Case history of shaft currents on a 1500 hp motor on PWM supply

This scenario starts with a 1500 hp forced draft fan boiler application in a paper mill powerhouse. It runs well for a year, then the installation is modified and "shaft current" motor bearing problems are identified. The installation (cabling and grounding) issues are corrected, but the bearing problem persists until a shaft grounding brush is installed. Even with the "shaft brush" installed, bearing frosting occurs within several months. This paper attempts to document an example of the "classic" EDM (electrostatic discharge machining) shaft current phenomena that leads to bearing fluting and ultimate failures. It does not cover the theory behind the shaft currents as this has been well covered in many previous papers. While answering several questions concerning this phenomena; many additional questions regarding practical solutions have arisen.