For either brand-new motors or those already in service, “best practices” means that informed technicians can make use of the latest diagnostic techniques (vibration analysis, thermography, shaftvoltage testing, etc.) to prevent electrical bearing damage — either at the very beginning or very quickly thereafter. If done correctly, the work need only be done once.
Variable frequency drives (VFDs — or inverters) can save 30% or more in energy costs. Because of this,
they have been identified as a key technology for those wishing to make their processing plants, automated assembly lines, HVAC systems, and other equipment more energy-efficient (“green”). Unfortunately, whether used to control a motor’s speed or torque, VFDs often induce voltages and currents that can damage bearings. In fact, the costly repair or replacement of failed motor bearings can wipe out any savings a VFD yields and severely diminish the reliability of an entire system.
Bearing failure rates vary widely, but evidence suggests that a significant portion of these failures occur only three to 12 months after system start-up. 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.
The high switching frequencies of today’s VFDs produce parasitic capacitance between a motor’s 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 (Fig. 1).
These discharges are so frequent that they create millions of tiny fusion craters. Before long, the entire bearing race wall 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 (Fig. 2). This causes noise, vibration, increased friction — and catastrophic bearing failure.
As the bearings degrade, the tiny metal particles blasted from the fusion craters intensify friction and abrasion,heat up the bearings, and burn the contaminated grease. Too often, the end result is bearing failure and costly,unplanned downtime.
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, technicians should:
• Inspect the bearing cavity, retaining a sample of the
grease in case further analysis is warranted to detect
contaminants, signs of excessive heat, etc.
• Cut the outer race in half.
• Clean the bearing’s components with a solvent.
• With a microscope, inspect the race walls for electrical