Evaluation and Testing of Turn Insulation of Large AC Motors

This paper reviews the results of tests performed on the stator windings of a 13.8 kV, 15000 hp variable speed slipring induction motor in order to assess its insulation condition and quantify the impulse strength of the motor winding. A surge voltage distribution test showed severe nonlinearity of turn voltage where up to 120% of the applied voltage was achieved across the first coil with 0.1-μs risetime. The effect of a surge capacitor was also demonstrated when a substantial reduction of the first coil voltage could be obtained at low risetime of 0.1 μs. Measurements of breakdown voltage (BDV) of coils at different locations gave indirect evidence that turn insulation degradation was most likely surge related. Finally, the paper contains a proposal of an alternative testing criteria to quantify impulse strength of turn insulation of large ac machines.