High-Frequency Modeling of Electrical Machines and Cables with the Help of an Impedance Analyzer


Since July 2022 an impedance analyzer from Keysight is part of the measurement equipment in the IsoLab at the IEM. It is used to measure the impedance spectrum of cables and electrical machines up to 100 MHz so that high-frequency models of cables and electrical machines can be parametrized and validated. 

  Picture of measurement setup Copyright: © IEM Impedance measurement of an induction machine

Power converters based on wide bandgap devices, including gallium nitride (GaN) and silicon carbide (SiC), are attractive for manufacturing electric drives for electric vehicles. Compared to standard silicon devices, SiC switches can operate at higher electrical field strength and temperature. In addition, owing to the extremely short turn-on and off times (10-20 ns), switching frequencies of up to 100 kHz can be realized. These features allow electrical machines to operate at higher voltages and modulating frequencies, thus achieving large power densities. However, these advantages come at a cost. The interactions between power converters and motors raise the electrical stress on the insulation system of electrical machines. The voltage impulses of the converter get reflected at the motor terminals since the cable characteristic impedance is not matched to the impedance of the stator. In the worst case, the voltage at the motor terminals can be twice the DC bus voltage. More importantly, an uneven distribution of electrical potential along the winding results. An accurate estimation of the maximal voltage difference between turns is crucial for designing the insulation thickness and maximizing the copper filling factor.

With the help of the impedance analyzer, high-frequency models of electrical machines and cables can be parametrized and validated. The influence of materials, environment factors, control algorithms, construction of electrical drives on the voltage distribution, and voltage overshooting behaviors can be accurately investigated.