Excellent article on the many things which must be looked out for wen handling or installing bearings in electric motors. Published in the M+R (maintenance + Reliability) section of Maintenance Technology Magazine, the article, also available on line at http://www.maintenancetechnology.com/2016/09/handle-bearings-care/  lists 9 different conditions or practices and gives great advice on each. Of course, each section is a topic in and of its self and the one I would like to discuss in some more depth is on:

Electric Current Arcing:


Shaft Voltage Discharge

Shaft Voltage Discharge

Most pitting in a motor’s bearings comes from one of 2 sources: (1) Capacitive coupled voltage from the stator to rotor through parasitic capacitance on all motors operated by a variable frequency drive (VFD)from small 1 HP to the largest medium voltage motors and (2) high frequency circulating currents which can occur on motors over 100 HP. Separate mitigation is necessary to protect the bearings from these two sources of bearing currents.

The phenomenon for the creation of electrical discharges is similar for both. Essentially the voltage on the motor’s shaft creates a potential high enough to overcome the dielectric of the oil film in the motor’s bearings. When this happens an electrical arc shoots through from the inner race via the rolling element to the outer race which is connected to ground.

The energy in this arc is great enough to melt the hardened bearing steel and create an electrical discharge machining (EDM) pit in the bearing race. The steel surface melts and the metal hardens on the rim of the EDM pit. Then the rolling element can either deform the bearing race surface or break the metal particle and contaminate the bearing. Last but not least, the lubrication burns and deteriorates.

This process can occur MILLIONS of times per hour and after just a short while the bearing surface is pitted and a phenomenon known as “fluting” starts. This is a washboard type pattern along the bearing race creating vibration, heat and eventual bearing failure.

How do we stop this from happening?

Motors up to 100 HP

Motors up to 100 HP

Motors over 100 HP

Motors over 100 HP

First: Download the AEGIS Bearing Protection Handbook!

Low-Voltage Motors up to 100 HP: Install AEGIS® Shaft Grounding Rings for these motors usually on the drive side, either internally or externally.

Motors over 100 HP: Install one AEGIS® Shaft Grounding Ring on the drive end AND and insulated bearing on the non-drive end opposite the shaft grounding ring which prevents circulating currents that may be present in addition to capacitive induced shaft voltage.

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

Pitting Frosting and Fluting Damage

Pitting Frosting and Fluting Damage

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.