The erratic nature of partial discharge measurements indicate surface contamination may be present on the windings of this motor.
Surface contamination can cause rapid winding failure. The contaminant bridges the gap between the high-potential surface and ground through microscopic cracks in the insulation. Each subsequent discharge helps to widen these cracks, moving rapidly inward — a process known as tracking.
In their Standard Handbook for Electrical Engineers, Fink and Beaty thoroughly describe the relationship between surface contamination, discharges, and insulation tracking:
Tracking damage has been traced entirely to the locally intense heat caused by leakage currents. These currents flow through any contaminated moisture film on the bridging insulating surface. As long as this film is fairly broad and continuous, the heat associated with the leakage current is spread over a wide area and is dissipated. However, heating promotes film evaporation. This causes the film to break up into small pools or islands. Each break in the film tends to interrupt a segment of the leakage current, causing a tiny arc. Even though the arc is small, severe local heating results. The intense heat of the leakage current arc is sufficient to cause a molecular and chemical breakdown of the underlying insulation.
Suggested actions
- Review partial discharge trends to determine the extent to which tracking already exists
- Run a polarization (Megger) test. Depending on the results, send the motor for cleaning & refurbishment. If cleaning cannot produce sufficient polarization index results, you may need to rewind the motor. When cleaning, specifically request that the shop avoid compressed air, which can drive dirt into the winding insulation, and solvents, which can dissolve or weaken the insulation.
- Review the cause of the surface contamination with your shop. For example, a common cause of surface contamination is grease that has become too thin and escaped the bearing, as well as overgreasing the bearing, which pushes grease out the back-side and into the motor’s winding cavity. If this is the cause of your contamination, discuss bearing grease/oil selection and lubrication service frequency with your shop with the goal of preventing future contamination of this motor.