Field Experience Identifying Electrically Induced Bearing Failures

This paper evaluates field test measurements and installation details from a field case study of low voltage AC induction motor bearing failures when operated under a pulse width modulated (PWM) voltage source inverter. Recent experience suggests the steep wave fronts increase the magnitude of the above electrical problems, leading to motor bearing material erosion and early mechanical failure. Until recently, bearing failures from shaft currents were predominantly electromagnetically induced. This paper presents three sources for bearing currents and under PWM operation can be both electrostatic and electromagnetic induced. A field methodology for identifying each source and recommended mitigation is presented. First, a brief history of bearing failures is discussed. Next, the application and a system model presented. An extensive field case study from several motor locations is presented following discussion of a methodology and bearing model found useful in field investigations. Field measurements are reviewed indicating both electrostatic and electromagnetic fields are induced within the motor. The measurements and installation details collected from the various sites quantify the variables that contribute to the different measurement results between motor locations. Effects of system components on shaft voltages and bearing currents are evaluated from field measurements and compared to theory. Quantitative results of solutions to shaft voltage and bearing currents are presented.