Investigation of LC Filter Unbalance in an Inverter-Fed Permanent Magnet Synchronous Machine Drive

Permanent magnet synchronous machines (PMSMs) are usually controlled through two-level voltage source PWM inverters. The output voltage of the inverter is in the form of voltage pulses between the positive DC-bus voltage and the negative DC-bus voltage. To smooth this inverter output voltages, different types of filters, typically LC-filters, are used in electric motor drives to mitigate the negative impact of the voltage pulses on the motor. The LC filters in electric motor drives are located between the inverter output terminals and the motor terminals. Theoretically, the filter elements, the inductance and capacitance are considered identical in each phase. However, in a practical scenario, it is difficult to have identical filter elements for all three phases. This non-ideal condition of filter elements amongst the three phases is considered as filter unbalance and this paper will investigate its impacts on the PMSM drive system. More specifically, the impact of LC filter unbalance on motor drive performance, including the current and torque ripples as well as the open loop frequency response, will be investigated through simulations and validated through experimental tests on a laboratory surface-mounted PMSM (SPMSM).