A method for common mode filter (CMMF) optimisation in the PWM inverter supplied motor drives

The negative effects of PWM switch-mode inverters on the electrical motors, such as failures in the motor windings isolation and electrical discharge machinery (EDM) leading to motor bearing failures are well known. Long cables between inverter and motor, very often found in, for example, railway applications, further increase these effects due to the cable reflections. The aim of this paper is to present an easy to use tool for practicing engineers providing mathematical analysis, design and optimisation of the LRC common-mode filtering network at the inverter output. This topology has been chosen after a literature research and considering the practical experiences with different CMMF topologies Firstly, a CMMF operation is explained, after that the filter is mathematically analysed using Laplace domain, leading to the definition of the filter optimisation criteria and in the end the simulation and experimental results are presented. This procedure was used on converters on different trains and has confirmed its value in operation.