End-turn grading for PWM-driven machines

Abstract

The end turn grading of a form-wound machine is coupled capacitively to the stator conductor so that only derivatives of the conductor voltage are relevant to the performance of the end turn grading. In other words, the key factors are the “slew rate”, i.e., the derivative of the conductor voltage with respect to time, and the capacitance per unit length between the stator conductor and conductor armor tape (CAT) and nonlinear stress grading (SG). Both the CAT and SG can participate in grading for high slew rates, and the peak field along the SG can be estimated from fundamental considerations.