Characterization of the Permittivity of Enameled Wires in Inverter-Driven Low-Voltage Electrical Machines

The use of inverters with high slew rates (du/dt) poses challenges to the insulation system of low-voltage electrical machines. Due to the fast-rising voltage pulses, a high overshoot at the terminals of the electric machine is expected. The electrical stress on the insulation system is determined by the electric field. Besides the applied voltage, the electric field is also depending on the permittivity of the insulation materials. If the critical electric field is exceeded, partial discharge can occur leading to premature failure of the machine. In the design process of the insulation system, the permittivity is usually considered as a constant value taken from data sheets or other literature. To predict the electric stress on the insulation appropriately, the permittivity has to be determined for the particular operation conditions. In this work, an approach to determine the permittivity at fast-rising bipolar voltage pulses is developed.