Detection of stator winding stress relief coating deterioration in conventional and inverter fed motors and generators

There seems to be an increasing incidence of significant surface partial discharges (PD) in stator windings rated 6 kV and above. This surface PD causes bright white deposits on the stator coils, and is usually caused by poorly-made or poorly-applied partly-conductive and silicon carbide coatings. The problem is even more likely if the winding is connected to a voltage source-pulse width modulated inverter used in variable speed drives. If uncorrected, this surface PD leads to high ozone levels within the machine which degrades most metallic and organic components within the motor or generator enclosure. Eventually, a stator ground fault or rotor failure may occur. This paper reviews the mechanisms that may lead to surface PD in stator windings and discusses how PD can be detected using visual examination and ultraviolet imaging, as well as using condition monitoring tools such as electronic ozone monitors and on-line partial discharge measurements.