Voltage Stress Modeling and Measurement for Random-Wound Machine Windings Driven by Inverters

This article studies the voltage stress in random-wound machine windings under steep-fronted voltage excitations from inverter drives. The voltage stress in this article includes overshooting voltages between any two conductors in the windings, and overshooting voltages between individual conductors and the stator ground wall. This stress may accelerate insulation degradation in inverter-fed machines compared to traditional AC machines. A high-fidelity finite element (FE) model was used to analyze individual conductors of random-wound windings and predict the voltage stress during switching transients. Experimental validation of the model was conducted using a purpose-made random-wound three-phase winding, designed to allow access to multiple coils and parallel strands. Firstly, the voltage propagation and the voltage stress for both single-phase and three-phase windings were simulated and tested under Pulse Width Modulation (PWM) voltage excitations without a rotor. Both voltage propagations among all the coils and individual turns were measured and compared with simulated results. The voltage stress on each part of the machine stator winding insulation system was tested and studied. Then, similar simulations and tests were conducted with a demagnetized rotor installed to study how the rotor saliency affects the voltage propagation and voltage stress. The measured winding voltage propagations for all scenarios generally agree with the FE simulated results.