Shaft current on large wound rotor motor: Generation, detection, measurement and mitigation

This paper reports the discovery of elevated vibration level of one of six 5000 HP gearbox of a cement vertical roller mill (VRM) due electro-erosion damage on the gear teeth. The gearbox has been in operation for only less than two years and the lead time for ordering is at least 18 months. Initial investigation by third party consultant showed that the destructive current was not coming from the shaft of the main drive motor but from a VFD-driven motor of the separator indirectly linked to the gearbox of the VRM. After an exhaustive in-house measurement campaign, it was discovered that the voltage was generated by the rotor of the main drive motor despite the fact that the motor had an insulated bearing. The potential has enough energy to generate high current micro sparks to cause micro pitting on the gear surface. The current had no damaging effect on the motor sleeve bearings but migrated to the gear through the non-isolated coupling. All the data from the shaft current, harmonics and motor current signature measurement were analyzed to determine the source and mode of generation of the shaft voltage. After which, a permanent solution was implemented to mitigate the damaging effect of the generated voltage on the shaft.