Learn About Electrical Bearing Damage and How to Prevent It… While Enjoying Lunch on Us!
Lunch-and-learn presentations should be educational, not simply promotional. To benefit those who commit their time to them, lunch-and-learns should address topics that are not well understood and should provide the latest information on them as well as solutions to problems. And we know from experience that shaft voltages and the damage they can cause to motor bearings result in unplanned, unwanted, and expensive downtime. So, learning why they occur and how to prevent them is critical to design engineers and to plant maintenance people.
The problem is that the damage caused by VFD-induced shaft currents manifests itself as physical damage. And all too often those who deal with such damage assume it was caused by poor shaft alignment, inadequate lubrication, or other physical factors.
When a variable frequency drive (VFD or inverter) is used to control a motor, it alters the waveform of the power to the motor. VFDs change balanced sine wave power into a series of unbalanced positive and negative pulses that create capacitively coupled common mode voltage on the shaft of the motor. Without a low resistance path to ground, this voltage will discharge through the motor’s bearings. These voltage discharges cause electrical discharge machining in the form of pitting (tiny fusion craters in metal surfaces), frosting (widespread pitting), and fluting (washboard-like ridges in the walls of the bearing race resulting from the operational frequency of the VFD).
Without proven long-term bearing protection, these discharges can destroy motor bearings — often in as little as 3 months!
At AEGIS®, we have been educating motor users, motor repair shops, plant maintenance personnel, specifying engineers, and contractors on the causes and prevention of electrical bearing damage for years. And through the years we have come across more than a few products that claim to protect against electrical bearing damage, but don’t actually live up to these claims. We know what works and what doesn’t. And because we have tested them, we can tell you where and how they fall short.
We have also developed best practices for diagnosing electrical bearing damage and preventing it in motors large and small, low or medium voltage. We have even detailed these best practices in a 56-page Bearing Protection Handbook that discusses in detail the causes of electrical bearing damage, how to diagnose it, and step-by-step best practices for protecting motors from it.
Yet, despite these efforts to inform and educate motor users, we still find many who are unaware that the problem even exists or are unsure why or how to address it.
So if you would like to learn about the causes of electrical bearing damage and best practices for preventing it, we have a 1-hour presentation on the subject. And we would welcome the opportunity to make this presentation to as many of your people as you would like — from a handful to a room full — at whatever time you like. We’ll even bring lunch or other refreshments.
To request a Lunch-and-Learn training at your facility, click here.
VFD-induced shaft voltages damage motor bearings and shorten motor life
If you’re using variable frequency drives (VFDs) or inverters to control motors, the motors are at risk of electrical bearing damage that can dramatically shorten their lives. VFDs
induce harmful voltages on motor shafts — voltages as 40 volts peak — that can destroy bearings in as little as 3 months!
Through electrical discharge machining, VFD-induced discharges can blast millions of pits in metal bearing surfaces. These discharges burn and contaminate bearing grease, drastically reducing its effectiveness. They also result in fluting, bearing failure, and costly unplanned downtime. And while most motor manufacturers offer “inverter-duty” or “inverter-ready” models, these motors have inverter-rated insulation to protect their windings, but nothing to protect their most vulnerable components — their bearings.
Vertical motors run on variable frequency drives are susceptible to electrical bearing damage and premature failure. When any motor, vertical or horizontal, is run on an inverter, the drive’s output causes a capacitive voltage on the motor shaft that can discharge through the bearings, causing electrical damage and premature failure. This has brought grief to more than one water treatment plant; you can read one of their stories here.
GE Power Conversion is aware of this problem, so they offer their Ultra™ Vertical Pump Motors with an AEGIS® Shaft Grounding Ring as an option. AEGIS® shaft grounding gives VFD-produced shaft voltages a low-impedance path to ground, drawing damaging current away from the bearings, and so protecting them against electrical damage.
For more on GE Ultra™ Vertical Pump Motors, see this brochure. To learn how AEGIS® shaft grounding helped with a Kansas treatment facility’s chronic bearing failure problem, read this case study. And for fuller background on electrical bearing damage due to VFDs, see the AEGIS® Bearing Protection Handbook.
This video clearly demonstrates that common mode chokes (inductive absorbers) installed on a VFD do not eliminate damaging shaft voltage, and that these chokes are no substitute for AEGIS® Rings.
In this video, we installed the common mode chokes exactly as recommended by the manufacturer and then operated the motor, with no AEGIS® Ring installed, while monitoring shaft voltages to see if bearing discharges occurred. With common mode chokes and no AEGIS® Ring, the oscilloscope detected continual shaft voltage discharge through the bearings. These discharges cause cumulative bearing damage, and lead to eventual bearing failure.
Then we simply stopped the motor, installed the AEGIS® Ring, and turned the motor back on while again monitoring the shaft voltage. All the shaft voltage was gone – as expected – since now the AEGIS Ring was discharging the voltage away from the motor’s bearings to ground. By providing a low-resistance discharge path to shaft voltages, the AEGIS® Ring protects the bearings from electrical damage.
Conclusion: Common mode chokes (cores) have zero benefit in reducing or eliminating shaft voltage, which can discharge through motor bearings and cause electrical pitting damage.
In contrast, AEGIS® Shaft Grounding Rings were found to eliminate the bearing discharges and effectively protect the motor’s bearings.
Electro Static Technology-AEGIS® is proud to announce that BUILDINGS has selected the AEGIS® Shaft Voltage Tester™ as a 2016 Money-Saving Product winner in the Energy Measures category. The Shaft Voltage Tester™ is a member of an elite group of 76 products showcased in the June 2016 issue of BUILDINGS and online at www.buildings.com.
Finalists were evaluated by the BUILDINGS editorial staff for the money-saving qualities they offer to building owners and facility managers in areas such as energy efficiency, water savings, and maintenance. The AEGIS® Shaft Voltage Tester™ saves money by detecting the hazard of VFD-sourced electrical bearing damage to motors, allowing users to head off expensive repairs and downtime by taking preventive measures before damage passes the point of no return. Continue reading