As you know, a popular, and effective, way to operate and control electric motors is through a variable frequency drive (VFD). VFDs provide a long list of benefits, yet have a few drawbacks. Among them shaft currents, which can lead to ball bearing failure.
Preventing shaft currents is the job of an AEGIS grounding ring. The effectiveness of the AEGIS ring is on display at a Wisconsin paper mill and is the focus of an excellent white paper authored by the fine folks at Electro Static Technology, the AEGIS creators.
First, some background.
At a paper mill in the Midwest, every new motor controlled by a variable frequency drive (VFD) is tested, and 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 ball bearing failures.
The maintenance history of one motor tells the story. The large 1,000 HP motor is part of a system that turns pulp into “parent rolls,” which are later cut into well-known brands of paper towels, napkins, and other products. The mill, one of many owned by a major paper company, employs more than 1,800 people in collecting and converting about 430,000 tons of wastepaper per year. The problem with the motor, which runs at up to 1,200 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 the plant’s maintenance contractor, L&S Electric Inc., the shaft currents would destroy the motor’s ball 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 their shop for reconditioning, which included disassembling it; cleaning, testing, and replacing both ball bearings; re-assembling and transporting it back to the mill, and reinstalling it. 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 the problem just moved to the drive-end bearing.
Each reconditioning took a chunk out of the maintenance budget — $8,000 to $10,000, not counting the cost of bringing the motor’s production line to a halt.
“It is a real pain to pull these motors,” notes the reliability engineer. “Many of them are in tight spaces. Rigging is an issue if you try to change the bearing in place. The coupling must be broken, the motor moved, the coupling removed from the motor shaft, the end bell taken off, etc. It might save us a few hours to change the ball bearings in place, but then there’s the potential of introducing contaminants into the bearing, and it’s still quite labor-intensive — we’re talking 12 to 16 hours. Either way, it’s a lot of downtime for us.”
Protecting the Ball Bearings Using an iPro Ring
The motor needed more reconditioning in the summer of 2009. At that time, L&S Electric added an insulated bearing housing and, at the reliability engineer’s request, installed an #AEGIS #ShaftGroundingRing. However, a few months later an on-site test showed there was still enough voltage to damage the bearings. When that happened, L&S Electric technicians installed two copper-bristle grounding brushes on opposite sides of the drive-end shaft.
The regional EST sales representative 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 AEGIS® SGR had been too small to divert all the current from the shaft of such a big motor, the sales rep explained. He offered an iPRO ring free of charge. In May of 2011, L&S installed the iPRO on the motor in place at the mill. The split-ring iPRO fit around the motor shaft, eliminating the need to decouple attached equipment.
Two and a half years later, the motor was still running smoothly — the longest stretch ever without a bearing replacement. A “pigtail” lead installed by L&S allows the reliability engineer to check the shaft voltage easily and safely, with a portable oscilloscope. He reports that the readings have stayed under 5 volts.
In addition to checking the motor’s shaft voltage, the manufacturer of the iPRO ring 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 of any corrosion or oxidation, and re-coated 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 AEGIS iPRO ring.
“I’m really ecstatic over the results,” comments the reliability engineer, who has since had AEGIS rings installed on several of the plant’s other motors.
Breaking down Damages to the Ball Bearing
Because VFDs are growing in popularity in a variety of industries, there have been a growing frequency of ball bearings becoming damaged because of shaft currents. The savings VFDs offer can sometimes be mitigated by frequent repair expenses.
It is now widely known 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 ball 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 shape the frosting into washboard-like ridges across the bearing race and causing noise, vibration, increased friction, and ball bearing failure.
This is what happened to the paper mill’s motor, over and over again until the AEGIS iPRO solved the problem.
The paper concludes:
Unlike conventional single-point-contact brushes, the AEGIS 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 very low impedance from shaft to frame, bypassing the motor ball bearings entirely and bleeding off damaging currents safely to ground.
For more information regarding VFDs, shaft current, and the benefits of an AEGIS grounding ring, contact L&S Electric.