Monthly Archives: November 2017

The new motor you installed 6 months ago is making the kind of noise you only hear in horror movies.  So, you purchase and install a replacement motor, hoping that it will not suffer the same fate.

But you’re not really sure what killed the original motor.  The best way to determine the cause of the motor’s failure is to perform an autopsy on it.  This includes cutting the motor bearings to see what happened to them.

Determining the cause of bearing failure can inform decisions on how to prevent such failures in the future and increase the reliability of VFD-driven motor systems.  Cutting and inspecting motor bearings can provide invaluable information about what caused the damage to them.  In fact, ANSI/EASA AR100-2015 recommends cutting and inspecting the bearings for evidence of EDM discharges whenever a VFD-driven motor is being repaired.

    1. Start by inspecting both bearings — inside and outside — for contamination, signs of excessive heat, hardening of grease, color of grease (look for abnormal coloration, e.g., blackened grease), and excess grease or oil escaping the bearing.  Remember to save a sample of the lubricant for analysis.
    2. Then, after removing the seals or shields, cut the outer race in half.  Follow all established safety precautions and use personal protective equipment including eye protection, hearing protection, a face shield, gloves, and protective clothing.
    3. Inspect the grease and look for any contamination in the bearing.  Burnt grease is typically a sign of continuous electrical arcing in the bearings that heats the oil component of the grease beyond its temperature capacity, causes rapid deterioration of the grease’s lubricating capability, and results in bearing race damage.  Arcing also loosens small particles of metal from the ball bearings and bearing races.  These particles are abrasive and will cause premature wear to bearing surfaces.
    4. Clean bearing components with a degreaser or solvent.
    5. Inspect bearing components for evidence of electrical discharge machining (EDM).  Each time the electrical voltage overcomes the dielectric of the bearing lubrication, it arcs through the inner race and through the rolling elements to the outer race, blasting tiny pits in the bearing surfaces. These pits are typically 5-10 microns in diameter, and can be easily seen under a microscope.
    6. Look for frosting of bearing surfaces.  To the naked eye, frosting appears as a discolored grey line around all or a portion of the inner or outer bearing race.  It is actually the result of millions of tiny pits, marring the race surface.  Since it can be caused by mechanical wear as well as EDM, discoloration should be examined under a microscope to determine its actual cause.  If the motor was controlled by a VFD, there is a high likelihood that the frosting was due to EDM.
    7. Look for fluting damage.  Fluting is a distinctive washboard-like pattern on the inside of the outer bearing race that results from the operational frequency of the VFD.  Fluting is visible to the naked eye or can be seen with 10X magnification.  Since fluting is sometimes confused with mechanical bearing damage (such as brinelling/false brinelling), careful consideration should be given to electrical fluting when assigning a cause to the bearing damage.

    If, as a result of these inspections you determine EDM to be the cause of the bearing damage, then be sure to install an AEGIS® Shaft Grounding Ring on the motor when the bearings are replaced.  AEGIS® Rings have been proven effective in protecting motor bearings (and those of attached equipment) in millions of installations worldwide.  So don’t settle for ineffective, partial, or temperamental bearing protection — insist on AEGIS® Rings.

If you are controlling motors with variable frequency drives (VFDs) – aka variable speed drives (VSDs) or inverters – those motors are at risk of electrical bearing damage.  VFDs convert pure sine wave power into a series of positive and negative pulses that combine to induce capacitive voltages on motor shafts. Without a safe path to ground, these shaft voltages can discharge through motor bearings and, through the process of electrical discharge machining, or EDM, will destroy the bearings — often in as few as 3 months!

According to NEMA MG1 Part, capacitive shaft voltages of 10-40 volts peak (or 20-80 volts peak-to-peak) can cause electrical discharges in motor bearings.

The best time to test for damaging shaft voltages is as soon as motors are put into operation — right after new motors are installed, immediately after a damaged motor is repaired or its bearings are replaced, or upon commissioning of motors in newly constructed buildings or installation of new production equipment.

But it’s not always easy to convince building owners or managers of the need for shaft grounding before an actual problem — and the expense of fixing it — arises.  And until now, the only way to test motors and document whether they were at risk of bearing damage was to buy or rent an oscilloscope and make or buy some sort of shaft voltage probe.

Now, the AEGIS® Shaft Voltage Tester comes complete and specially configured — right out of the box — with everything you need to take and record accurate voltage readings from the spinning shafts of motors.  In fact, the Shaft Voltage Tester’s screen capture feature lets you save actual voltage readings to a USB drive  for subsequent analysis and presentation, providing proof to building owners or corporate management of the need for shaft grounding.

So don’t wait until you need to repair or replace damaged bearings before checking to see if your motors are at risk.  Test them as soon as they are operational with an AEGIS® Shaft Voltage Tester.  And if they are at risk, protect them with AEGIS® Shaft Grounding Rings.

Special Offer:

From now until the end March, 2018, if you purchase an AEGIS® Shaft Voltage Tester with an extended warranty (AEGIS-OSC-9100-W2), Electro Static Technology will include a probe holder, a magnetic base, a shaft grounding simulator (to demonstrate how an AEGIS® Ring would reduce the shaft voltage), and 3 extra shaft voltage probe tips — a $336 value — at no charge!  Testers typically ship within 2 days of order.

In addition, your local AEGIS® representative will provide free training on the use of the Shaft Voltage Tester — at your facility and at your convenience — free of charge!

The AEGIS® Shaft Voltage Tester is the only handheld oscilloscope that comes with everything you need to monitor electric motors for electrical bearing damage from VFDs.

From now until the end of the year, the extended warranty AEGIS-OSC-9100-W2 will include, at no extra charge, a magnetic base, probe holder, shaft grounding simulator (to see the effect an AEGIS® ring would have on the shaft voltage), and three extra shaft voltage probe tips.  These accessories would have a list price of $331.00, so this is a great deal.  And every Shaft Voltage Tester also comes with a free training from your area AEGIS® representative, at your facility, and at your convenience.

The Shaft Voltage Tester with 2-year warranty now comes with a free SVP-KIT-9100MB.

To take advantage of this deal, just contact your local or preferred AEGIS® distributor and order an AEGIS-OSC-9100-W2, and we’ll add an SVP-KIT-9100MB at no extra cost.  Like most other AEGIS® parts, Shaft Voltage Testers usually ship within a day or two, so you can get started testing shaft voltage without delay.