New inverter output filter configuration reduces common mode and differential mode dv/dt at the motor terminals in PWM drive systems

In this paper a new inverter output filter is proposed for PWM drive systems. The proposed filter is shown to effectively reduce both the differential mode and common mode dv/dt at the motor terminals even in the presence of long motor leads. Reducing differential mode dv/dt reduces over voltages at the motor terminals and lowers the stress on the motor insulation. Lowering common mode dv/dt is shown to significantly reduce high frequency leakage currents to ground and induced shaft voltage in the motor. An important advantage of the proposed concept is that the filter can be installed within the inverter enclosure to achieve both the differential mode and common mode dv/dt reductions at the motor terminals. Thus the use of the proposed filter can contribute enhanced bearing life and improve reliability of PWM drive systems. Analysis, design equations and experimental results on a 480V, 20hp, PWM drive system are presented. The proposed filter configuration is an excellent candidate for many new and retrofit PWM drive systems.