Investigation of Turn Insulation Failure Mechanisms in Large AC Motors

In old, large ac motors, turn insulation failure may result from rubbing of the turns against one another. A purely electrical failure might be puncture of the turn insulation (usually in line-end coils) by steep-fronted surges impinging on the motor terminals. Another mechanism which could cause a similar failure has been identified. This third mechanism involves degradation of the turn insulation by partial discharges (which are usually most severe in line-end coils) in the stator end winding area. Results of destructive tests on two motors as well as early results from a program to monitor voltage surges in service are presented. This work was partly funded by EPRI.