Least Resistance

Bearing protector rings at a Kansas facility reduce the risk of variable-frequency drive voltage causing bearing distress.
Least Resistance
Don Koci, superintendent of water treatment systems, checks for vibration on a vertical hollow-shaft pump motor. (Although the pump is below the motor, the top of the motor provides thrust to the head shaft and is considered the drive end.)

Interested in Energy?

Get Energy articles, news and videos right in your inbox! Sign up now.

Energy + Get Alerts

Two months after the 10 mgd (design) Reverse Osmosis Water Treatment Center opened in Hutchinson, Kan., operators heard the whine of fluted bearings coming from a high-service, 250 hp vertical hollow-shaft motor that pumped to the city’s water towers.

The manufacturer replaced the bearings, but when the same problem occurred in several other motors, the company knew it wasn’t at fault. Electrical engineer Dan Biby, of Professional Engineering Consultants in Wichita, shipped some motors to a repair shop. Shortly after they were reinstalled, the whine returned. “We had never experienced this phenomenon in this magnitude,” says Biby.

Biby and Superintendent of Water Treatment Systems Don Koci searched the Internet for answers and found Independent Electric Machinery Co., a motor repair shop in Kansas City. It had developed the Vertical Motor Solution, a process that helps keeps stray variable-frequency-drive-induced shaft voltage from damaging bearings in vertical hollow-shaft motors.

Shaft grounding

Pumps at the facility lift contaminated water from an industrial area. Reverse osmosis and aeration remove most volatile organic compounds, and the water then is mixed with water from uncontaminated wells before being pumped to the water towers. Variable-frequency drives control the motors, adjusting flow rate and pressure.

“Isolating the problem with the pumps was an exhausting process,” says Biby. “Scott Wilkins and his team from Independent Electric were a great help.” When a technician measured shaft voltage, everyone looking at the repetitive and extremely rapid pulses applied to the shaft agreed that the drives were inducing harmful voltages and damaging the bearings.

“Once voltages exceed the resistance of the bearing lubricant, they discharge through the motor’s bearings, causing fusion craters, severe pitting and fluting damage,” says Wilkins. “Diverting the current makes these motors sustainable.”

Koci shipped one or two motors at a time to the shop for rehabilitation. After Wilkins’ team replaced the bad bearings, they installed AEGIS shaft-grounding protection rings (Electro Static Technology) on the head shafts above the lower guide bearings.

To prevent voltage from transferring to attached equipment before current was removed, technicians flame-spray welded a ceramic coating on the thrust-bearing carriers. They ground the carriers to tight tolerances to minimize wear on the coated surface and ensure proper bearing fit. The finished carriers have a hardness of Rockwell 50 C and more than 1 gigohm resistance at 1,000 volts.

Reliable pumping

The grounding ring’s design includes nanogap technology — rows of continuous, overlapping conductive microfibers encircling the motor shaft to provide hundreds of thousands of discharge points. As the fibers wear slowly, they conform to the shaft for excellent electrical contact throughout the life of the bearing. They also divest small amounts of grease, oil film and dust away from the shaft.

Electron transfer technology involves three processes working simultaneously to ground across gaps less than 2 nm, between gaps from 2 nm to 5 nm, and across gaps greater than 5 nm.

As the first 10 reconditioned motors returned, an Independent Electric representative at the plant confirmed that voltages were below the level at which they could damage the bearings. Once the motors were installed, Koci sent the remaining seven out for repair. For the plant’s four horizontal pump motors, technicians installed a grounding ring at the nondrive end and ceramic-coated bearings at the drive end.

The pumps have run without problems for more than a year. “The shaft-grounding rings weren’t on the market when we designed the plant,” says Biby. “The cost is insignificant when compared with the cost of new facilities. Furthermore, while variable-frequency drives can save 30 percent or more in energy, repairing or replacing motor bearings can wipe out those savings while diminishing the reliability of the entire system.” wso


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.