A Novel Output Filter Topology to Reduce Motor Overvoltage

When an induction machine is supplied with a voltage-source pulsewidth-modulation (PWM) inverter, a ringing voltage overshoot appears at motor terminals due to steep PWM voltage pulse propagation along the motor cable. It has been reported that in particular situations, when very long motor cables are used and retrofit installations, the overvoltage could stress motors with poor insulation. This paper presents a novel inverter output filter topology consisting of an LC filter which has the star point of the common-mode capacitors clamped to the + and - dc-link rails by the means of two diodes. The proposed filter drastically reduces the voltage overshoot at the motor terminals, thus reducing motor insulation stress. This is a cheap alternative to more known industrial solutions.