A method to reduce common mode and differential mode dv/dt at the motor terminals in PWM rectifier/PWM inverter type adjustable speed drive systems

Abstract

In this paper, it is shown via analysis and experiments that common mode dv/dt transitions at motor terminals is twice as severe in a regenerative (PWM rectifier/PWM inverter) type ASD compared to a nonregenerative (diode rectifier/PWM inverter) type ASD system. It should be noted that higher common mode dv/dt at the motor terminals results in: (i) higher induced shaft voltage at each dv/dt transition, which can result in bearing currents and cause pitting/fluting of the bearings, and (ii) higher leakage currents to ground which can interfere with the plant zero sequence protection system. The paper proposes a method to reduce these effects. An input/output filter topology with interconnection to DC-link midpoint is proposed. This method reduces both common mode and differential mode dv/dt at the motor terminals, permits the use of long motor leads, and the filter components can be installed within the rectifier/inverter enclosure. Finally experimental results are presented on a 20 kW PWM ASD test stand.