Output filters for AC adjustable speed drives

The standard industrial solution for adjustable speed drives (ASD) is the use of induction motors (IM) fed by voltage-source inverters (VSI). The inverter generates a pulsewidth modulated (PWM) voltage, with dv/dt values of about 6 kV/mu s or even more. In three-leg inverters for three-phase applications the occurrence of common-mode (CM) voltage is inherent due to asymmetrical output pulses [1]. Consequently, several secondary effects arise at the inverter's output: high-frequency conducted and radiated emissions, leakage current, motor insulation stress due to wave reflection [2], bearing stress due to bearing currents, acoustic switching noise. Depending on the specific application, the mitigation of some of these effects (or all) might be necessary. The common solution for mitigating the secondary effects at the output of PWM-VSI is the use of output filters [3],[5],[6]. Several types of output filters are used and this paper presents an overview of the various output filter solutions typically employed in industrial applications. The paper does not propose any new solutions and it is intended as a guide for practicing engineers and a source of information for researchers.