Surge Test-based Identification of Stator Insulation Component with PD Activity for Low Voltage AC Motors

Partial discharge (PD) has been identified as one of the root causes of stator insulation failure even for low voltage (LV) ac motors. For reliable operation of variable frequency drive (VFD) motors, a standard acceptance test procedure (IEC STD 60034-18-41) to ensure PD-free operation of LV motors was recently developed. Although the test can verify whether PD exists in the stator for a voltage surge with predetermined voltage level and risetime, the information on where PD activity is present in the stator insulation is not clearly provided. In this paper, a test method for applying variable risetime surge voltage is proposed for identifying the component of stator insulation where PD occurs in the LV random wound motor stator. The proposed test allows the user to locate whether PD is in the turn, phase, or ground insulation. This information is critical for lowering the PD inception voltage for improving the motor reliability through insulation system design/manufacturing or motor/drive system reconfiguration. The test is implemented with a 4.5 kV variable risetime surge generator prototype, and 4 LV ac motors are tested to show where PD is likely to occur in commercial LV motors.