Shaft Grounding

Testing VFD driven motors for shaft voltage identifies potential bearing damage risk and avoids down time

Innovative companies like AKF Analysis & Testing are on the leading edge of technology services to assist building owners in ensuring their mechanical and VFD driven motor systems stay up and running.  They deploy highly skilled and experienced vibration analysts and testing specialists from the best in the industry with members from Local 638 and Local 94 and focus on building operations such as chiller plants, air handlers, pumps, and critical system units.

In the case study which follows you will see how shaft grounding rings were successfully implemented to improve reliability for a New York City landmark building:

Making Motors Last at Time & Life Building in new York City

Making HVAC fan motors last was the goal for for tenants of the Time & Life Building, in New York’s Rockefeller Center: Shaft grounding rings were installed to prevent an on-going bearing failure problem with remarkable success.  Installed in 2008, these motors are still running today!

Time and Life Building, New York City

A preventive maintenance plan that reduces the total life-cycle cost of operations in a prominent building at the heart of New York City serves as a good example of how the push for greater reliability and up-time in building management has led to finding a solution to a chronic, widespread problem with HVAC motors – electrical bearing damage.

The problem is all too common in AC motors controlled by variable frequency drives (VFDs), which are also known as inverters, adjustable speed drives, etc. These drives are widely used because they can save energy, especially in applications with varying loads. Because many centrifugal fans and pumps run continuously, their motors will use less power if the input is modulated by VFDs.  For example, a 20% reduction in fan speed can reduce energy consumption by nearly 50%. With rising energy costs, the use of throttling mechanisms to restrict the work of a motor running at full speed would be wasteful.

However, efficiency alone is not enough if equipment keeps breaking down. That is what was happening at the 48-story Time & Life Building, one of 19 buildings in the Manhattan business and entertainment complex built and partially owned by the Rockefeller Group and known as Rockefeller Center.

More than 100 VFDs control the speeds of the 240 motors that run the building’s HVAC fans and pumps. Unfortunately, a large portion of the savings from these systems has been wiped out by maintenance costs because, in addition to their intended function, VFDs induce powerful, unwanted currents which cause electrical bearing discharges and, ultimately, premature motor failure.

SHAFT GROUNDING

Proper tuning of a drive’s frequency output range and proper grounding of a VFD-controlled motor’s frame are paramount. Only recently has it become clear that without an effective shaft-grounding device as well, stray currents can wreak havoc with bearings, causing premature motor failure. Ironically, some products designed to protect bearings, such as conventional
metal grounding brushes, require extensive maintenance themselves. Others, such as insulation, can shift damage to connected equipment.

One of the newest and most promising bearing-damage mitigation devices uses patented shaft grounding ring technology to safely bleed off these damaging currents to ground.  Engineered with special conductive microfibers, the AEGIS shaft grounding ring safely discharges VFD-induced shaft voltages by providing a very low impedance path from shaft to frame, bypassing the motor’s bearings entirely.

For more than 20 years, since the installation of the first modern VFDs, the Time & Life Building’s maintenance department has dealt with chronic motor and bearing failure. At times, the bearing damage had advanced to the noisy stage, at which an unpleasant, high-pitched sound was transmitted through duct-work. Thanks to the efforts of AKF Analysis & Testing, an engineering firm hired by the Rockefeller Group Development Corp. to periodically test and tune (with harmonic filters) the building’s VFDs, the rate of motor/bearing replacements has dropped from 90 to 20 per year, but in today’s economy that is still too costly. Other attempts to mitigate the problem, including the installation of ceramic bearings on some motors, have produced mixed results, usually proving too costly for the meager improvements gained.

THE SOLUTION AT LAST

Late in 2007 AKF Analysis and Testing read about the AEGIS shaft grounding ring  and began the process that could eventually end the bearing damage problem at the Time & Life Building once and for all.   AKF A&T decided to recommend the installation of a single shaft grounding ring on the most problematic of all the HVAC motors at Time & Life.  Ron Perez, the building’s manager of engineering, consented to the experiment, and follow-up testing showed the ring to be so successful at diverting harmful shaft currents that O’Connell decided to make his company a distributor for the ring.

Fan motor in the Time & Life Building with AEGIS Ring installed by AKF Analysis and Testing

It was an unprecedented move.  AKF A&T specializes in vibration monitoring and analysis, acceptance testing, critical speed testing, and motor current waveform analysis for preventive maintenance and energy management on behalf of government agencies and businesses in a multitude of East Coast buildings, including hotels, hospitals, laboratories, and office buildings.

Never before had the company endorsed a particular product.  The whole phenomenon of electrical bearing damage is so misunderstood that some maintenance managers have lost their jobs over it because replacing a fan motor is a big expense.   In an office tower, a motor can be running critical equipment
that supplies air to 30 floors where the tenants are paying as much as $110 a square foot.  They have the right to expect the temperature and quality of their air to remain constant.  AKF A&T was so convinced of the ring’s effectiveness that they recommended it be eventually installed on all HVAC motors in the Time & Life Building and in other buildings for which they have contracts.

Because AKF Analysis and Testing usually visit a client’s building three or four times a year to run diagnostic tests on the HVAC equipment and tune the VFDs, they have seen the progression of motor bearing damage.  Now, the typical procedure is to recommend installation of an AEGIS shaft grounding ring whenever a replacement motor is installed or a motor’s bearings are replaced to ensure that harmful bearing currents have been eliminated and the VFD is running at its optimal performance.

THE EXPERIMENT

Shaft Voltage readings before and after one year

On February 6, 2008, before the ring was installed on the problematic motor, AKF A&T used a shaft voltage probe and an oscilloscope to measure the discharges from the motor’s shaft at 59.2V (peak-to peak) and 37.2V (peak-to-peak), at two different oscilloscope settings (10μsec/ div and 2μsec/div, respectively), for an average of 48.2V (peak-to-peak). The oscilloscope screen showed rapid dv/dt voltage collapse at the trailing edge of the waveform – typical of the electrical discharges that damage bearings. On February 20, 2008, two weeks after the ring was installed, AKFA&T took a ground-reference reading, for baseline comparison, of 9.21V (peak to-peak) with the oscilloscope set at 40ns/div. Minutes later, AKF A&T took two more shaft-current readings at the same setting: 8.86V (peak-to-peak) and 11.2V (peak-to-peak).  A little more than a year later, on March 9, 2009, the motor was checked again by AKFA&T technicians. This time the shaft voltage was even lower: 4.8V (peak-to-peak).  The readings may speak for themselves and the Building engineering manager Perez agrees the ring “seems to have resolved the issue.” Based on the positive results, he has installed AEGIS shaft grounding rings on additional fan motors in the Time & Life Building.

The article below corrects and adds to the EASA Current’s article from the Dec 2017 issue:  “Motor bearings: Electrical damage simplified.”

An article on electrical bearing damage recently appeared in EASA Currents Magazine. While it raised a lot of good points, it also contained information on shaft grounding practices and shaft grounding rings which needs correction or further examination. As we discuss electrical bearing damage, please note that we are referring to motors operated by variable frequency drives (VFD).

The title of the article was “Motor bearings: Electrical damage simplified.” True to this title, the article tries to stay as “nontechnical” as possible. Unfortunately, staying “nontechnical” is not always desirable since a thorough understanding of bearing currents and solutions is critical for successful repairs.  As the inventors of conductive microfiber shaft grounding rings – the AEGIS® products – we’ve been discussing electrical bearing damage and its solution for over twelve years. It is my opinion that in trying to keep it simple, the article sometimes oversimplifies the problem.

When talking about shaft grounding, it is a mistake not to differentiate capacitive shaft voltage discharge currents (EDM currents) and circulating currents (be they low or high frequency). The author states that his preferred method of bearing protection is to “break the electrical circuit on the opposite drive end (ODE)” of the motor, and install shaft grounding at the drive side. This mirrors our recommended approach for AC motors over 100 HP and for all medium voltage motors. However, circulating currents are not an issue in small AC motors (under 100 HP), so bearing insulation is unnecessary for them and adding a shaft grounding ring to the DE or NDE is sufficient to protect the motor’s bearings.

But the author then states that “if the [shaft] voltage can be clamped low enough with just a grounding brush, the ODE insulation is not always necessary.” This is false. Shaft grounding cannot prevent circulating currents. So, 100 HP+ and medium voltage motors would still be at risk of circulating currents. These motors will always need both shaft grounding (preferably the drive end), and some type of insulation at the opposite end (preferably the ODE).

There is another confusing or confused statement about circulating currents in the same paragraph: “This looping is also the reason [not to] install shaft grounding on the ODE as the loop can extend into the driven machine” and thereby damage its bearings. This is easily misconstrued to mean that shaft grounding should never be installed at the ODE. But it is important to know that this applies only when circulating currents are a potential problem, i.e. motor over 100 HP and medium voltage motors.

When discussing circulating currents potentially extending “into the driven machine” we advise referring to the IEEE 112 which recommends insulating the NDE to avoid a circulating current path in the driven machine. Last but certainly not least, it is important to remember that installing a shaft grounding ring on the DE is still necessary to prevent capacitive voltages from causing EDM damage to the steel (non-insulated) bearing and/or going down the shaft to the driven equipment.

Motor under 100 HP only needs a shaft grounding ring to protect the bearings.

Motor over 100 HP and need insulation (NDE) with a shaft grounding ring on the opposite end (DE)

 

Now for a discussion on shaft voltages: The author gives a maximum safe shaft voltage level of 5 V peak to peak, and correctly points out that there is no universally applicable “safe” level of shaft voltage. In fact, it the peak voltages which harm the bearings so discussing “peak” voltage is key. The NEMA MG1 document gives a range of 10 to 40 volts peak (20 to 80 volts peak to peak) as the shaft voltage level where bearing discharge can occur. But every system is different; every motor in every state of operation will have its own bearing breakdown voltage where discharge through the bearing occurs. With the lack of a consistent safe level across systems, neither peak voltage nor peak-to-peak measurements are a reliable measure of risk.

Therefore, we recommend that machine owners or motor repair technicians take shaft voltage readings with a high-speed oscilloscope (at least 100 MHz bandwidth). With this equipment, you can unambiguously tell whether discharge is occurring in the bearings. The smoking gun is a discharge pattern in the shaft voltage waveform (a slow voltage buildup followed by a rapid transition down).

While the article recommends shaft voltage testing, it only recommends measuring peak-to-peak voltage, even though it is not the most important measure. The sample shaft voltage reading in the article has a timescale of 2 ms/div, which is far too long to detect discharges, which occur on a timescale of microseconds. More on this in the Bearing Protection Handbook.

The shaft grounding ring section starts off well, but goes awry in the second sentence by referring to “the standard single row ring.” This refers to the AEGIS® SGR, for low voltage motors, but SGRs have two rows of conductive microfibers, not one. Next the article states that the standard SGRs are “usually” sufficient for smaller motors, but “may not always provide a low enough resistance circuit [sic] to ground.” An SGR with two fiber rows is extremely effective and is proven in millions of applications worldwide on low voltage motors under 500 HP (and as noted earlier, motors over 100 HP also need bearing insulation in addition to the SGR ring).

The article does better when talking about the six fiber-row AEGIS® PRO Rings, saying “These rings are the best option for the least amount of maintenance.” AEGIS® PRO rings are in fact the best and recommended option for medium voltage motors and LV motors over 500 HP.

The author does seem confused, though, about the purpose of the extra rows of conductive microfibers in the PRO ring. He seems to think the extra rows are to decrease the PRO’s resistance relative to an SGR. But in fact, the extra rows serve to increase current carrying capacity, not resistance/impedance per se. This extra capacity is needed for medium voltage and large low voltage motors (500 HP+) because they have higher shaft currents and the PRO ring’s extra rows accommodate that increased current.

The article later claims that “they can be susceptible to wear, heat, and grease contamination.” It is unclear whether “they” refers to PRO rings or SGRs, but the claim is inaccurate for either product. Any properly installed AEGIS® ring is not susceptible to wear. It is designed as a “wear-to-fit” device; the fibers slowly wear until they just-touch the shaft surface, a process which can take 200,000 hours or more. Electrical contact is maintained, by physical contact and near-contact electron transfer, so the ring does not stop working. As for heat tolerance, the manufacturer spec states that AEGIS® rings can withstand temperatures up to 410° F (210° C), and we know of at least one customer who uses AEGIS® rings in an oven at 400 degrees.

As for grease, the AEGIS® fibers cut through and tolerate light grease, but this is a moot point. As the article mentions, AEGIS® rings can be mounted inside motors, away from external grease and grime, and this is where motor repair shops should install them. One bit of information the article left out is that there is even a UL-approved process for installing AEGIS® rings inside explosion proof motors, which is unique among shaft grounding technologies.

Now we come to the author’s preferred shaft grounding method, the copper bristle brush. These devices have a few shortcomings that went unmentioned. The brush’s springs may clog and jam, copper is easily oxidized to its nonconductive green/blue oxide, and the shaft can glaze. Also, we are rather skeptical about the use of grease on the brush.

The article only mentions common mode chokes (CMC) briefly, but manages to get a few errors into those three sentences. The most important one to correct is the misleading statement that they “do not always reduce the shaft voltage to a low enough level to eliminate bearing damage.” In fact, common mode chokes barely affect shaft voltage at all and do nothing to eliminate EDM bearing discharges from the capacitive induced shaft voltage. CMCs are just not useful against the shaft voltage discharge currents that shaft grounding protects against. They may be able to reduce high-frequency circulating currents, but CMCs are not as effective as insulating one bearing, as described above. At any rate, common mode chokes cannot take the place of shaft grounding.

All that said, the article ends with some good preventive maintenance advice, including making regular shaft voltage measurements, and advising specifying engineers to call for motors with preventive measures installed. It ends, “Good service centers will identify electrically damaged bearings and strive to recommend the best method to prevent further issues.” We agree wholeheartedly. Bearing inspection is required by the ANSI/EASA Standard AR100-2015, and by the EASA warranty checklist, and is also one of the processes required of accredited EASA motor repair shops.

Ultimately, this article was written to educate motor repair shops and more importantly, to help them improve their processes and so improve their business. We agree with these goals and hope that the original article succeeds in encouraging an enlightened discussion and seeking the most accurate information when considering bearing protection. And we hope that this article, too, will help advance those goals.

AEGIS Rings have worked for years to improve reliability and sustainable operatuion of HVAC and Pump motors.   Case in point: Harvard University’s Campus services Facility Maintenance Operations (FMO) “offers Motor Shaft Grounding, a maintenance program that prevents bearing failure and significantly extends motor life.”

https://www.campusservices.harvard.edu/system/files/documents/950/Motor%20Shaft%20Grounding.pdf

Variable frequency drives (VFDs) present a compelling option for energy savings in sustainable building projects. Power usage in continuously running centrifugal pumps and fans decreases notably with input modulation by a VFD. For example, a 20% reduction in fan speed can reduce energy consumption by nearly 50%. VFDs also introduce their own sustainability problems, however. VFD-induced shaft currents can damage bearings, leading to shorter motor life and costly repairs. Shaft grounding technology such as the AEGIS Shaft Grounding Ring offers a sustainable solution to this problem. Unfortunately, most new construction projects including VFD-driven motors do not utilize this effective option.

The university maintenance department has spearheaded a campus-wide sustainability drive starting with their own Platinum LEED certified headquarters. A major component of this has been a testing program for VFD-driven HVAC motors. Maintenance technicians employ oscilloscopes and voltage measuring probes to ascertain the presence of shaft voltage. When harmful voltage levels are detected, the maintenance department may recommend the installation of an AEGIS ring. Already successfully employed in multiple new buildings on campus, expanding the AEGIS ring to existing motor setups that require bearing protection continues to increase campus-wide sustainability.

The University Test Case

In December 2009, the ring’s manufacturers installed their product on two VFD-controlled HVAC motors in the maintenance headquarters building as a demonstration of the new program. The identical three year old Baldor 7.5HP motors respectively powered a chilled water pump and an air supply fan. With the VFD set at 60HZ, the first motor was running at 1,776rpm. The oscilloscope measured peak-to-peak discharges of 61 volts. Results showed rapid voltage collapses at the trailing edge of the waveform, typical of the electrical discharges that damage bearings. After the reading, technicians cleaned the shaft and installed a split AEGIS ring. Follow-up test results displayed the discharge plot as a straight line, indicating that the AEGIS ring diverted shaft voltage discharges.

Pump Motor with AEGIS Ring installed with EP2400 Conductive Epoxy

The second motor was tested under identical conditions and measured 50.8 volts peak-to-peak shaft discharge. Due to the limited accessibility of this motor, application of a hand-held heater sped epoxy curing in the AEGIS ring installation. After the complete installation, a new test read only 380 millivolts peak-to-peak, again indicating the AEGIS ring successfully diverted shaft voltage discharges to ground.

Conclusion

VFDs provide a compelling option for energy savings in sustainability-minded design. However, unless a product such as the AEGIS Shaft Grounding Ring mitigates the risk of bearing damage, repair costs could outweigh any savings. While this problem remains best addressed in the design stage of the system, the university case study effectively demonstrates the potential to retrofit previously installed motors with shaft grounding technology. Once installed, an AEGIS ring requires no maintenance and lasts for the life of the motor, providing effective protection against shaft voltage.

An array of motors with AEGIS factory-installed http://www.est-aegis.com/oems.php

Most motor manufacturers have product lines with AEGIS Rings factory installed.

More information about the university case study can be found here.

Using variable frequency drives (VFDs or inverters) to control the speed of motors large and small is not only a flexible, cost-effective means of improving process/system control, it can also greatly reduce energy costs.

But VFDs have their drawbacks.  They can induce voltages on the shafts of the motors they control.  Without an alternate path to ground, these VFD-induced voltages will discharge through motor bearings causing electrical discharge machining (EDM) that blasts pits in bearing and race surfaces.  This process eventually leads to frosting and fluting (washboard-like ridges on the bearing race), and bearing failure — often in as little as 3 months!

While some large motors can be repaired on site, many must be pulled out of service, loaded onto a truck (often requiring a crane), and transported to a motor shop for repairs.  This can be a time-consuming and very expensive process.  And the larger the motor, the greater the cost of replacing damaged bearings.

But the cost of repairing motors often pales in comparison to the staggering costs of downtime and lost production.  Larger motors are typically used in high volume processing applications, where the cost of every minute of lost production is greater.

AEGIS® PRO Series Rings protect medium-voltage motors, generators, and turbines, as well as large-frame low-voltage AC motors (500 HP or greater) and DC motors (300 HP or greater).  PRO Rings are vailable in solid- and split-ring versions for shaft diameters from 2.5″ (50 mm) to 31.5″ (800 mm). They have 6 rows of conductive microfibers that completely encircle the motor shaft, providing millions of contact points that channel harmful shaft voltages away from bearings and safely to ground.

Solid PRO Rings are ideal for installation on new motors before they are put into service or on older motors when they are repaired. Split PRO Rings come in mating halves that slide around the shafts of coupled motors for fast, easy field installation.

There’s even an AEGIS® PRO Ring for protecting and monitoring remotely located or hard-to-access motors. The AEGIS® PROMR combines an AEGIS® PRO Ring with an AEGIS® monitoring ring (separated by an insulated phenolic spacer). It provides bearing protection and remote monitoring in a single, compact unit.  And Universal Mounting Brackets — designed to accommodate shaft shoulders, slingers, or other end bell protrusions — simplify and speed the installation of any size PRO Series Ring.

For horizontally mounted motors with single-row radial ball bearings at both ends of the motor, best practices include:
• At the non-drive end: Isolate the bearings using an insulated sleeve or coating; or install insulated ceramic or hybrid bearings to disrupt circulating currents.
• At the drive end: Install one AEGIS® PRO Bearing Protection Ring either internally (on the back of the bearing cap) or externally (on the motor end bracket).
• Whether the ring is installed internally or externally, be sure to apply AEGIS® Colloidal Silver Coating to the motor shaft where the ring’s conductive microfiber will make contact.
• For low- or medium-voltage motors where both bearings are insulated, install one AEGIS® Ring, preferably on the drive end to protect the bearings of attached equipment (e.g., gearbox, pump, fan bearings, etc.).

For more information on AEGIS® PRO Series Rings, see the PRO Series product page. You can also read an ROI analysis of PRO Rings, or peruse our case studies on how PRO Rings solved real-life problems in a number of applications.


Variable frequency drives (VFDs) are increasingly the norm in heating, ventilation, and air conditioning (HVAC) applications. These inverters reduce energy consumption by allowing motors to run at lower speeds. While yielding potential energy savings of 30%, VFDs introduce electrical currents which can shorten motor life if not properly addressed. VFDs provide such savings because HVAC fans and pumps run continuously at reduced loads. Reducing a fan’s speed by half cuts horsepower requirements to one-eighth. For this reason, VFDs represent a superior solution to the traditional throttling mechanisms used in such applications.

Most of today’s HVAC motors are built with sealed bearings. As a result, the primary cause of bearing failure in VFD-controlled motors is electrical damage. This damage occurs because voltages build up on the shaft and then discharge in short bursts along the path of least resistance–through the bearings. Over time, this discharge can lead to pitting and fluting. The National Electrical Manufacturers Association (NEMA) officially recommends bearing insulation to address these problems.  AEGIS Bearing Protection Rings provide a maintenance-free solution to bearing insulation in HVAC applications. Two rows of conductive microfibers lining the entire inner circumference of the ring provide safe avenues for discharge. Additionally, AEGIS rings qualify as sustainable technology under the Federal Energy Management Program.

AEGIS Rings are already widely used in a variety of HVAC installations. Electro Static Technology (EST), the manufacturers of AEGIS rings, recently ran a test on a rooftop air conditioning unit to demonstrate their effectiveness. Running the unit at the same conditions, peak-to-peak discharges from the shaft totaled 44.8 volts without an AEGIS ring. AEGIS ring installation reduced this reading to 3.76 volts peak-to-peak, well below levels that would damage bearings. A Chicago hospital recently observed the same results after introducing AEGIS rings to an air handling unit for operating rooms.

All VFD-driven motors are vulnerable to electrical damage. As VFD-driven motors continue to grow in HVAC applications, the AEGIS Bearing Protection Ring provides a compelling solution to mitigate the risk of bearing damage through electrical discharge. Motors protected by AEGIS rings will both enjoy reduced energy consumption and reduced maintenance costs. For more information about this sustainable, cost-saving solution, including the two effectiveness tests mentioned above, consult the following white paper.