A Transfer Function Approach to a Common Mode Filter (CMMF) Optimization in the PWM Inverter Supplied Motor Drives

The negative effects of pulse width modulated switch-mode inverters on electrical motors, such as failures in the motor windings isolation and electrical discharge machinery 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 optimization of the load ratio control common-mode filtering network at the inverter output. This topology has been chosen after a literature research and considering the practical experiences with different common mode filter (CMMF) topologies. First, a CMMF operation is explained. After that the filter is mathematically analyzed using Laplace domain, leading to the definition of the filter optimization 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.

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