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.

Pulse Width Modulated (PWM) power sources, also known as variable frequency drives (VFD), adjustable speedy drives (ASD), “inverters,” or “drives,” cause bearing current problems within electric motor bearings.   The most common types of bearing current are capacitively induced shaft voltage discharge, or EDM current, and in larger motors, electromagnetically induced high frequency circulating currents.

The impedance of the bearing to high frequency discharges is a function of the oil film between the rolling element and the bearing race, load and system design variables, the type of bearing, speed of rotation, and other factors.  The bottom line is that as the shaft rotates an oil film forms between the rolling elements and the bearing race.  This oil film is dielectric and would not normally not allow current to flow.  However, because it is very thin, only 5 to 10 microns thick, when the induced shaft voltage is high enough, [10 to 40 volts peak per NEMA MG1] the oil film will break down and allow an arc to pit the motor’s bearing.

Shaft Voltage Discharge

Bearing current is the current that arcs through the electric motor bearings and leads to mechanical damage. This damage is evident from the pits in the bearing.  While the electric motor is in operation, arcing occurs thousands of times per second.  In addition to pitting the bearing, arcing also oxidizes the lubrication, which further decreases the bearing life.

Two potential mechanisms for bearing damage when operating on pulse width modulated variable frequency drives are dv/dt (the change in voltage divided by the change in time), which contributes to EDM currents caused by the parasitic capacitive coupling from stator to rotor, and in larger motors (over 100 HP / 75 kW), the high frequency circulating currents are caused by high common mode current levels.

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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!


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