A preventive plan for bearing protection
Perhaps someday all motors will be so well built that there will be no more electrical bearing damage. Until that day comes, motor repair shops will continue to replace bearings eroded by voltages induced by variable frequency drives (VFDs). If the customer has to send the same motor back for new bearings again in six months, he is likely to develop serious doubts about the shop’s competence.
End users of inverter-driven motors have every right to expect uptime and reliability. After all, VFD-induced electrical bearing damage can be prevented, not just repaired. When bearings fail, proper repair practices can fix the problem for good, but value-added services such as inspection, testing, and analysis can prevent the need for repairs in the first place.
On the other hand, a repair shop that fixes a motor’s bearing problem properly only has to do it once and is therefore more likely to earn customer loyalty. Better yet, a shop that offers the latest diagnostic services (vibration analysis, thermography, shaft-voltage testing, etc.) can show a customer how the right preventive measures can head off electrical bearing damage or nip it in the bud.
Working at the customer’s plant, either on a brand new motor prior to its installation or on a motor already in service, personnel who know what they are doing can now protect bearings for the life of the motor. This is what we mean by “best practices.”
By now it is widely understood that induced shaft voltages discharge through the bearings of many VFD-controlled, alternating-current (ac) motors (see Figure 1). The high switching frequencies of today’s VFDs produce parasitic capacitance between a motor’s stator and rotor. Once the resulting shaft voltages reach a level sufficient to overcome the dielectric properties of the bearing grease, they discharge along the path of least resistance — typically through the bearings (see Figure 2).
During virtually every VFD switching cycle, induced shaft voltage discharges from the motor shaft to the frame via the bearings, leaving a tiny pit (usually 5 to 10 microns in diameter) in the bearing race.
These discharges are so frequent (millions per hour) that through the process of electrical discharge machining, they create millions of fusion craters, or pits. Before long, the entire bearing race can become marked with countless pits known as frosting. A phenomenon known as fluting may occur as well, shaping the frosting into washboard-like ridges across the bearing race (see Figure 3), which can cause noise, vibration, increased friction, and catastrophic bearing failure.
As the bearings degrade, high temperatures can cause bearing grease to burn, degrade, and fail, causing decreased bearing life and premature failure. The arcing blasts tiny particles of metal from the race wall, and these contaminate the grease, intensifying abrasion. Too often, the end result is costly, unplanned downtime.
Failure rates vary widely, depending on many factors, but evidence suggests that a significant portion of failures occur only 3 to 12 months after system startup. Because many of today’s motors have sealed bearings to keep out dirt and other contaminants, electrical damage has become the most common cause of bearing failure in ac motors with VFDs.
Cutting and carefully inspecting the bearings of motors needing repair will often provide information that can be used to prevent a recurrence of the problem. Following established safety precautions, repair shop technicians should:
- Inspect the bearing cavity, retaining a sample of the lubricant in case further analysis is warranted to detect contaminants, signs of excessive heat, hardening or blackening of grease, or grease that has escaped the bearing.
- Cut the outer race in half.
- Inspect the grease inside more closely, again searching for signs of contamination.
- Clean the bearing’s components with a solvent.
- With a microscope, inspect the race walls for electrical pitting/frosting/fluting.
If inspection of the old bearing indicates electrical damage, the best way to protect replacement bearings is to install a shaft grounding ring that has a full 360 degrees of circumferential conductive microfibers touching the motor shaft. Properly installed, such a ring will conduct harmful shaft voltages away from the bearings and safely to ground. With a ring installed, voltage will travel from the motor shaft, through the ring’s conductive microfibers, to its housing, then through the motor’s housing to ground (see Figures 4 and 5).
All paths must be conductive, so paint on the motor’s faceplate must be removed. Likewise, the motor’s shaft must be clean down to bare metal, free of any coatings (see Figure 6). Depending on its condition, the shaft may require scrubbing with emery cloth or a similar material.
Even when the shaft appears clean, wiping it with a non-petroleum-based solvent will remove unseen residues. After cleaning, the conductivity of the shaft should be checked with an ohm meter. If the reading at the portion of the shaft that will contact the ring’s microfibers is higher than 2 Ωs, the shaft should be cleaned again.
A grounding ring should never operate over a shaft keyway, which has sharp edges and could reduce conductivity. On some motors, the dimensions of the spacer and mounting screws can sometimes be adjusted/changed to avoid a keyway. If this is not feasible, the portion of the keyway that will contact the ring’s microfibers should be filled with epoxy putty.
Conductivity should be further enhanced by lightly but evenly coating with colloidal silver any portion of the shaft that will contact the ring’s microfibers. This will also help retard corrosion.
Threadlocking gels and liquids other than conductive epoxy are not recommended for the screws that mount the ring to the motor, as they might compromise the conductive path to ground.
The ring should be centered on the motor shaft so that its microfibers contact the shaft evenly.
After installation, testing with an ohm meter is again recommended. The best method is to place one probe on the ring and one on the motor frame. (The motor and drive must be grounded to common-earth ground in accordance with applicable standards.)
For environments where the motor will be exposed to excessive amounts of dirt, dust, or other debris, it may be necessary to protect the ring’s fibers with an o-ring or v-slinger or install the ring inside the motor’s housing. Bearing isolators with built-in circumferential grounding rings are also available.
Most of the recommendations above also pertain to grounding rings mounted to a motor’s housing with conductive epoxy instead of screws, split rings designed to slip around an in-service motor’s shaft instead of over its end, larger rings designed for higher voltage motors and generators, and rings press fitted, bolted, or bonded (with conductive epoxy) into a bearing retainer or custom bracket inside a motor’s housing.
For internal installations, an additional machined spacer can keep the ring farther away from the bearing grease cavity. Metal-to-metal contact is still essential, of course, so a bearing retainer must be free of any coatings or other nonconductive material where it will touch the ring.
For horizontally or vertically mounted motors with horsepower of 100 (75 kW) or less and single-row radial ball bearings on both ends, a shaft grounding ring can be installed on either end. For horizontally mounted motors with horsepower greater than 100 and single-row radial ball bearings on both ends, the bearing housing at the nondrive end must be electrically isolated to disrupt circulating currents. Options for achieving such isolation include insulated sleeves, nonconductive coatings, ceramic bearings, or hybrid bearings. The grounding ring should be installed at the drive end.
For any motor in which the bearings at both ends are already insulated, the drive end is preferred for installation of a grounding ring, to protect bearings in attached equipment such as a gearbox, pump, fan, or encoder.
For any motor with cylindrical roller, Babbitt, or sleeve bearings, the end with such bearings should be electrically isolated, and the grounding ring should be installed at the opposite end.
Testing and analysis
Measuring shaft voltage on a VFD-driven motor provides valuable information for determining whether there is a risk of electrical bearing damage. The best time to take such measurements is during the start-up of a new or recently repaired motor. Every motor has its own unique parameters. Combined with vibration analysis, thermography, or other diagnostic services, results (including saved oscilloscope-screen images) can be presented in a report to the customer. Results should also be used in developing preventive and predictive maintenance programs.
Shaft voltages are easily measured (using appropriate safety procedures) by touching an oscilloscope probe to the shaft while the motor is running. The best probe will have a tip of high-density conductive microfibers to ensure continuous contact with the rotating shaft. A portable oscilloscope with a bandwidth of at least 100 MHz should deliver accurate waveform measurements (see Figures 7, 8, 9, and 10). Probe/oscilloscope kits are available.
Just as shaft voltage measurements can show that a motor’s bearings are in danger of electrical damage, they can confirm that a shaft grounding ring is working. If a proven ring has been properly installed, typical discharge voltage peaks should be less than 10 V, depending on the motor.
Adam Willwerth is sales and marketing manager for Electro Static Technology.