Bearing protection for breakdown protection
A preventive maintenance plan at the prominent Time & Life Building in New York City serves as an example of the widespread problem of electrical bearing damage in HVAC motors. The 48-story building is one of 19 buildings in the Manhattan business and entertainment complex known as Rockefeller Center. The problem is related to the ac motors that are controlled by variable frequency drives (VFDs). Since VFDs induce unwanted currents, electrical bearing discharges, and premature motor failure, the savings generated by the VFDs are erased by the maintenance costs associated with them.
The Rockefeller Center's HVAC system can be powered by either natural gas or electricity, and more than 100 VFDs control the speeds of the 240 motors that run the building's HVAC fans and pumps. Proper tuning of a drive's frequency output range and proper ground of a VFD-controlled motor's frame help avoid premature motor failure and system downtime.
The Rockefeller Group Development Corp . initially selected AKF Analysis & Testing (AKF A&T) LLC, New York City, to periodically test and tune the building's VFDs using harmonic filters. The testing resulted in a drop from 90 motor/bearing replacements per year to 20. However, the Rockefeller Group felt the 20 motor/bearing replacements were still too expensive, so the group contracted AKF A&T to find an alternative solution to the motor protection problem.
After other mitigation techniques proved unsuccessful, AKF A&T partner Tom O'Connell Jr. selected the AEGIS SGR Bearing Protection Ring from Electro Static Technology , Mechanic Falls, Maine. Engineered with conductive microfibers, the mitigation device uses electron transport technology to safely discharge VFD-induced shaft voltages by providing a low-impedance path from shaft to frame, which completely bypasses the motor's bearings. O'Connell suggested installing the protection ring on the Time & Life Building's most problematic HVAC motor—a 50-hp fan motor that had already failed three times (Figure 1).
“The whole phenomenon of electrical bearing damage is so misunderstood that some maintenance managers have lost their jobs over it,” O'Connell said. “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 sq ft. They have the right to expect the temperature and quality of their air to remain constant.”
AKF A&T conducted a vibration analysis that revealed the problematic motor's failures were due to shaft current induced by the VFD controlling the motor. In February 2008, before the protection ring was installed on the motor, AKF A&T used a voltage probe and an oscilloscope to measure the discharges from the motor shaft at 59.2 V (peak-to-peak) and 37.2 V (peak-to-peak), at two different oscilloscope settings (10 s/div and 2 s/div, for an average of 48.2 V (peak-to-peak). Two weeks after the ring was installed, AKF A&T took a ground-reference reading, for comparison, of 9.21 V (peak-to-peak) with the oscilloscope set at 40 ns/div. One year later in March 2009, the team tested it again and the shaft voltage was 4.8 V (peak-to-peak) and the motor was running at 100 degrees cooler. Based on the positive results, Ron Perez, the Time & Life Building's manager of engineering, installed 13 AEGIS protection rings on additional fan motors in the building.
The use of VFDs in buildings can provide sizable energy and cost savings, but without adequate shaft grounding, the results are indiscernible. The Time & Life Building managers' decision to experiment with the bearing protection ring resulted in cost savings and a reduction in energy usage.
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