Dan Biby learned the hard way. An ordeal concerning several inverter-controlled pump motors with bearing damage made him a believer in the need for shaft grounding.
Although destructive currents can occur in any motor, they are considerably more common in motors controlled by energy-saving inverters, also known as variable frequency drives or simply drives. VFDs can save 30% or more in energy costs, but whether used to control a motor’s speed or torque, they often induce motor-shaft voltages that damage bearings.
This cumulative degradation of bearings apparently is caused by repetitive and extremely rapid pulses applied to the motor from a modern VFD’s non-sinusoidal power-switching circuitry. The many names used to describe this phenomenon include parasitic capacitance, capacitive coupling and common-mode voltage.
In fact, the costly repair or replacement of failed motor bearings can wipe out any savings that a VFD yields and severely diminish the reliability of an entire system. Consulting-specifying engineers end up with unhappy customers who discover only after bearings fail that most warranties do not cover electrical bearing damage. This leads to finger-pointing, and typically the CSE and the end user get stuck with the repair costs.
Case in point
Biby, an engineer with Professional Engineering Consultants in Wichita, Kan., helped design a new water treatment plant for the city of Hutchinson, Kan. The Reverse Osmosis Water Treatment Center has the capacity to pump 10 million gal. of drinking water per day.
The result of more than 20 years of negotiations between government agencies and polluters, the plant is the solution to a groundwater contamination problem. Pumps lift contaminated water from beneath an industrial area and remove most of its volatile organic compounds through reverse osmosis and aeration. The water then is diluted with water from uncontaminated wells across town before it is pumped to the city’s water towers.
All the pump motors are controlled by VFDs that provide adjustability in flow rate and pressure.
Unfortunately, within two months of the plant’s completion in 2009, one of the high-service, 250-horsepower vertical motors that pump to the water towers was making a lot of noise. It was the telltale whine of fluted bearings, and Biby soon found himself in charge of a remediation project that would last more than two years.
“The motor manufacturer replaced the bearings on that first motor,” Biby recalls. “But when we soon had the same problem with the rest of the motors, the manufacturer said, ‘This isn’t a bearing problem, it’s something else.’”
Biby shipped some motors to a repair shop, but soon after they were reinstalled the noise started again. All parties agreed the repairs had been inadequate and Biby was able to recoup some of the cost. He then turned to Independent Electric Machinery Co., a Kansas City, Mo.-based motor repair shop.
“We had never experienced this phenomenon in this magnitude,” he says. “It was a pretty exhausting process to find out what was causing it. Once we finally agreed that drive-induced shaft voltage was the culprit, we started shipping the motors to IEMCO for repair.”
A proven solution
Through trial-and-error and hard work, IEMCO developed the Vertical Motor Solution, a process to prevent electrical bearing damage caused by stray shaft currents in vertical hollow-shaft motors.
For six years, Scott Wilkins, IEMCO’s manager of motor shop operations, has overseen the reconditioning of hundreds of vertical motors (most run pumps), none of which has experienced repeat bearing failure.
“The ‘green’ movement has dramatically increased the use of drives to save energy, especially in new construction,” Wilkins notes. “As a result, we often see the problem in the motors at new water or wastewater treatment plants.”
IEMCO usually works on motors with failed bearings, so the bearings have to be replaced during the process. After replacing the ruined, pitted bearings, Wilkins’ team installs a shaft grounding ring next to a vertical motor’s guide (lower) bearing and, using proprietary techniques, applies ceramic insulation to the carrier that holds the thrust (upper) bearing in place at the motor’s drive end.
For medium-voltage motors, Wilkins uses the larger AEGIS iPRO grounding ring, designed to accommodate generator/motor shafts up to 30 in. diameter.
Sharing the costs
Coordinating the remediation with Biby was Don Koci, Hutchinson’s superintendent of water treatment systems. The engineering firm and the city shared the costs. To keep the water plant running, they shut down and shipped only one or two motors at a time.
Before IEMCO worked its magic on each ailing motor, the shop sent a technician to Hutchinson to measure the shaft voltage. Back in Kansas City, the team replaced the bearings, coated the thrust-bearing carrier and installed a shaft-grounding ring next to the guide bearing. An IEMCO representative also was present at the plant for the reinstallation and startup of each reconditioned motor to confirm voltages were below the level at which they could damage bearings.
After the first 10 damaged motors had been successfully reconditioned, Koci sent the water plant’s remaining seven pump motors directly to IEMCO, at the city’s expense. To some degree, this was for preventive maintenance, since bearing damage was less advanced in some of these motors than in others. Four were horizontal pump motors, in which IEMCO installed an AEGIS ring at the nondrive end and ceramic-coated bearings at the drive end.
Today, the motors continue to run without problems.
Wilkins has seen serious electrical bearing damage in vertical motors as small as 30 horsepower, though he says the damage is more rapid and evident in motors above 100 horsepower. However, the elephant in the room is the growing awareness throughout the industry that these motors — all motors, in fact — could be built to withstand shaft currents in the first place. A few forward-looking motor manufacturers have recently added the AEGIS SGR Bearing Protection Ring, the same brand IEMCO uses, as a standard feature on certain models, but retrofitting still is the most common way to prevent electrical bearing damage.
When Biby and his colleagues started designing the water plant, AEGIS grounding rings were not yet on the market. But the Hutchinson experience made them rethink the way they deal with potential bearing damage.
“We’ve learned a lot,” he says. “We have since adjusted our motor specifications so that all new motors connected to VFDs will be equipped with shaft grounding rings. The main reasons are the costs of the downtime, the replacement costs and the work that’s necessary. If you’re specifying drives and motors, you should be dealing with this up front. If you install the rings beforehand, you can avoid these problems altogether. The cost is pretty insignificant when you look at the total cost of a large project.”
About the author: Adam Willwerth is the sales and marketing manager for Electro Static Technology and has extensive experience in industrial product development and commercialization.