Grounding rings on plant motors mitigate potentially damaging shaft voltages

Monarch Cement Company’s plant in Humboldt, Kansas.

It seemed as though nothing could stop the squealing. Not that Monarch Cement’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 wasn’t 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 can produce 1.3 million tons of cement a year. Since 2001, when the ball mill was new, its motor bearings had to be 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, in the summer of 2009, I decided to try something else.”

This is the ball mill on which the first two grounding rings were installed at the Monarch Cement Company plant. The 5,000-hp motor bearings failed regularly until an AEGIS iPRO Bearing Protection Ring (inset) at each end of the motor diverted damaging voltages. These on-site retrofits were accomplished without the need to decouple the motor from the mill, thanks to the use of a split-ring version of the iPRO.

The “something else” was the Aegis iPro bearing protection ring, manufactured by Maine-based Electro Static Technology (EST). By channeling harmful electrical currents away from bearings to ground, the iPro can extend the lives of medium-voltage motors and generators, thus improving the reliability of entire systems in which they are used. The rings are available in a range of sizes to accommodate generator/motor shafts to 30 in. diameter.

Maintenance-free, the Aegis iPro is used for medium-voltage motors that drive pumps, compressors, mixers, shredders, conveyors and other machinery in mining, food processing, wastewater treatment, petrochemical refining and many other high-current applications. The iPro also protects generator bearings 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 (IEMCO), a local motor and equipment repair shop. Wilkins recommended the iPro, and Riebel had IEMCO install two on the ball mill motor. EST recommends installing an iPro in the drive end and insulation on the non-drive end of most large motors. For some large motors, especially those that don’t have insulation designed into them or where insulation can’t be installed easily, EST recommends installing iPro rings at both ends of the rotor shaft.

Riebel and Wilkins chose the iPro split-ring model, which is designed to facilitate field retrofits. The mating halves install 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 regional service centers. 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

An AEGIS iPRO Bearing Protection Ring the IEMCO shop installed in an internal bearing retainer as part of a motor overhaul for Monarch Cement Company machinery.

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 minimize electrical resistance, the iPro’s entire inner circumference is lined with multiple rows of conductive microfibers. Locked securely in the ring’s patented Aegis FiberLock channel, the microfibers completely surround the motor shaft, providing millions of discharge points for shaft currents and forming the path of least resistance that diverts currents away from bearings to ground.