For more than 20 years, since the installation of the first modern variable-frequency drives (VFDs) at the Time & Life Building in New York City, the maintenance department had coped with chronic HVAC-fan-motor-bearing failure, the result of VFD-induced shaft currents.
VFD-induced shaft currents discharge through bearings, leaving fusion craters in ball bearings and bearing race walls. The discharges are frequent enough that, before long, the entire race is riddled with pits known as frosting. Concentrated pitting at regular intervals along a race wall can cause washboard-like ridges called fluting, a source of noise and vibration. By that time, bearing failure often is imminent.
“The whole phenomenon of electrical bearing damage is so misunderstood that some maintenance managers have lost their jobs over it,” Tom O'Connell Jr. of AKF Analysis & Testing (AKFA&T), an engineering firm hired by Rockefeller Group Development Corp. to periodically test and tune the building's VFDs, said. “It's critical because a single motor might supply air to 30 floors, where the tenants are paying $110 a square foot and expect the temperature and quality of their air to remain constant.”
Late in 2007, O'Connell suggested installing an AEGIS SGR Bearing Protection Ring from Electro Static Technology on the shaft of what Building Manager Ron Perez called “one of the most annoying motors in the building,” a 50-hp fan motor notorious for running hot and making a lot of noise because of short-lived bearings.
The AEGIS SGR Bearing Protection Ring uses the principles of electron tunneling and ionization to bleed off damaging currents safely and efficiently. It provides a low-impedance path from shaft to frame, bypassing a motor's bearings. To boost the rate of electron transfer, conductive microfibers line the ring's entire inner circumference.
The AEGIS SGR Bearing Protection Ring requires no maintenance, lasts for the life of a motor, and qualifies as sustainable technology under the Federal Energy Management Program.
Prior to installing the SGR in February 2008, AKFA&T used a voltage probe and oscilloscope to confirm that VFD-induced currents were indeed discharging from the motor shaft at voltages high enough to damage bearings.
Since the AEGIS SGR was installed, the motor has run quietly and at least 100°F cooler. Tests two weeks after the installation showed negligible shaft currents. A year later, the voltage was even lower.
To date, Perez has installed AEGIS SGRs on 41 fan motors in the Time & Life Building.
Estimating it will reduce repair costs by approximately 10 percent, O'Connell recommends the SGR be installed on all existing HVAC motors and specified for new buildings.
Information and photographs courtesy of Electro Static Technology.
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