Operations & Management
A motor’s best friend: Cement maker goes green with bearing protection ring
February 14, 2011 by Electro Static Technology
It seemed as though nothing could stop the squealing. Not that the Monarch Cement Company’s huge ball mill wasn’t already loud. Powered by a 5,000-HP motor, it pulverizes 100 tons of clinker (a burned mixture of limestone and shale) per hour. But the squealing was not what Randy Riebel wanted to hear. As electrical supervisor at Monarch’s plant in Humboldt, Kansas, he knew the noise meant the motor’s bearings were going – again.
In fact, the sound of chronic bearing damage was all too familiar at the plant, which has the capacity to produce 1.3 million tons of cement a year. Since 2001, when the ball mill was new, its motor bearings had been replaced three times.
"We kept greasing those bearings, but they kept on squealing," Riebel recalls. "We knew that if we waited too long, the bearing race walls would become fluted like they had in the past, and we weren’t looking forward to another replacement because of all the expense and downtime. It takes at least 10 days to pull that motor – it’s a major production. Sometimes we have to hire help, rent a hoist to put it on a truck, and take it away to be rebuilt. So this time [summer of 2009] I decided to try something else."
The "something else" was the AEGIS iPRO Bearing Protection Ring, manufactured by Maine-based Electro Static Technology (EST). By safely channelling harmful electrical currents away from bearings to ground, the iPRO extends the lives of medium-voltage motors and generators, thus improving the reliability of entire systems in which they are used. It is available in a range of sizes to accommodate generator/motor shafts up to 76 cm in diameter.
Maintenance-free, the AEGIS iPRO is ideal for medium-voltage motors that drive pumps, compressors, mixers, shredders, conveyors and other machinery used in mining, food processing, wastewater treatment, petrochemical refining, and many other high-current applications. The iPRO also protects the bearings of generators in both utility and on-site power generation systems.
Riebel had been discussing electrical bearing damage with Scott Wilkins, manager of Motor Shop Operations for Independent Electric Machinery Company (IEMCO), a local motor and equipment repair shop. Wilkins recommended the iPRO, and Riebel had IEMCO install two of them on the ball mill motor. While for most large motors EST recommends installing an iPRO in the drive end and insulation on the non-drive end, for some large motors – especially those that do not have insulation designed into them or where insulation cannot be easily installed – EST recommends installing iPRO rings at both the drive end and the non-drive end of the motor.
Riebel and Wilkins chose the iPRO split-ring model, which is designed to facilitate field retrofits. The mating halves of each iPRO were installed around the motor shaft without the need to decouple the motor from the mill.
A family-owned business founded in 1906, IEMCO sells, services, repairs and tests motors, generators, hoists, welders and electrical distribution switchgear. IEMCO’s main office in Kansas City, Kansas, has a fully equipped machine shop. The company also has five other service centres in the region. Because they deal with large motors routinely, IEMCO’s personnel are well aware of the severe damage shaft currents can cause to motor bearings.
Mitigating electrical bearing damage
If not diverted, shaft voltages can discharge through bearings, pitting the balls and race walls. Without long-term bearing protection, concentrated pitting at regular intervals along a race wall can cause washboard-like ridges called fluting, a source of noise and vibration. The eventual result is motor failure.
Ironically, some products designed to protect bearings, such as conventional spring-loaded grounding brushes, require extensive maintenance themselves. Others, such as insulation and ceramic bearings, can shift damage to connected equipment.
To boost the electron-transfer rate, the iPRO’s entire inner circumference is lined with multiple rows of conductive microfibres. Locked securely in the ring’s patented AEGIS FiberLock channel, these microfibres completely surround the motor shaft, providing millions of discharge points for harmful shaft currents and creating the path of least resistance that effectively diverts these currents away from bearings to ground. The microfibres themselves are specially engineered for exceptional flexibility to prevent breakage and ensure that the ring will last for the life of the motor.
A nagging, widespread problem
Contractors and retail home-improvement stores in six midwestern states depend on the Monarch plant, which sends cement by truck and train to its terminals in Des Moines, Iowa, and Dodge City, Kansas. The terminals distribute the cement to 13 Monarch subsidiaries, which sell it in their respective areas. Some of the cement is sold in bulk, some in bags. And some of it is further processed by the subsidiaries, which fabricate building products or add stone and sand to produce ready-mix concrete.
When Monarch was founded in 1908, chunks of blasted limestone ("shot rock") up to one metre across were loaded by hand into mule-drawn carts. Now this limestone is moved by huge front-end loaders, 50-ton dump trucks, and conveyors to be processed by a series of computer-controlled crushers, kilns and mills until it is as fine as face powder.
Most of the processing machinery is powered by electric motors, and the problem of chronic bearing damage is by no means limited to the plant’s ball mills. Many of the motors are controlled by variable frequency drives (VFDs), which induce additional high-frequency currents on motor shafts. A fan or pump motor tends to use less power if the input is modulated by a VFD, but the benefits of improved efficiency are lost if the motor keeps breaking down.
Such breakdowns were recurring headaches for Riebel, but because the two AEGIS iPRO rings installed in 2009 appear to be protecting the bearings of the ball-mill motor, he has since had IEMCO install the iPRO on nine more motors that had to be removed from service.
A case in point is a VFD-controlled cooler-vent fan where the 300-HP motor had to be replaced frequently for almost eight years. Every time, the kiln had to be shut down for at least a day. The old motor was removed, and the rebuilt spare motor had to be aligned and coupled.
"We’d send the pulled motor out to be rebuilt, but then three to six months later we’d have to do the same thing all over again," says Riebel. "For the cooler vent fan, motor we tried insulation on both bearings. With the insulation, the motor lasted two years between breakdowns. The shunted electricity might have hurt the bearings in the fan itself. Insulation just pushes the problem on down the line. The electricity has got to go somewhere if it’s not grounded. The iPRO has given this fan a fresh start."
"We didn’t really realize what the problem was," Riebel reflects. "There wasn’t much information available about electrical bearing damage. We just knew that bearings would fail and the motor would overheat, but we were not looking to see why. Again and again, we just sent the motor out, got it rebuilt, and put it back in service. We didn’t know the root cause."
Solved by a green technology
It is now Monarch policy to have IEMCO add the iPRO ring in the shop whenever a VFD-controlled Monarch fan motor is overhauled. Another such installation was on the 2,250-HP motor for an ID (induced-draft) fan that pulls kiln-heated air through a roller mill, to dry the limestone and shale during the raw grinding process. Other motors that now have iPROs include four at the plant’s kilns, where air is forced in and out: two 2,000-HP ID fan motors and two 1,000-HP baghouse (dust-collecting) fan motors.
Also, because a cement plant is a very dusty place and many motors are outdoors, Monarch has begun to specify that some of its new motors must come equipped with the AEGIS Severe-Duty SGR Bearing Isolator Shaft Grounding Ring, another Electro Static Technology product. As in the iPRO, the secured conductive microfibres of the AEGIS Severe-Duty SGR completely surround the motor shaft for efficient grounding, but this model has a built-in IP56 non-contact isolation seal to provide extra protection from dust, water and other contaminants.
Monarch’s maintenance manager, Mark Pily, authorized the purchase of the plant’s first motor with a factory-installed Severe-Duty SGR, after consulting IEMCO’s Wilkins. As of this writing, a 200-HP air-compressor motor is the only such motor in operation at the plant.
"We want to keep the bearings clean because we push that motor really hard," Riebel explains. "We usually lose that motor because of winding failure. I think most of that is caused by the bearings starting to fail, which causes the motor to overload."
An ounce of prevention
Using a voltage probe and oscilloscope, Riebel periodically takes shaft voltage readings on all of the plant’s motors with grounding rings. He is pleased with the results because the readings are low, indicating that the rings have reduced potentially damaging shaft voltages.
Time will tell exactly how much money the rings will save Monarch overall, but Riebel is convinced the AEGIS iPRO provides effective, long-term bearing protection that reduces the costs of downtime and motor maintenance.
"So far, so good," he says. "On the 5,000-HP, since the last set of bearings only lasted a year, chances are we would have noticed problems by now, but we haven’t had any – no squealing."