System electrical parameters and their effects on bearing currents

Abstract

This paper examines AC motor shaft voltages and resulting bearing currents when operated under pulse width modulation (PWM) voltage source inverters. The paper reviews the electrical characteristics of bearings and motors that cause shaft voltages and bearing currents. A brief review of previous work is presented, including a system model for electrical analysis of bearing currents. Relying on the work of a companion paper, the propensity for electric discharge machining (EDM) is determined by a design equation that is a function of system components. Pertinent machine parameters and their formulas are presented and values calculated for machines from 5 to 1000 HP. The effects of system elements on shaft voltages and bearing currents are evaluated experimentally and the results compared to theory. Finally, the paper presents quantitative results for one solution to the shaft voltage and bearing current problem.