Bearing Currents and Shaft Voltages in Electric Motors and Their Relationship to Pulse Width Modulation Drives

Modern variable frequency drives (VFD) have become the foundation of industrial electric motor control. However, observation of harmful effects has been found from the integration of these drives into existing systems. This thesis presents a comprehensive study on the causes and mitigation techniques of damaging phenomena within electric motors operated from pulse width modulation (PWM) drives. Primarily, this will include an investigation on high-speed rise times (dV/dt) produced by drives which cause damage to various motor components. Voltage wave-reflections and peak-voltages are reasons for the breakdown of cable insulation and stator winding burning. Other such phenomena stem from common-mode voltages; a source of unbalanced PWM voltage waveforms. Due to parasitics, shaft voltage accumulation and current flow throughout the motor enclosure. Bearing currents are of chief interest due to electrical discharge machining (EDM) that occur damaging the races and rolling-balls. Subsequent discharging of electrical current cause significant maintenance of machines. Models of each subsystem will be investigated to identify root causes of destructive events. Mitigation techniques will also be presented and will attempt to solve premature failings including, but not limited to: insulated bearings, conductive grease, shaft grounding brushes, PWM techniques, and dV/dt line filters