Slot-discharge activity in air-cooled motors and generators

In large motors and generators, the surfaces of the stator coils and bars are finished with a partially conducting coating to suppress discharge activity between the insulation surface and the core. However, slot discharge activity has been reported in some air-cooled motors and generators where significant areas of this coating have been eroded from the insulation surface at the high-voltage end of the winding. This paper considers how slot discharge damage may develop even where the surface coating is in good electrical contact with the stator core. Although the carbon-loaded paints often used to coat epoxy resin/mica paper insulation have acceptable surface resistivities, they are vulnerable to mechanical damage. It is shown that the removal of even a small area of the coating will initiate discharge activity and lead to the type of damage seen in service. The conditions necessary for slot-discharge activity are calculated and the results compared with measurements on stator coils removed from service.