Automated Testing for Early Identification of PD in the Stator Insulation of Low Voltage VFD Motors

Electrical stresses in industrial variable frequency drive (VFD) motors are increasing with the higher voltage and dv/dt levels, and with the advent of wide bandgap power devices. This increases the likelihood of partial discharge (PD) in VFD motors, and is expected to increase the risk of stator insulation failures in low voltage (LV) motors for which the insulation is not resistant to PD. To ensure that PD does not occur during operation, LV VFD motors are qualified for PD-free operation in the design or manufacturing stages. However, the PD inception voltage of qualified LV motors decreases with insulation aging exposing them to the risk of PD-induced insulation failure. In this paper, an automated off-line test method for VFD-embedded PD testing is proposed. The main idea is to perform PD testing whenever the motor is stopped at a voltage higher than the operating voltage to identify PD activity in the insulation before it occurs during operation. A series impulse and common mode voltage tests are proposed for stressing the different components of insulation for early identification of PD in terminal-/neutral-end ground, phase, and turn insulation. Testing on a LV, VFD motor verifies that PD can be identified whenever the motor is stopped to provide early warning of insulation failure.