A compact active filter to eliminate common-mode voltage in a SiC-based motor drive

This paper presents an active compensation device for common-mode (CM) voltage elimination in 3-phase space-vector pulse-width-modulated (SVPWM) inverters. The proposed device consists of a single-phase 2-level inverter (H-bridge) which supplies a compensating voltage to the inverter via a step-up common-mode transformer tied to all three phases at the output. The H-bridge active filter is supplied by a low voltage bus and switched several orders of magnitude faster than the inverter switching frequency. This device takes advantage of the direct knowledge of the switching pulses sent to the inverter to predict and generate the compensating voltage. A technique is employed to subtract the low frequency harmonics from the modulation of the H-bridge which allows for the size of the common-mode transformer to be reduced significantly. Small passive components are added to attenuate the active filter's PWM frequency content and thus produce an effective compensating voltage. This paper will review existing common-mode voltage compensation techniques and demonstrate that the proposed method is a logical choice for certain drive applications. Design considerations are included to provide understanding and guidance for implementation of the device, as well as MATLAB/Simulink simulation results to demonstrate the operation of the active compensation device. Final validation is presented through experimental results from a hardware prototype.