Motor maintenance is critical to maintaining an industrial plant’s power supply, but many important steps help to keep a plant’s motors optimized and running.
“There are other obvious items to consider such as plugged air vents or excessive heat, lubrication, or power quality, but plants normally do not have programs in place to regularly check vibration and alignment,” says Charles Dix, engineer and co-owner, Carolina Hydro Technologies (www.carolina-hydro.com) in Providence, North Carolina. “These items are done on the initial installation, but not checked on a regular basis.”
Kirk Blankenship, CMRP, is senior asset care engineer at MillerCoors (www.millercoors.com) in Fort Worth, Texas. His experience is primarily with smaller NEMA frame motors. “I’ve seen studies say that the leading cause of motor failure is bearing failure to the tune of more than 50%,” says Blankenship. “This is certainly the case in my experience, but for me it’s been closer to 75% or 80%. So now the question becomes how do we maintain the motor bearings?”
While many mechanics will almost instinctively say the most important thing is lubrication, Blankenship says they’re dead wrong. “The first, most overlooked and underrated step in maintaining motor bearings is correcting for soft foot and proper alignment,” he explains. “Coincidently, alignment is also one of the primary steps of optimizing motor performance. This can be easily verified by recording the Ampere draw before and after a good laser alignment. Now the second step would be proper lubrication, right? Wrong. Most places I’ve been have two lubrication programs — over-lube and under-lube. This is driven by the two major lubrication philosophies it seems that most of us were taught at some point: ‘lube it until fresh grease comes out’ or ‘two shots is all you need.’” These extreme lubrication philosophies seem to prevail regardless of the size of the bearing or the amount of grease required to get fresh grease to come out.”
Few people understand what the right amount of lubrication is, explains Blankenship. “The real problem is that it is often no one’s job to figure out how much grease is the right amount,” he says. “Motors add complexity to the situation, as there are numerous types, sizes, and orientations that make it nearly impossible to have a single best practice on how to properly lubricate a motor. The point is that there is some research and documentation that needs to occur before a motor can be properly lubricated. Developing specific, well-defined lubrication and inspection instructions is the second most important step in maintaining a motor and keeping its performance-optimized.”
Grease lubrication for electric machines has a long and controversial history, explains Howard Penrose, Ph.D, CMRP, vice president engineering and reliability services for Dreisilker Electric Motors (www.dreisilker.com). “It is one of the most important topics in machine reliability,” he says. “The No. 1 cause of electric machine failure is over-greasing bearings, a close second is under-greasing. Most machine manufacturers subscribe to a relubrication process involving the motor rpm and time with specific amounts of grease applied to ‘replenish’ based upon the bearing size. It is always recommended that grease is not purged from a bearing, especially while the machine is running.”
The optimal method to grease a bearing is to perform replenishment based upon grease condition, suggests Penrose. “This would require grease sampling or very exact vibration or similar technology, which would detect the signs of lubrication breakdown,” he explains. “One relatively recent method for testing and replenishment is ultrasound lubrication. There are several challenges here. Some bearing manufacturers will not warranty bearings lubricated in this fashion. However, the same companies will often sell the technology. Second, this process involves adding lubrication until a sound level drops. This would mean that ‘soap,’ or grease, is being forced between the rotating components, which are not designed to manage this. Internal components of a bearing are oil-lubricated, with the grease being a carrier for the oil. Third, most motor manufactures call for bearings to be lubricated with the motor stopped and tagged out. Ultrasound lubrication calls for the machine to be operating.”
Motor testing off-line and on-line are the most critical steps for optimization and performance, explains Geoff Generalovic, a retired maintenance electrician with more than 35 years of industrial experience. “Each test picks up on different parameters in the motor circuit,” he says. “Off-line actually tests the whole motor circuit from starter to the motor; any changes over time due to motor deterioration or cable problems will be picked up doing this test. On-line testing under load is important to see how the motor performs electrically, showing a balanced system phase to phase is important. This also tests to see if the mechanical components are affecting the system adversely.”