Proper Shaft Grounding Solves Motor Bearing Problems at Ethanol Plant
Something was destroying the motor bearings of the centrifuge and ventilation fans at the Red Trail Energy Ethanol Production Plant in North Dakota. Plant maintenance personnel realized the bearings on these large motors were failing long before they should have been. The high cost of replacing the bearings soon caught the attention of the plant manager, who called in a specialist to determine what was causing the damage.
When the Red Trail plant's grounding systems had previously been evaluated, stray voltages on the cabinets of variable frequency drives (VFDs) that control the speeds of various motors in their production process were discovered. Without proper grounding, stray electrical currents can do tremendous damage to a plant's systems. The National Electric Code requires system resistance to be at the 25-ohm level, but in some cases can be down to 5 ohms or less.
Diagnosing the problem
Shaft voltage readings were taken on the 30 HP and 150 HP centrifuge motors and the 500 HP ventilation fan motor. These motors exhibited premature bearing damage - long before reaching the L10 life of their bearings. The cause was diagnosed as stray currents - VFD-induced voltages that were building up on motor shafts and discharging through motor bearings.
Implementing a long-term cure
VFD-induced shaft currents discharge to ground along the path of least resistance, and without long-term shaft grounding, that path is typically through the motor bearings. It was recommended to install shaft grounding rings (SGRs) on all motors controlled by VFDs, to channel the harmful discharges away from bearings and safely to ground.
SGRs come in mating halves to 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, the rings were installed on the 30 HP and 150 HP centrifuge motors using conductive epoxy.
Because of its large size, rings with 6 rows of conductive microfiber brushes providing high current-capacity were installed with mounting brackets on the 500 HP ventilation fan motor.
Once the installations were completed, follow-up readings were taken on the running motors to check shaft voltage levels. Readings that were as high as 19 volts before the rings were installed, dropped to less than 1.3 volts (too low to cause premature bearing damage).
In order to guarantee stable resistance through the changing seasons, the recommendation was made to add the protection of electrically "bonding" all motors to an electrolytic deep-well ground rod system.
Since the installation of the rings, Red Trail has not had any problems with their motor bearings; no more high-pitched squealing from damaged bearings, and costly downtime has been minimized.
Adam Willwerth is the sales and marketing manager at Electro Static Technology
Electro Static Technology | www.est-aegis.com