Turn Insulation Capability of Large AC Motors Part 2 - Impulse Strength

The breakdown voltage of the turn insulation in seventeen motors of various types was measured for 0.1μs risetime impulses. The motors covered a wide range of voltage class, power rating, insulation type, age and winding design. For most motors, the breakdown voltages were 5 pu or higher which is many times the 1.5 pu strength for 0.1 μs surges suggested in an IEEE guideline [1]. Testing of individual coils indicated that some coils had very low breakdown voltages probably due to imperfections acquired during coil forming or stator winding. In general, the withstand strength of the motors was 50 to 100% higher than that of individual coils. The interface between the turn insulation and the groundwall was involved in most failures. Tests on two new motors indicate that manufacturers have the capability to build motors with impulse strengths of 10 pu or more.