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.”[pullquote]
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.”
Keeping the cooling fan shrouds and motor fins cleaned and free of debris helps tremendously in keeping the motor cool and therefore reducing the chances of the internal windings and other components from breaking down due to high temperatures, and cleaning the contacts regularly is a plus, says Billy Knox, RCM technician at Georgia-Pacific (www.gp.com) in Dudley, North Carolina.
Gary Finchum, electrical engineer and project manager at the AES (www.aes.com) E.W. Stout Generating Station in Indianapolis agrees. “The first step is to keep the motors cool,” he says. “Keep the motors clean and air flow paths clear. The second is having a vibration monitoring plan to find early signs of bearing failures.”
Optimizing electric motor life would be best achieved by addressing the most common root cause of failures from mechanical issues, explains Hal Scott, district manager at Baldor Electric (www.baldor.com). “Contamination concerns and mechanical bearing failures result in the majority of premature motor failures,” he says. “Contamination issues can be addressed using enhanced motor enclosure designs. The industry-standard, totally enclosed, fan-cooled motors have several optional enhancements to resist environmental conditions that be detrimental to causing premature motor failure. Features such as moisture-resistant paint, use of stainless steel housing designs, additional gaskets, shaft slingers, shaft seals, and encapsulating motor windings help to prevent premature failures. Several wash-down designs, severe-duty, and IEEE-841upgraded motors are all readily available to address different contamination concerns.”
Premature bearing failures can be reduced by incorporating a proactive predictive maintenance program, performing vibration analysis on a consistent basis, suggests Scott. “Proper alignment and balanced loads — couplings, impellers, fan wheels, sheaves — directly affect bearing life,” he says. “Critical loads that run continuously should have vibration analysis performed monthly. Motors that run approximately eight hours per day and are critical would be recommended to have vibration analysis performed on a quarterly basis. Three vibration measurements begin a trend which best defines the condition of a load and should allow for a window of time to repair or replace the motor or driven device.”
On intermittent-duty motors, protect the idle motor from condensation buildup, continues Scott. “On a TEFC motor, condensation buildup causes excess wear on the bearings and premature failure,” he warns. “To minimize the effect of condensation buildup, there are a couple of solutions. Where a consistent electrical supply is available, internal space heaters are recommended. Another solution is to run an intermittent-duty motor periodically. This allows the motor to heat up and therefore dissipate the moisture buildup. It is always important to maintain motors with the manufacturer’s recommended grease.”
Spare motors that are kept in inventory might need to be tested or turned periodically. “I don’t know that a motor would have issues sitting on the shelf, but I would recommend turning the rotor by hand once every six months on all motors in storage,” offers Carolina Hydro Technologies’ Dix.
Another option is using a tester, such as PdMA’s MCE motor circuit evaluator, says Generalovic. “To get a good read on a spare motor on the shelf, look for balanced readings, phase to phase, for both resistance and inductance,” he suggests. “Starting a motor-testing program using something like the PdMA tester is very cost-effective and beneficial.”
A megger tester is another option. “Putting the meg on the motor on a scheduled inspection can help check the motor and make sure that you don’t have an issue with wires drying out and breaking down causing a short and can help in detecting moisture or condensate inside the stator and winding,” says Georgia-Pacific’s Knox. “If moisture is present, this could cause a lot of problems such as rust, insulation breakdown, and other issues that can result in premature failures.”
AES’s Gary Finchum says testing of motors isn’t needed, as long as they’re kept in a clean, dry environment. “However, motors with antifriction bearings, if stored for more the six months, should be rotated 180° to prevent bearing damage,” he warns.
Baldor’s Scott agrees with the advantages of storing spare electric motors in a clean, dry, heated environment. “I have used the baseline of rotating the motor shafts every six months a minimum of three revolutions and placing the shaft keyway in a different position than the original position,” he adds. “It would be best not to place the motors directly on concrete and place on shelving resistant to any vibration caused from machine operation or vehicle traffic.”