Proper shaft grounding solves motor bearing problems at Red Trail Energy’s ethanol plant. This contributed article appears in the April print issue of Ethanol Producer Magazine.
Something was destroying the motor bearings of the centrifuge and ventilation fans at the Red Trail Energy, 50 MMgy ethanol plant in Richardton, North Dakota. Plant maintenance personnel realized that the bearings on these large motors were failing long before they should. And the high cost of replacing the bearings soon caught the attention of the plant manager, who called in a specialist, Scott Fisher of Sustainable Grounding Systems, to determine what was causing the damage.
Fisher had worked at the Red Trail plant before when a North Dakota electrical contractor, Ystaas Electric Services, brought him in to evaluate the plant’s grounding systems. Ystaas enlisted Fisher’s services when plant workers discovered stray voltages on the cabinets of variable frequency drives (VFDs) that control the speeds of various motors in the production process.
An expert on the causes and prevention of equipment damage that result from stray currents and inadequate grounding, Fisher is well-known for his work with major companies involved in oil exploration, drilling and refining, petrochemical processing, grain elevators and even the Los Angeles train system. He learned his craft while working in Europe for a company that was involved with grounding systems for various applications including high-speed rail and military installations.
“Without proper grounding, stray electrical currents can do tremendous damage. In fact, these types of currents are becoming a huge problem, and dealing with them is a highly specialized field,” Fisher notes. Because the problems are not maintenance oriented, most electrical contractors don’t know how to solve them. The contractors can get system resistance down to the 25 ohm level required by National Electric Code. Fisher was trained to get systems down to 5 ohms or less.
Diagnosing the Problem
When Fisher arrived, he took shaft voltage readings on the 30 HP and 150 HP centrifuge motors and the 500 HP ventilation fan motor. These motors exhibited bearing damage long before reaching the L10 lifespan. He knew right away that the culprit was stray currents—VFD-induced voltages that were building up on motor shafts and discharging through motor bearings. And
he knew just how to deal with them.
Fisher knew that VFD-induced shaft currents discharge to ground along the path of least resistance. He also knew that without long-term shaft grounding, that path is typically through the motor bearings. He recommended installing AEGIS Shaft Grounding Rings on all motors controlled by VFDs to channel these harmful discharges away from bearings and safely to ground.
Fisher installed split grounding rings on the 30 HP and 150 HP centrifuge motors. These rings come in mated halves that allow fast, easy installation on in-service motors without having to decouple the motor from attached equipment. After removing any dirt, corrosion and paint that might interfere with adhesion and conductivity, Fisher installed the rings using conductive epoxy.
Because of its large size, Fisher recommended an AEGIS PRO Series Ring for the 500 HP ventilation fan motor. Designed for large low-voltage motors and medium-voltage motors, these rings have six rows of conductive microfiber brushes that provide high current capacity. Fisher installed the larger grounding rings using universal PRO Series mounting brackets.
Once the installations were completed, he took follow-up readings on the running motors to check shaft voltage levels. Readings that were as high as 19 volts before the grounding rings were installed, dropped to less than 1.3 volts—too low to cause premature bearing damage.
In addition, Fisher recommended the added protection of electrically “bonding” all motors to an electrolytic deep-well ground rod system that would guarantee stable resistance through the changing seasons.
“Red Trail has not had any problems since we installed the AEGIS Rings—no more high-pitched squealing from damaged bearings. And costly downtime has been minimized,” Fisher says.