Design and analysis of an integrated differential-common mode filter for on site motor bearing problems

In this paper an integrated differential-common mode filter is introduced for solving on site motor bearing problems that are induced by Pulse Width Modulated (PWM) inverters. This topology significantly reduces or eliminates common mode voltage impressed on the motor bearing race that causes fluting as a result of PWM switching action. Prior filtering techniques required either a special inverter design with dc midpoint accessible or a special motor design using a Wye connected machine with available neutral point to solve the bearing fluting problem. The new filter does not require any modification in inverter power structure or motor if applied in motor-drive systems. It also eliminates the need for an isolation transformer that is also used to mitigate the fluting problem. This topology can be included in the power structure design stage or utilized as an optional add-on module for troublesome field applications. Although the introduced filter topology is applicable to any application that involves a PWM inverter, discussion and analysis in this paper is restricted to motor-drive applications. Theoretical background, description of the bearing problem and filter design, and a complete set of supportive experimental results is included.