Challenged to reduce energy consumption, facility managers are installing variable frequency drives (VFDs, also known as inverters) in heating, ventilation, and air-conditioning (HVAC) systems. By allowing motors to run at less than full speed, these drives can yield energy savings of 20-30 percent, but they also induce currents that can damage bearings and shorten motor life. The resulting repair costs can wipe out any savings from their use. To make HVAC systems sustainable as well as energy-efficient, a reliable method of bearing protection is required.

VFD-Induced Currents Destroy Bearings, Kill Motors

Many HVAC fans and pumps run continuously, but often at reduced loads. Because the energy consumption of such devices correlates to their flow rate cubed, the motors that drive them will use less power if controlled by a VFD. In fact, reducing a fan’s speed by half cuts the horsepower needed to run it by a factor of eight.

Although the energy-saving potential of VFDs was never in dispute, for years the true cause of VFD-induced bearing failure was often misdiagnosed. Eventually, repair shops and testing consultants proved that the high-peak voltages, fast voltage rise times, non-sinusoidal shaft currents, and parasitic capacitance associated with typical pulse-width-modulated VFDs lead to the cumulative erosion of bearings.

Without mitigation, voltages repeatedly build up on the motor shaft to a certain threshold, then discharge in short bursts along the path of least resistance, which all too often runs through the motor’s bearings. The discharge rate tends to increase with carrier frequency.

Figure 1:Pitting of a bearing race wall (magnified) — the result of VFD-induced electrical discharges from the motor shaft.

Continued discharges result in the pitting (Figure 1) of the balls and race walls through electrical discharge machining (EDM). Concentrated pitting at regular intervals along the race wall can cause washboard-like ridges called fluting (Figure 2), a source of vibration and noise that can reverberate through ductwork. And while damaging voltages can be measured by touching an oscilloscope probe to the shaft while the motor is running, by the time such damage can be heard, bearing failure is often imminent.

Figure 2:Taken from a failed motor, the “fluted” bearing race wall (left) resulted from VFD-induced bearing currents. Protected by an AEGIS SGR Bearing Protection Ring, the bearing race on the right is undamaged.

Protecting Motor Bearings from Electrical Damage

Some bearing protection products, such as insulated or ceramic bearings, can shift the damage elsewhere. In fact, Standards issued by the National Electrical Manufacturers Association (NEMA) highlight the need for extra bearing protection for VFD-driven motors but point out that bearing insulation alone will not prevent damage to other connected equipment. When the path to the bearings is blocked, the damaging current seeks another path to ground. That other path often runs through a pump, gearbox, tachometer, or other device, which can consequently wind up with bearing damage of its own.

Many so-called “inverter-duty” motors offer beefed-up winding insulation, but do not guard against bearing damage. If they are to be truly ready for use with VFDs, these motors also need bearing protection.

Ironically, some other products designed to protect bearings from electrical damage, such as conventional spring-loaded grounding brushes, require extensive maintenance themselves.

One of the most promising protective devices diverts damaging currents safely to ground, bypassing the motor’s bearings. Installed around the shaft of a VFD-controlled motor, the maintenance-free AEGIS SGR Bearing Protection Ring extends motor life and qualifies as sustainable technology under the Federal Energy Management Program.

Preventive Maintenance

As preventive maintenance for motors already in service, the AEGIS ring can be quickly and easily retrofitted onto virtually any motor regardless of shaft size, horsepower, or end-bell protrusion using conducive-epoxy and/or a new Universal Mounting Kit (Figure 3). Conductive microfibers, which line the ring’s entire inner circumference in two rows, boost the electron-transfer rate.

Figure 3:The Universal Mounting Kit includes brackets and hardware that simplify mounting of the AEGIS SGR Bearing Protection Ring.

The AEGIS ring has been proven in hundreds of thousands of installations. HVAC applications include rooftop systems, indoor and outdoor air handling units, ventilation fans, fan walls, air- or water-cooled chillers, chilled water and other pumps, condensing fans, and compressors (Figure 4).

Figure 4:To save energy and help make buildings “green,” the AC motors in many types of HVAC equipment today are controlled by variable frequency drives and thus are vulnerable to bearing damage. This diagram shows some of these applications where shaft grounding makes motors and systems sustainable as well as energy-efficient.

Shaft Grounding Proven Effective on Rooftop Unit

Electro Static Technology (EST), the manufacturer of the AEGIS ring, recently demonstrated the ring’s effectiveness on a typical rooftop air conditioning unit at a Michigan department store (Figure 5). EST technicians took voltage readings from one of the unit’s VFD-controlled fan motors both before and after installing the ring on the motor’s shaft.

Figure 5: Rooftop HVAC unit in Michigan where a newly installed AEGIS SGR Bearing Protection Ring reduced shaft currents by 92 percent — to levels well below those that damage bearings.

In a continuous 60-μsec trace with the motor running at 3,600 rpm (80 Hz output), discharges from the shaft measured 44.8 volts peak-to-peak, high enough to cause pitting of the motor’s bearings (oscilloscope settings: 10v/div, 5 msec/div). After the grounding ring was installed, a follow-up test at 3,600 rpm showed that discharges had dropped to only 3.76 volts peak-to-peak (Figure 6), well below levels that damage bearings (oscilloscope settings: 2v/div, 500 μsec/div).

Figure 6: Motor shaft currents before (left) and after (right) installation of the AEGIS SGR Bearing Protection Ring on a motor in an HVAC unit on the roof of a Michigan department store.

New Line of Motors with Bearing Protection Inside

While retrofits are still the most common way to protect bearings from VFD-induced shaft currents, a small number of forward-thinking OEMs are now offering motors with shaft grounding rings factory installed. For example, Baldor Electric Company recently introduced its Super-E NEMA Premium line of general-purpose motors with AEGIS SGR Bearing Protection Rings installed inside the motor housing. Internal mounting (Figure 7) protects the motor against bearing damage from the outset and minimizes their exposure to moisture, dust, and other contaminants.

Figure 7: The AEGIS SGR Bearing Protection Ring can also be installed internally by the motor’s manufacturer, to protect a motor’s bearings from the outset.

Sustainable Energy Savings

All VFD-driven motors are vulnerable to electrical bearing damage. A savvy specifier will choose new motors that are truly equipped for use with today’s fast-switching VFDs.  But for motors already in service, retrofitting with an economical device such as a shaft grounding ring is the best approach.

Operations and maintenance costs can account for 60-80 percent of a facility’s life-cycle costs.  When HVAC equipment does not have to be repaired or replaced as often, that percentage drops significantly. VFDs hold the promise of sizable energy savings, but without effective, long-term bearing protection for the motors they control, those savings could be wiped out by high maintenance costs. By diverting bearing currents safely to ground, AEGIS SGR Bearing Protection Rings promote the reliable, long-term operation of VFD-driven HVAC systems, locking in energy savings to make these systems sustainable and truly “green.”

For more information visit www.est-aegis.com.