WEB EXCLUSIVE: Extending Motor Life
This Web Exclusive article is contributed by Electro Static Technology, maker of AEGIS shaft grounding products.
Increasing Sustainability Of Motors In Manhattan’s Time & Life Building
A preventive maintenance plan that reduced the total life cycle cost of operations in a prominent building at the heart of New York City serves as an example of how the push for more sustainable building management has led to a growing awareness of 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 had been happening for two decades 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. The building’s HVAC system can be powered by either natural gas or electricity, enabling management to switch back and forth to take advantage of whichever source costs less at the time.
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 had 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.
The Role Of Shaft Grounding
Proper tuning of a drive’s frequency output range and proper grounding of a VFD-controlled motor’s frame are paramount. Only in recent years 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 advanced electron transport technology to safely bleed off these damaging currents to ground. Engineered with special conductive microfibers, the AEGIS SGR Bearing Protection 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 had 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 ductwork. Thanks to the efforts of AKF Analysis & Testing, LLC (AKFA&T), an engineering firm hired by the Rockefeller Group Development Corporation to test and tune (with harmonic filters) the building’s VFDs periodically, the rate of motor/bearing replacements has dropped from 90 to 20 per year, but that was still too costly. Other attempts to mitigate the problem, including the installation of ceramic bearings on some motors, produced mixed results, usually proving too costly for the actual improvements gained.
The Solution At Last?
Finally, late in 2007, Tom O’Connell Jr., a partner in AKFA&T, read about the AEGIS SGR Bearing Protection Ring and began the process that could eventually end the bearing damage problem at the Time & Life Building. He called the ring’s manufacturer, Electro Static Technology, to discuss this and soon after decided to recommend the installation of a single SGR shaft grounding ring on the most problematic of all the HVAC motors at Time & Life.
Ron Perez, the facility’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. This was an unprecedented move. AKFA&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.
“The whole phenomenon of electrical bearing damage is so misunderstood that some maintenance managers have lost their jobs over it,” O’Connell explains. “Replacing a fan motor is a big expense. And let me make it clear—it is not just a motor. In an office tower, a motor can be running critical equipment that supplies air to 30 floors where 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. Now, with [this bearing protection ring], I can show a customer, in person, how the ring improves his equipment—immediately. The before and after are dramatic and indisputable.”
O’Connell has since recommended the bearing protection rings be eventually installed on all HVAC motors in the Time & Life Building and in other buildings for which he has contracts. Estimating it will reduce repair costs by approximately 10%, he also recommends the ring be included in the HVAC specifications for new buildings.
Because AKFA&T usually visits 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 install an SGR whenever a replacement motor is installed or a motor’s bearings are replaced. Follow up testing and more critical frequency tuning ensure that harmful shaft currents have been eliminated, and the VFD is running at its optimal performance. AKFA&T does no actual repairs or installations themselves, to avoid a potential conflict of interest.
Notorious for running hot and making a lot of noise due to short lived bearings, the 50-HP fan motor that O’Connell convinced Perez would be a good candidate for the first SGR was “one of the most annoying motors in the building,” Perez recalls. Three times the motor had failed. The Rockefeller Group had replaced the motor twice, the bearings once. Vibration analysis by AKFA&T indicated the failures were due to shaft currents induced by the VFD controlling the motor. The maintenance crew tried different lubricants and line reactors, to no avail. With this motor, even AKFA&T’s VFD frequency tuning did little to stop the destructive currents.
Today, the motor is quiet and runs at least 100° cooler, reports O’Connell. Oscilloscope tests show that shaft currents have dropped to a negligible voltage.
O’Connell and AKFA&T employee Brian Melvin, a vibration analyst, have both seen electrical bearing damage again and again, wherever motors are controlled by VFDs. “The problem exists in every building we’ve worked in, which is more than 200 buildings in New York City alone,” says Melvin.
Every drive/motor interface is different, so sometimes the damage starts when the motor is two months old, sometimes when it is six years old, but it is too often a problem. The two men are equally familiar with other mitigation measures; their periodic testing shows how ineffective such measures usually are over the life of a motor.
On February 6, 2008, before the ring was installed on the problematic motor, AKFA&T used a voltage probe and an oscilloscope to measure the discharges from the motor shaft at 59.2 volts (peak-to-peak) (see readings above) and 37.2 volts (peak-to-peak), at two different oscilloscope settings (10μsec/div and 2μsec/div, respectively), for an average of 48.2 volts (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 9.21 volts (peak-to-peak) with the oscilloscope set at 40ns/div. Minutes later, AKFA&T took two more shaft-current readings at the same setting: 8.86 volts (peak-to-peak) (see readings below) and 11.2 volts (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: 4.8 volts (peak-to-peak).
Building engineering manager Perez agrees the ring “seems to have resolved the issue.” Based on the positive results, he has installed AEGIS SGRs on 13 additional fan motors in the Time & Life Building. Asked if he expects other buildings in Rockefeller Center to follow suit, he replies, “Based on our success, I am sure it would be something they would consider.”
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