Shaft Voltage Testing

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.


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.


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.


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.

Pitting Frosting and Fluting Damage

Electric motors are prone to many sources of damage, some more common, and some better known.  An increasingly common, but still not too well known, problem facing these motors is bearing current.  Bearing currents damage the ball bearings with electrical discharge machining (EDM) resulting in motor failure.  In addition to the repair or replacement costs of the electric motor, there is the possible loss of production caused by that motor failure.  Adding it all together results in one expensive failure!

Bearing currents have been a thorn in the side of motor users for years.  However, incidents are on the rise for a simple reason: more people are using pulse width modulated variable frequency drives (VFDs).  We are going to look at what bearing currents do and how to avoid them.

Bearing Currents Cause Problems

Shaft voltages create electrical bearing damage when they discharge through the bearings in a tiny but destructive electrical arc. Pitting, frosting, and fluting are some of the damages caused by that current.  These types of damages lead to premature bearing failure, which then leads to the motor failing as well.  In addition, if a conductive coupling is connected to the motor drive end shaft, shaft voltage can discharge through the coupled equipment damaging its bearing with electrical arcing.

Balanced 3 phase sine wave line voltage

Unbalanced PWM pulses – Common Mode Voltage

Three-phase power is what operates most AC motors.  On line power, the three phases always sum to zero volts.  The sum of the three phases is also called common mode voltage.  The common mode voltage is zero on line power because each phase is a smooth sine wave voltage 120 degrees out of phase with the other two phases.  But when a VFD is in control, each phase consists of a series of positive, zero, and negative voltage pulses.  The three phases don’t usually sum to zero, so the common mode voltage is continuously changing with the pulses driving the motor.  This nonzero common mode voltage creates shaft voltage by parasitic capacitive coupling between stator and rotor.

And when voltage is induced onto the motor’s shaft, that voltage “looks for” a least-resistance path to ground. Usually, the path of least resistance leads through the motor’s bearings.  As described above, shaft voltage discharging through the bearing causes EDM and cumulative electrical bearing damage.

Why Test Motors for Shaft Voltage?

If your facility relies on electric motors, include testing for shaft voltage in your preventative maintenance (PM) program. These tests are performed on site and you receive an analysis of any potential problems.

The idea behind an effective PM program is obvious:  to stop issues before they become problems.  Using an effective testing method for shaft voltage leads to lower cost options than repairing or replacing an expensive motor. Reducing the risk of shaft voltage leads to a reduced risk of bearing failure and presents an opportunity to save substantial expenses.

Testing for shaft voltage is a relatively simple process.  At an exposed area of the motor’s shaft, a conductive microfiber probe tip safely touches the motor’s spinning shaft and captures shaft voltage data for analysis.  Using that information, recommendations are made for courses of action.  Using the AEGIS Shaft Voltage Tester is the simplest and easiest way to take shaft voltage readings.  The AEGIS Best Practices Handbook explains the step-by-step procedure as well as many other important topics to protect your motor bearings!


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.

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.