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Variable-frequency drives (VFDs), 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.
If downtime occurs, savings from efficiency increases can vanish.
Shaft currents induced by VFDs can wreak havoc on bearings, dramatically shortening motor life and resulting in repairs. A reliable method of shaft grounding is essential to mitigate these currents and realize the full potential of the VFDs.
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. It has served as a model for the more than 300 buildings serviced by its staff.
Among the department’s recent 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, may require maintenance themselves. Other methods, such as bearing isolation, can shift electrical damage to connected equipment.
When harmful discharge levels are detected, the school’s maintenance department typically recommends the installation of a bearing-protection device (in this case, the Aegis™ SGR bearing protection ring) that bleeds off the damaging currents. This shaft grounding ring 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 and 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.
The split ring was installed on two VFD-controlled HVAC motors in the basement of the school’s 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 V, with rapid voltage collapses at the trailing edge of the waveform — typical of the electrical discharges that damage bearings.
After the readings were taken:
1. The shaft was cleaned with fine-grit sandpaper and wiped with an alcohol-dampened rag.
2. Paint was removed from the motor end bracket with a Dremel® rotary tool, exposing enough bare metal to mount the split ring.
3. The two-part conductive epoxy (included in the kit) was mixed and applied to the back of the ring.
4. To enhance conductivity, the motor shaft adjacent to the end bracket was coated with Aegis colloidal silver.
5. The ring was opened, centered around the shaft, taped closed, and pushed against the end bracket (see Figure 1, page 7). The entire installation took about 10 minutes.
Approximately an hour after installation, the epoxy had cured sufficiently to allow testing of the motor again with the oscilloscope and probe. This time the oscilloscope discharge plot was essentially a flat line, indicating that shaft voltage discharges were being diverted by the ring to ground.
On Motor 2, preinstallation peak-to-peak shaft discharges measured 50.8 V. The shaft was prepared. It was difficult to reach and only a couple of inches of it were exposed between the end bell and a sheave, making the split ring’s versatility and ease of installation key benefits.
A handheld heater helped speed the epoxy’s cure; about half an hour after installation the shaft voltage was measured again. This time, the reading was 380 millivolts (peak-to-peak).
For years, VFD-induced bearing failure was often misdiagnosed, until repair shops and testing consultants proved that the high peak voltages, fast voltage rise times, and nonsinusoidal shaft currents lead to the cumulative erosion of ball bearings and race walls.
For motors already in service, the ring can be retrofitted on any NEMA or IEC motor regardless of shaft size, horsepower, or end-bell protrusion. Key to the ring’s success are conductive microfibers that line the entire inner circumference of the ring in two rows, completely surrounding the motor shaft. Once installed, the ring requires no maintenance and lasts for the life of the motor, the company said, regardless of rpm.
Baldor Electric Co. recently introduced its Super-E® NEMA Premium® line of inverter-ready 1-50 hp general-purpose motors with Aegis grounding rings factory installed inside the motor housing (see Figure 2, page 7), with a choice of totally enclosed fan-cooled (TEFC) or open drip-proof (ODP) configurations.
Operation and maintenance costs can account for 60-80 percent of a building’s lifecycle costs. When equipment does not need to be repaired or replaced as often, that percentage drops significantly.
Publication date: 08/29/2011