Grounding Rings Rescue Paper Mill’s Bearings

Bearing-protection rings neutralize damaging VFD-caused electrical currents in the mill’s motor shafts, preserving the bearings and uptime.

At a paper mill in the Midwest, every new motor that is to be controlled by a variable frequency drive (VFD) must be tested. If shaft voltages are found, the motor must be equipped with an AEGIS® Bearing Protection Ring to divert damaging currents to ground. The plant’s electrical reliability engineer established this money-saving specification after many years of frustration from recurring bearing failures.

Fig. 1. Pitting of a bearing race wall (magnified) is the result of electrical discharges from the motor shaft.

The maintenance history of one large 1000 HP motor tells the story. It is part of a system that turns pulp into “parent rolls,” which are later cut into paper towels, napkins and other products. The mill is one of many owned by a major paper company, and employs more than 1800 people. The operation collects and converts about 430,000 tons of wastepaper per year. The problem with the motor, which runs at up to 1200 rpm, stemmed from its VFD, which induces stray currents that travel through the motor’s shaft.

Despite the efforts of the plant’s in-house maintenance staff and maintenance contractor (L&S Electric, Inc.), the shaft currents would destroy the motor’s bearings within two years. Seeking ground, the currents blasted the bearing balls and races with countless fusion craters. The arcing that created these pits released tiny particles of metal that contaminated bearing grease, causing friction and high temperatures that burned the grease.

Following the industry-standard routine of the time, L&S Electric would take the motor to its shop for reconditioning. This included disassembly, cleaning, testing and replacement of both bearings; reassembly; transportation back to the mill; and reinstallation. Over the years, L&S tried insulating the opposite-drive-end bearing and installing carbon-block grounding brushes inside the drive-end bearing cap. But this just moved the problem to the drive-end bearing. Each reconditioning cost as much as $10,000, not counting the cost of downtime on that motor’s production line.

Fig. 2. Inspecting the recently installed AEGIS® iPRO Bearing Protection Ring on a key motor at the Midwestern paper mill. The split-ring iPRO was installed without decoupling the motor.

Finding the right ring

When L&S took the motor to their shop for a reconditioning in June 2009, the contractor’s reliability engineer tried something new. L&S added an insulated bearing housing to the motor’s drive end and, at the engineer’s request, installed an AEGIS SGR shaft grounding ring, manufactured by Electro Static Technology (EST). The ring channels harmful currents away from bearings to ground. Hopes for this solution were dashed three months later when an on-site oscilloscope reading indicated the voltage was over 5 volts—still high enough to damage the bearings. When the bearings were replaced again, L&S added two copper-bristle grounding brushes on opposite sides of the drive-end shaft.

EST then suggested the mill try another type of AEGIS ring: the iPRO. Specially designed for high-current applications, the iPRO is ideal for protecting medium-voltage motors, generators and turbines against electrical bearing damage. The previous model had been too small to divert all the current from the shaft of such a big motor. In May 2011, L&S installed the iPRO on the motor in place at the mill. The split-ring unit fit around the motor shaft, eliminating the need to decouple attached equipment.

Fig. 3. Two views of the iPRO Bearing Protection Ring. Prior to installation, the section of the motor shaft to be contacted by the grounding ring’s microfibers was cleaned and coated with colloidal silver to enhance conductivity and retard corrosion.

Two and a half years later, the motor was still running smoothly—the longest stretch ever without a bearing replacement. A “pigtail” lead allows the L&S reliability engineer to check the shaft voltage easily and safely, which he does regularly with a portable oscilloscope. He reports that readings have stayed below 5 volts.

In addition to checking the motor’s shaft voltage, EST recommended periodic inspections be done to ensure the motor shaft remained conductive in this harsh environment. After the first six months of operation, the split-ring iPRO was removed and the motor shaft was cleaned to remove corrosion, and recoated with colloidal silver to ensure high conductivity. This routine shaft maintenance is now scheduled to coincide with regularly scheduled plant maintenance shutdowns and has enhanced the effectiveness of the iPRO ring.

Fig. 4. A technician uses an oscilloscope to confirm that motor-shaft voltages remain below 5 volts after installation of an iPRO Bearing Protection Ring. A special “pigtail” lead allows maintenance personnel to periodically check the voltages (and the likely condition of the bearings) safely and easily, without exposing the shaft.

Be aware of ‘parasitic capacitance’ from VFDs

Because VFDs can save 30% or more in energy costs, they have been cited as a key technology for those wishing to make their commercial HVAC systems, automated assembly lines and other processes more energy-efficient. But whether used to control a motor’s speed or torque, these drives often induce voltages that can damage bearings. In fact, the costly repair or replacement of failed motor bearings can wipe out any savings a VFD yields and severely diminish the reliability of an entire system. It is now widely understood that the high switching frequencies of today’s VFDs produce “parasitic capacitance” between a motor’s stator and rotor. Once the resulting voltage pulses reach a level sufficient to overcome the dielectric properties of the bearing grease, they discharge rapidly and repeatedly along the path of least resistance between shaft and frame—typically through the bearings.

Without mitigation, these discharges can be so frequent that, through the process of electrical-discharge machining, they create millions of fusion craters. Before long, the entire bearing race can become eroded with countless pits known as frosting. Cumulative degradation known as fluting can also occur, shaping the frosting into washboard-like ridges across the bearing race and causing noise, vibration, increased friction and bearing failure. This is what happened to the paper mill’s motor repeatedly—until the iPRO solved the problem. MT

A Maintenance-Free Solution for Dangerous Shaft Currents

Unlike conventional single-point-contact brushes, the iPro grounding ring’s conductive microfibers line its entire inner circumference, encircling the motor shaft with contact points for far greater effectiveness. This design boosts the ring’s electron transfer rate, providing low impedance from shaft to frame, bypassing the motor bearings entirely, and bleeding off damaging currents safely to ground.

Because the ring’s conductive microfibers work with little or no physical contact, they do not need the regular maintenance/replacement that metal or carbon-block grounding brushes require. Patented Nanogap Technology maintains electrical contact between the microfibers and the motor shaft, thus diverting harmful shaft voltages, whether or not the microfibers are touching the shaft. This ensures that the ring will last the life of the motor regardless of motor speed, and will work effectively even in the presence of dirt, grease or other contaminants.

The maintenance-free iPRO grounding ring is available in a range of sizes to accommodate generator/motor shafts up to 30” in diameter. Embedded securely in the AEGIS FiberLock channel along the inner circumference of the ring, six rows of conductive microfibers completely surround the motor shaft, providing millions of discharge points for harmful shaft currents and effectively diverting these currents to ground. As part of a preventive maintenance program, the iPRO can be installed on in-service motors or whenever bearings are replaced.