Motors on VFD

An ethanol plant in North Dakota was burning through motor bearings at an alarming rate. 

Read the entire story on line at ChemicalProcessing.com

Maintenance personnel at Red Trail Energy’s ethanol plan in Richardson discovered that ball bearings inside several large motors were failing much sooner than they should.

Ethanol Plant

Failing Motors

Writing at Chemical Processing, Red Trail Energy’s Kent Glasser illustrated the problems the plant faced while trying to keep up failing motors. Replacing motor bearings isn’t cheap. In addition to paying a service center for parts and labor, the amount of time a motor is out of service quickly adds up to lost production and revenue.

The manager of the ethanol plant became concerned with those frequent high repair costs and called in a grounding system specialist to troubleshoot the three problematic motors.

Processing Motor

Scott Fisher, the Chief Operating Officer at Sustainable Grounding Systems, had a working history with Red Trail when a North Dakota electrical contractor hired him to evaluate the grounding systems of the plant.

Fisher arrived at the plant in May 2017 and took initial readings from a 30 HP and 150 HP centrifuge motors and a 400 HP fan motor. He discovered that the bearings in these motors had premature damage. These damages occurred much sooner than the ordinary wear and tear normally displayed by bearings operating properly.

Sustainable Grounding Systems is a nationally recognized company providing electrical grounding design, installations and inspections throughout the United States.  Contact Scott Fisher at  +1 (701) 721-3306 or email at [email protected]

A Familiar Suspect

His leading suspect? Stray currents from VFD-induced voltages built up on the motor shafts and discharged through the path of least resistance: the bearings.

Fisher’s solution was installing shaft grounding rings from AEGIS®. Split AEGIS® SGR rings were installed on the centrifuge motors and an AEGIS PRO Series Ring on the 400 HP motor.

AEGIS® PRO Series Rings

The AEGIS® PRO Series Rings are especially well-suited for these environments. They are designed for large frame, low-voltage motors of 500 HP or greater, medium-voltage motors, and 300 HP or larger DC motors.

AEGIS<sup>®</sup> PRO Series Rings for Medium Voltage MotorsTo learn more about how AEGIS® PRO Series shaft grounding rings offer maximum bearing protection, download this guide to best practices.

The follow-up readings from the motors showed significant improvement. Measurements as high as 19 volts dropped to less than 1.3 volts, which is too low to create premature damage to the bearings.

Reaping the Benefits

By starting a program to improve control of its production process, Red Trail Energy installed VFDs on all their motors. The company reaps several levels of a rapid payback on their investment.

First, VFDs provide several energy-saving functions that, with the money saved from using less energy, it eventually pays for itself. Playing the role of a soft starter, a VFD prevents damage to the motor caused by the sudden onrush of current during the startup procedure.

AEGIS® Rings – Prevent electrical fluting damage in bearings

In Sum

Finally, AEGIS® grounding rings protect motor bearings from damaging shaft currents. By preventing shaft current from reaching and damaging the bearings, shutdowns due to constant motor repair saves the company hundreds of thousands of dollars in lost revenue.

Read the entire story in Chemical Processing on-line

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 31.4.4.3, 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!

Continuing our annual tradition, Electro Static Technology employees donated their time and labor to this year’s United Way campaign in September.

Employees generously pledged almost $6,000 to the United Way of Androscoggin County, a 50% increase over 2016 contributions.  Some of the money will be used to support 18 food banks that provide meals for children, families, and seniors in need.  And some of the funds will be used for the education of pre-school children, K-12 students, high school dropouts, and students with learning disabilities.

Employees also identified local residents who could use a helping hand.  One group offered their time and hard work to clean up the yard of an older woman in Poland, Maine.  Another group helped an elderly widower in Minot, Maine, clean his house — inside and out.

Several of us commented that while contributing money to the United Way Campaign feels great, helping those in need on a face-to-face, one-on-one basis is even more gratifying.

In our last post, we described how VFDs cause electrical bearing damage.  So, how do you protect motors from this damage?

Capacitive EDM currents are caused by electrons’ need to go through the bearings to travel between the shaft and ground.  So to prevent EDM currents, you only need to provide an alternative, low-resistance path from shaft to ground.  AEGIS® Bearing Protection Rings provide such a path, channeling these currents away from bearings and safely to ground.

For low-voltage motors (up to 100 HP), the best practice is to install an AEGIS® SGR Bearing Protection Ring – either internally or externally – on either the drive end or non-drive end of the motor to discharge capacitively induced shaft voltage.

For motors greater than 100 HP, capacitive EDM currents will still be present, but now high-frequency circulating currents will occur as well.  The best way to prevent HF circulating currents is to interrupt them with insulation or isolation of the bearing at the opposite end from the AEGIS® Ring.  For motors between 100 and 500 HP, install an AEGIS® SGR Ring on the drive end, and isolate the bearing housing on the non-drive end with an insulating sleeve or coating, or an insulated ceramic or hybrid bearing to disrupt circulating currents.

For low-voltage motors greater than 500 HP or medium-voltage motors, use an AEGIS® PRO Ring instead of an SGR Ring.

For all motors with both bearings insulated, install the correct AEGIS® Ring (SGR or PRO) on the drive end (either externally or internally) to protect the bearings in attached equipment.

Regardless of the type of AEGIS® Ring or where you install it, be sure to apply AEGIS® Colloidal Silver to motor shaft circumference in the area where the ring’s microfiber brushes will contact it.