Partial Discharge Testing of Motor Stators Using an Impulse Tester and Oscilloscope
Fig. 2 illustrates the locations where there is a risk of partial discharge in a stator. When testing these at-risk locations while the stator is connected to the neutral point (Fig. 3), testing between shorted coils on the same phase can only be performed by applying an impulse voltage. By contrast, testing between coils and the core (ground), which are mostly
isolated, can be carried out at a high AC voltage such as that used by a withstand voltage tester, rather than at an impulse voltage.
Impulse testers are designed to detect layer shorts (shorts occurring across multiple turns caused by coil damage).These instruments have not provided functionality for detecting partial discharges. Demand for the ability to detect partial discharges at impulse voltages is rising due to the trend towards higher-voltage inverters.
An impulse tester is connected between the coils of a stator that is connected to the neutral point (Figs. 4 and 5). Two CT probes are attached to the ground side. CT1 measures from DC to 120 MHz, while CT2 measures from 4.8 kHz to 400 MHz. The current waveforms from these two CTs are observed using an MR6000.
• The current waveforms did not exhibit significant disturbances at a test voltage of 2000 Vpeak.
• A significant disturbance resembling a pulsating current was observed at a test voltage of 2600 Vpeak. That disturbance indicates a partial discharge.
• Partial discharges can be detected by observing current waveforms during impulse testing.
• If there is a low level of insulation, the discharge starting voltage will decrease.
• It may be possible to automate testing using the oscilloscope’s trigger or spectrum analyzer function.
• However, it will be necessary to exercise caution concerning grounding and the noise environment.