that leverages high-resistance ground systems is presented in this article. The main concept is to apply the insulation test voltage using the VFD and measure the leakage current from the grounding resistor with high resolution. The capacitance (C) and dissipation factor (DF) are calculated and trended over time to identify ground insulation degradation before failure occurs. This test can be embedded in the VFD and performed automatically without motor access or disassembly whenever the motor is stopped. Test methods for assessing the insulation condition of the ac grid-, dc bus-, and motor-side of the VFD system are proposed and verified experimentally.