A preventive plan for bearing protection
Diagnostic services can extend the life of your bearings
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.