Turn Insulation Capability of Large AC Motors Part 3 - Insulation Coordination

The coordination of the surge strength of large AC motor stator windings with the surge voltages encountered during normal switching of the motors in utility generating stations is described in this paper. The surge strength of the turn insulation of motor windings was determined from destructive tests on 17 stators [6]. The surge voltage magnitude which can strike the motor's terminals was measured on 33 motors during normal operation in several different utility generating stations [5]. The surge strength of most motors exceeds the surge magnitudes which can occur during normal service. The only motors which have a surge strength below the highest surge voltages measured were either damaged from severe aging, or had poorly made turn insulation when compared to other coils in the same stator. Vacuum switchgear does not appear to pose a greater hazard to motor windings than conventional air-magnetic breakers, as long as the vacuum circuit breaker does not interrupt motor starting currents. Improved quality control tests are required to allow manufacturers to identify the few defective coils which may occur in a winding.