Partial discharges (PD) are a strong indicator of a motor’s stator insulation health. As the magnitude and count of PD increase, stator insulation deteriorates, or degrades, more rapidly. If not addressed, partial discharges can result in catastrophic stator winding failure.
PD levels tend to decrease during the first 12- to 18-months of a motor’s service life, level out, and then increase rapidly as the motor approaches failure. While exact time-to-failure cannot be predicted, a doubling in PD magnitude over a six-month period indicates rapidly deteriorating stator insulation.
The type of machine does not impact expected PD magnitudes, enabling us to compare this motor to Iris’s database of more than 60,000 PD test results. Motors@Work generates two sequential alerts based on comparing PD peak magnitude (Qm) to Iris’s database: The first alert occurs when Qm exceeds the 90th percentile, meaning the magnitude of discharge detected is higher than 90% of the test results on similar machines. The second alert occurs when Qm exceeds the 95th percentile, meaning the magnitude of discharge detected is in the top 5% of all test results on similar machines.
Suggested actions
- Download test data from your Iris device to your computer and use Iris’s PDTracPro & PDView software to review 2D and 3D plots of the detected discharges; pulse characteristics visible through these plots can predict void location and cause:
- PD pulses of equal magnitude and opposite polarity clump near 45° (negative pulses) and 225° (positive pulses) indicate voids within the groundwall insulation. Machines that were improperly impregnated, or that have been operated for several years at high temperatures, tend to develop voids within the groundwall insulation.
- PD pulses with an observable predominance of negative pulses clumped near 45°, during the positive portion of the AC cycle, indicate voids between the copper conductor and insulation. A machine with a frequently cycled load, or severe overheating, develops voids near the copper conductors.
- PD pulses with an observable predominance of positive pulses clumped near 225° indicates voids near the core iron. Loose coils, poor semi-conductive coatings, and problems with the grading/semicon interface can all lead to surface discharge between the stator bar and the grounded core iron, often called slot discharges.
- Erratic-looking PD pulses and those shifted 30° (based on the phase rotation of the machine) from classic phase positions (i.e., 15°, 75°, 195°, and 255°) indicate end-winding discharges. Surface contamination or inadequate spacing in the end arm area can lead to partial discharge activity in the end-winding. The erratic or phase-shifted nature of these pulses comes from their phase-to-phase – as opposed to phase-to-ground – reference.
- Conduct polarization index, surge, and/or hi-pot testing, as appropriate based on your motor type, to gauge overall stator insulation health
- Inspect your motor’s stator insulation visually, looking for signs of discoloration, cracking, loose blocking materials, ozone formation, white powder, and slot or turn-to-turn discharges
- If these additional tests and visual inspections indicate that PD predominantly occurs near the conductors, near the iron core, or in the end-windings, perform maintenance to lower PD activity, such as rewedging, cleaning, dip and bake, and/or repairing the voltage stress coatings. Unfortunately, rewinding is the only option for motors where PD predominantly occurs within the groundwall insulation.
For more assistance with understanding your partial discharge results, please contact Iris Power.