MECHANICAL CONTRACTING
Ivy League university uses shaft grounding rings to improve sustainability of HVAC motors
university demonstrates how the push for more sustainable “green” building management has led to growing awareness of a chronic, widespread problem with HVAC motors — electrical bearing damage. Variable frequency drives (VFDs),
A 80
also known as inverters, are widely used because they save energy, especially in applications with variable torque loads. Because many centrifugal fans and pumps run continuously, their motors use less power if the speed is controlled by VFDs rather than by using a damper for flow control. But if downtime occurs, savings from efficiency increases vanish. Shaft currents induced by VFDs can wreak havoc with bearings, dramatically shortening motor life and causing costly repairs. To mitigate these currents and realize the full potential of VFDs, a reliable method of shaft grounding is essential. The university’s maintenance
department kicked off a campus- wide push for sustainability with the renovation of its own headquarters. When completed in 2006, this complex received a Platinum LEED® rating, serving as a model for the more than 300 buildings serviced by its staff. Among the department’s recent
preventive maintenance program at a leading New England Ivy League
The AEGIS™ SGR Bearing Protection Ring can now be installed with epoxy.
efforts to foster sustainability is a testing and retrofitting program for VFD-controlled HVAC motors. Technicians use portable probes and oscilloscopes to test fan and pump motors for harmful shaft voltages. Ironically, some products designed to protect bearings, such as conventional grounding brushes, require extensive maintenance themselves. Others, such as bearing isolation,
can shift electrical damage to connected equipment. So, when harmful discharge levels are detected, the maintenance department usually recommends the installation of a
proven bearing-protection device that bleeds off the damaging currents — the AEGIS™ SGR bearing protection ring. A shaft grounding ring, the AEGIS safely redirects VFD-induced shaft voltages by providing a very-low-impedance path from shaft to frame, bypassing the motor bearings entirely. Manufactured and sold by Electro
Static Technology, the grounding ring is available in two versions — solid or split. Designed for installation with either brackets or conductive epoxy, the split ring used by the university speeds field installations, because it can be installed without uncoupling the motor from attached equipment. To demonstrate its effectiveness,
the AEGIS split ring was installed on two VFD-controlled HVAC motors in the basement of the maintenance headquarters, which is cooled in summer by ground-source heat pumps. Motor 1 powers a chilled water pump. Motor 2 runs an air supply fan. Both are Baldor 7.5 HP motors (NEMA frame size 213T). When a voltage probe was held
against the shaft of Motor 1, the oscilloscope indicated peak-to-peak discharges of 61 volts, with rapid voltage collapses at the trailing edge of the waveform — typical of the electrical discharges that damage bearings. After the readings, the shaft was
Prior to installing the ring, a narrow band of AEGIS™ colloidal silver coating was applied around the shaft where the fibers contact it, to improve the surface conductivity. The ring was then installed on the shaft and re-taped.
cleaned with fine-grit sandpaper and wiped with an alcohol-dampened rag.
e Continued on p 82
phc may 2011
www.phcnews.com
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