For six years, Scott Wilkins, manager of motor shop operations at Independent Electric Machinery Co. (IEMCO) in Kansas City, has overseen the reconditioning of hundreds of vertical hollow-shaft motors through a process known as the Vertical Motor Solution.
After replacing the bearings, his team installs a shaft grounding ring next to the motor’s guide (lower) bearing and applies ceramic insulation to the carrier that holds the thrust (upper) bearing in place at the motor’s drive end. Wilkins has not heard of a single motor that has failed since receiving this treatment.
Although destructive currents can occur in any motor, most bearing damage is in motors controlled by variable frequency drives (VFDs), also known as inverters or simply as drives. VFDs can save 30% or more in energy costs, but they often induce shaft voltages that discharge through the bearings, leaving fusion craters: pits in the bearing balls and race walls. Concentrated pitting at regular intervals along a race wall forms washboard-like ridges called fluting, which causes excessive noise and vibration.That usually means bearing failure is imminent.
This cumulative degradation is well-documented and believed to be caused by repetitive and extremely rapid pulses applied to the motor from the VFD’s non-sinusoidal power-switching circuitry. The phenomenon is called parasitic capacitance, capacitive coupling, and common mode voltage.
Unfortunately, repairing and/or replacing the bearings can wipe out any savings that a VFD yields.
“We often see the problem in the motors at new water or wastewater treatment plants,” says Wilkins. “General contractors and consulting-specifying engineers frequently end up with unhappy customers who discovered only after bearings failed that most warranties don’t cover electrical bearing damage. This leads to a lot of finger pointing, and typically the engineer and utility get stuck with the repair costs.”
Case in point: an RO plant
An electrical engineer with Professional Engineering Consultants (PEC) of Wichita, Kan., Dan Biby helped design a water treatment plant for the city of Hutchinson, Kan. The result of more than 20 years of negotiations between government agencies and polluters, the 10-mgd Reverse Osmosis Water Treatment Center is the solution to groundwater contamination. Pumps lift contaminated water from beneath an industrial area and remove most volatile organic compounds (VOCs) through reverse osmosis and aeration. That water is then diluted with water from uncontaminated wells across town before being pumped to the city’s water towers.
All of the pump motors are controlled by VFDs that provide adjustability in flow rate and pressure. Within two months of completion in 2009, one 250-hp vertical motor was making a lot of noise. It was the telltale whine of fluted bearings, and Biby found himself in charge of a two-year remediation project.
The motor manufacturer only replaced the first motor that went bad. A local repair shop botched repairs on others.