In an effort to determine OEM motor manufacture’s perceptions and beliefs about electrical bearing damage, DP&A Sales performed a literature review of sorts to answer the question: Do OEM motor manufacturers acknowledge electrically induced bearing damage as being a significant problem. Our search results were surprising. The following is a word-for-word excerpt from a major OEM motor manufacturer’s article on electrically induced bearing damage.
“On rare occasions, operators in the process industry may note that their low voltage motors experience a current flowing through the motor bearings, especially when they are used with a variable speed drive (VSD).”
Basically, the OEM motor manufacturer is strongly implying that electrically induced motor bearing failure in VFD motors is a rare event.
DP&A Sales and Shaft Grounding Systems, Inc. performed a large survey to determine the incidence rate of electrical bearing damage in 18-month-old HVAC equipment. Vibration analysis was used to successfully detect fluted or frosted varieties of electrical bearing damage typically caused by capacitive discharge of shaft-to-frame voltage through bearings. The study population was 1,150 small AC motors in the 10-100hp range. The method was to survey all AC motors running in a specified area of the facility. The service was generally ventilation fans in clean rooms. Of the 1,150 motors surveyed, 150 were not powered by variable frequency drives (VFDs). The following is a graph of the results:
After 18 months of continuous operations, 225 of the 900 VFD motors were found to have electrical bearing damage resulting in a 25% incidence rate. After 18 months of continuous operation, the non-VFD powered motors only suffered a 0.65% incidence rate of electrical bearing damage. This result clearly indicates VFD powered motors are much more prone to electrical bearing damage than non-VFD motors and that the bearing failure incidence rate of VFD powered motors is very significant. Of note, a follow-up survey at 30 months of 100 motors powered by VFDs were found to have an incidence of electrical bearing damage of 65%.
This survey clearly indicates the incidence rate of electrically induced bearing damage in VFD motors is a much more common occurrence than what a major OEM motor manufacturer would lead an end user to believe. Electrically induced bearing damage is a significant problem in many installations and its prevalence will only increase as industry becomes more reliant on VFD powered motors. To our knowledge, the only cost effective and reliable method of eliminating electrically induced bearing damage is to provide a low impedance pathway from shaft-to-motor frame. DP&A Sales and Shaft Grounding Systems, Inc. have been providing best in class SGSTM shaft grounding systems for 30 years. They are the only proven long-term fix for electrical bearing damage and are particularly suited for critical applications as well as industrial applications where reliability, performance and maintainability are required.
