The Influence of the Rotor on Surge PD Testing of Low Voltage AC Motor Stator Windings

Partial discharge (PD) has been identified as one the main root causes of stator winding failures in low voltage (LV) random wound AC motors fed by PWM inverters. To ensure that PWM inverter-fed motors (type I) are PD-free under impulse excitation, the test procedure for acceptance testing of the stator insulation system was developed (IEC TS 60034-18-41). Although many successful cases of applying the TS have been reported, some concerns regarding the test procedure have been raised, and the TS is currently under revision. It is implied in the TS that the rotor has no impact on the surge PD test result, and testing is often performed without the rotor; however, it was observed that the surge PD test results can change with rotor insertion for some types of machines. The off-line surge PD test must be performed under conditions representative of actual operation (with the motor assembled) to guarantee absence of PD. Therefore, it is important to investigate the influence of the rotor, and to incorporate the findings in the test procedure, if necessary. In this paper, the influence of the rotor on the off-line surge PD test is investigated for AC machines, and it is shown that the PD inception voltage (PDIV) can change significantly depending on the rotor design. A detailed analysis is given along with experimental test results on a number of induction and permanent magnet (PM) synchronous motor samples with different rotor designs. Based on the observations made in this work, suggestions on the TS that can provide surge PDIV measurements under conditions closer to that of the actual operation with the rotor are given to help reduce the likelihood of false indications with off-line surge PD testing.