If inspection of the old bearing indicates electrical damage, the most reliable and cost-effective way to protect replacement bearings is to install a modern shaft grounding ring. Unlike older single-point contact brushes, these rings completely surround a motor’s shaft with contact points.
Conductive microfibers should line the ring’s entire inner circumference, boosting the electron transfer rate (Fig. 3). A properly installed ring provides a very low impedance path from shaft to frame, safely bleeding off damaging voltages to ground and bypassing the motor’s bearings entirely. And because the microfibers work with little or no contact, they do not clog up and wear out like conventional grounding brushes.
A growing number of forward-looking motor manufacturers have recently added a factory-installed shaft groundingring as a standard or optional feature on certain models, but they are still exceptions to the rule. Many industrial supply houses and distributors of motors and bearings sell grounding rings that can be installed on new, refurbished, or inservice motors.
to maximize a grounding ring’s effectiveness, all electrical paths must be conductive; paint on the motor’s faceplate must be removed. Likewise, the motor’s shaft must be clean down to bare metal. Even after scrubbing with emery cloth, wiping the shaft with a non-petroleum-based solventwill remove unseen residues. After cleaning, the conductivity of the shaft should be checked with an ohm meter. If the reading at the section that will contact the ring’s microfibers is higher than two ohms, 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 beadjusted/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 (Fig. 4).Thread-locking gels and liquids other than conductive epoxy are not recommended for the screws that mount the ringto the motor, as they might compromise the conductive pathto ground.
The ring should be centered on the motor shaft so that itsmicrofibers contact the shaft evenly.When mounting the ring externally to an end bracket, splitrings designed to slip around an inservice motor’s shaft instead of over its end simplify installation.
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. Bearing isolators with built-in circumferential grounding rings are also available. For severe-duty environments such as many mining applications, however, mounting the shaft grounding ring inside the motor provides the best protection from contamination (Fig. 5). Using conductive epoxy orscrews, the ring can be mounted directly to a bearing retainer. An additional machined spacer will keep the ring away from the bearing grease cavity. Metal-to-metal contact is still essential, so the 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 installedon 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 non-drive end mustbe 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 electricallyisolated and the grounding ring should be installed at the opposite end.
Measuring shaft voltage on a VFD-driv-en motor provides valuable information for determining whether there is a risk of electrical bearing damage. The best time to take such measurementsis 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-screenimages) can be presented in a report to the supervisor/customer. Results should then 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 toensure continuous contact with the rotating shaft. A portable oscilloscope with a bandwidth of at least 100 MHz should deliver accurate waveform measurements; 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 also 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 volts.
In summary, end users of VFD-driven motors have every right to expect uptime and reliability. After all, VFD-induced electrical bearing damage can now be prevented, not just repaired.
Routine inspection, testing, and analysis can provide advance warning andinsight into the problem, and when bearings fail, proper repair practices can fix the problem — permanently.Motor shaft grounding rings such as AEGIS bearing protection rings can be installed during motor repairs or onnew motors before they are put intoservice.