Protect VFD-actuated motors from electrical bearing damage
Bearing seals only keep away the dirt, while stray voltage and current injury persists
By Adam Willwerth
|Prior to installation of a grounding ring, the motor shaft must be cleaned down to bare metal, free of any nonconductive material. Conductivity can be further enhanced by coating the part of the shaft that will contact the ring with colloidal silver.|
Until the day comes when all motors have built-in bearing protection, maintenance people and motor-repair shops will forever be replacing damaged bearings, so the common wisdom says.
But, the truth is, if a bearing problem is fixed right and proper mitigation applied, it only need be done once. Better yet, a good technician can use the latest diagnostic techniques – including vibration analysis, thermography and shaft-voltage testing — to head off electrical bearing damage from the get-go, nipping it in the bud, so to speak.
Whether it’s a brand new motor or one already in service, this is what’s meant by “best practices.”
Variable frequency drives (VFDs, also known as inverters) can save 30% or more in energy costs. The need to reduce energy usage leads to their use in plants, assembly lines, HVAC and other equipment.
Unfortunately, whether controlling motor speed or torque, VFDs often induce voltages and currents that can damage bearings. In fact, motor bearing repair or replacement can wipe out the savings a VFD yields and severely diminish system reliability.
Bearing failure rates vary widely, but evidence suggests that a significant portion of failures occur within three to 12 months of start-up. Because many motors today have sealed bearings to keep out dirt and contaminants, electrical damage is the most common cause of bearing failure in VFD-equipped AC motors.
VFD high switching frequencies produce parasitic capacitance between motor stator and rotor. By now it is widely understood that, once the resulting shaft voltages overcome the dielectric properties of bearing grease, they discharge along the path of least resistance — typically through the bearings (See Figure 1).
Discharges are so frequent they create millions of tiny fusion craters. Before long, the entire bearing race wall is marked with countless “frosting” pits. What’s called fluting may occur as well, shaping the frosting into washboard-like ridges across the bearing race (See Figure 2). This causes noise, vibration and increased friction. As the bearings degrade, tiny metal particles blasted from the fusion craters intensify friction and abrasion, heat the bearings, and burn contaminated grease. The result, sooner or later, is catastrophic bearing failure.
Cutting and carefully inspecting motor bearings needing repair often provide clues that can help prevent a recurrence.
With all due safety, technicians should:
- Inspect the bearing cavity, retaining a sample of the grease in case further analysis is warranted to detect contaminants, excessive heat signs or other important clues.
- Cut the outer race in half.
- Clean the bearing’s components with a solvent.
- With a microscope, inspect the race walls for electrical pitting, frosting or fluting.
If bearing inspection indicates electrical damage, the best way to protect replacement bearings is to install a modern shaft grounding ring. Unlike 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 (See Figure 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 motor manufacturers have added factory-installed shaft grounding rings 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 in-service motors.
Installation tips & tricks
To maximize grounding-ring effectiveness, all electrical paths must be conductive. Paint on the motor’s faceplate must be removed. The motor shaft must be clean down to bare metal. After scrubbing with emery cloth, wiping the shaft with a non-petroleum-based solvent will remove unseen residues. After cleaning, shaft conductivity 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, spacer dimensions and mounting screws sometimes can be adjusted to avoid a keyway. If this isn’t feasible, the keyway portion that will contact the ring’s microfibers should be filled with epoxy putty.
Conductivity should be further enhanced by a light but even coating with colloidal silver of any shaft portion that contacts the ring’s microfibers. This will also help retard corrosion (See Figure 4).
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.
When mounting the ring externally to an end bracket, split rings designed to slip around an in-service motor’s shaft instead of over its end simplify installation.
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 motors are exposed to dirt, dust or debris, it’s good 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 such as in mining, however, mounting the shaft grounding ring inside the motor is the best contamination protection (See Figure 5).
Using conductive epoxy or screws, 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 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 greater than 100 hp and single-row radial ball bearings on both ends, the bearing housing at the non-drive end must be electrically isolated to disrupt circulating currents by, for example, the use of insulated sleeves, nonconductive coatings, ceramic bearings or hybrid bearings. The grounding ring should be installed at the drive end.
For any motor in which bearings at both ends are already insulated, the drive end is preferred for grounding-ring installation, 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.
Starts at installation
Measuring shaft voltage on a VFD-driven motor provides good information for knowing the risk of electrical bearing damage. The best time to measure is during the start-up of a new or 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 to customers or management. Results support 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.
Users of VFD-driven motors expect uptime and reliability. After all, VFD-induced electrical bearing damage can be repaired and prevented. Insight borne of inspection, testing and analysis leads to advance warning, and when bearings fail, proper repair practices can fix the problem for good. Motor shaft grounding rings can be installed during motor repairs or on new motors before they are put into service.
Adam Willwerth, Sales and Marketing Manager for Electro Static Technology, has extensive experience in industrial product development. He is named on four patent applications pertaining to conductive microfiber shaft grounding technology and has presented seminars on the subject of bearing current mitigation at professional conferences in the U.S. and Europe.
For a 36-page handbook on the practices summarized in the above article, contact Electro Static Technology, 31 Winterbrook Road, Mechanic Falls, ME 04256-5724, TEL: (207) 998-5140, FAX: (207) 998-5143, www.est-aegis.com/bearing.