Influencing factors and risks associated with the use of shaft grounding systems

With the widespread adoption of inverter-fed, variable-speed drive systems, parasitic high-frequency side effects are increasingly observed. These can lead to current flow through bearings and gearbox components, resulting in premature component and system failure. To mitigate so-called EDM currents (Electric Discharge Machining), shaft grounding systems are used in both industrial and automotive applications. These systems are intended to establish a low-impedance connection between the motor shaft and the stator housing, thereby protecting the bearing from harmful current discharges. In inverter operation with steep switching slopes, not only the resistance but the frequency-dependent impedance of the connection becomes relevant. Manufacturers and existing publications describe shaft grounding systems as highly effective, versatile, and nearly maintenance-free. In contrast to the few available studies – mostly conducted on new systems or without replicating realistic electrical loads - investigations at IAL show that parameters such as type of electrical load, lubricant, temperature, grounding technology, shaft surface and peripheral speed can significantly affect contact impedance and accelerate aging effects to varying degrees. For many configurations, a noticeable degradation- and in some cases unsuccessful mitigation - can already be observed after a short runtime. This presentation highlights these influencing factors, presents experimental results, and discusses the associated risks in the practical use of shaft grounding systems.