Inverter Side RL Filter Precise Design for Motor Overvoltage Mitigation in SiC Based Drives

This study investigates the use of resistor-inductor (RL) filters for motor overvoltage issues caused by cables connecting inverters to electric machines. Using existing filter design formulae and fixing the desired peak motor voltage, an RL filter was designed for a 15 kW industrial drive. This filter was used with a Si IGBT inverter and a matching between the design targets and the experimental results was found. Using the same filter with an SiC MOSFET inverter, a detrimental effect in overvoltage mitigation task was found. From simulations, it was noticed that combination of filter resistor parasitic inductance and high dv/dt causes unpredicted additional high-frequency motor overvoltages. These phenomena require higher filter inductance values with respect to the standard one, thus previous filter design formulae cannot be used. New design formulae, valid when inverters with high dv/dt are used, are proposed and experimentally validated. Interestingly, using the proposed formulae, it was discovered that the best filter resistance is not equal to the characteristic impedance of the cable. A design guideline that permits to optimally design the RL filter without simulation tool, considering the resistor parasitic inductance and the inverter dv/dt is finally proposed.