Studies of tracking on end turns

The breakdown of end turn insulation in high voltage rotating equipment due to “tracking” appears to be an often unrecognized cause of failure. End turns insulation of form wound coils often becomes coated with dirt when machines operate in contaminating environments. In the presence of high humidity, the contaminant coating be comes conductive. If a leak in the insulation permits current to flow in the coating, tracking and erosion of the insulation can occur. The erosion through the insulation may lead to a short between two adjacent conductors or a breakdown to ground near the core. When a motor fails due to tracking, the coils are usually badly carbonized on the end turns and therefore the analysis of the true cause of failure is often difficult. Chemical or corona attack on the insulation, high voltage surges and mechanical damage are among other causes which might be suspected when failures occur in the end turn area. Although these causes undoubtedly contribute their share of failures, the presence of dirt on the end turns of a motor which has failed requires that the possibility of tracking be definitely considered.