Time, Life, and VFDs

Jan 1, 2010 12:00 PM

An HVAC motor at the Time Life building with a bearing protection ring installed. For VFD-equipped motors of less than 100 hp (75 kW), a single SGR on either end of the motor shaft is typically enough to divert harmful shaft currents.

More than 100 variable frequency drives control the 240 motors that run the HVAC fans and pumps in the Time Life Building in New York's Rockefeller Center. In the past, however, a large portion of the savings from these systems were wiped out by maintenance costs because the switching transistors in the VFDs induced powerful currents that discharged to ground through the motor bearings. This effectively caused a kind of electrical-discharge machining that eroded the bearing race walls and led to appreciable bearing noise and, ultimately, premature motor failure.

The problem was solved with a device that uses conductive microfibers to safely bleed off these damaging currents to ground. The AEGIS SGR Bearing Protection Ring provides a low-impedance path from shaft to frame, bypassing the motor bearings entirely. The device never wears out because it exerts no pressure on the motor shaft. Alternative technologies such as conventional metal grounding brushes, do wear out and tend to need extensive maintenance.

In one case the protection rings went on a 50-hp motor that had previously failed three times from induced bearing currents. With the rings installed, the motor ran much more quietly and cooler by at least 100° F. Before installation of the protection rings, discharges from the motor shaft measured as high as 59.2 V (peak-to-peak) After installation, readings dropped to 11.2 V (peak-to-peak) at worst. Used with a VFD, the AEGIS SGR Bearing Protection Ring also qualifies as sustainable technology under the Federal Energy Management Program (efficiency standards for federal-government buildings under the Energy Independence and Security Act of 2007).