Filtering techniques to minimize the effect of long motor leads on PWM inverter fed AC motor drive systems

In this paper, filtering techniques are investigated to reduce the motor terminal overvoltage, ringing and dv/dt in inverter fed AC motor drive systems where long leads are required. Analysis is presented to show that the distributed L-C of a cable and the PWM inverter switching action lead to motor terminal voltage reflections and ringing that stress the turn insulation and contribute to bearing currents. First and second-order shunt filter designs are analyzed and compared. Detailed design equations to match the filter impedance to the cable surge impedance and to determine the filter component values are presented. It is demonstrated that a second-order shunt filter connected at the motor terminals greatly reduces the overvoltage stress and ringing, and also lowers the dv/dt of the PWM switching pulse. The filter design is shown to be solely dependent on the cable type (L/C ratio) and is independent of the cable length. Simulation and experimental results are presented to verify the proposed filter designs for 50 ft and 100 ft cable lengths