Monitoring of shaft voltages and grounding currents in rotating machines

The phenomenon of voltages and currents on motor and generator shafts has been known for the last 100 years [1, 2]. It affects motors, hydro generators and turbo generators. There has been an increase in occurrences and intensity of shaft current damages as machines became larger, particularly on high speed turbo generators. The build-up of voltage on the shaft and the resulting currents may damage bearings and could result in catastrophic rotating machine failure. Various methods have been used to minimize the impact of harmful levels of shaft voltage and current. This paper will provide details on causes of shaft voltages, methods to minimize them, and monitoring tools available to detect harmful conditions.