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VFD-driven HVAC Motors Need Bearing Protection

Sept. 6, 2017
The technology that makes a heating and cooling system so efficient may also make It less reliable.

A major goal of modern heating, ventilation and air conditioning (HVAC) systems is to eliminate energy wasted on overheating or overcooling areas of a building or home.  Matching heating and cooling capacity to load can yield tremendous energy efficiency gains and dramatically reduce building energy costs. But the same technology that makes today’s heating and cooling systems so efficient may also be causing the motors in these systems to fail prematurely, resulting in high maintenance and repair costs.

A key tool in improving the energy efficiency of today’s HVAC systems is variable frequency drives (VFDs). By controlling the amount of electric power to motors, VFDs control their speed, saving 30 percent or more in energy costs. They are a simple, cost-effective means of precisely matching HVAC capacity to load within a building or within discrete areas of it, thereby eliminating the energy costs associated with conditioning and moving the air as well as the costs of over-conditioning it.  Because the energy required to run fan or pump motors correlates to their flow rate cubed, reducing the speed of these motors by half reduces the horsepower needed to run them by a factor of 8.  And restricting the work of a motor through the use of dampers or other throttling mechanisms is simply a waste of energy and money.

But while VFDs provide significant energy savings in HVAC applications, they can also damage motors. VFDs create voltages on the shafts of the motors they control ― voltages that can discharge through motor bearings, causing pitting (small fusion craters in metal  bearing surfaces), frosting (widespread pitting), fluting (washboard-like ridges on the bearing race), and complete bearing failure ― often in as short as three  months.

In the end, the cost of repairing or replacing the motor often cancels out the cost savings from using a VFD. And the owner must deal with the costs and inconvenience of being without heat or air conditioning.

How big a problem is VFD-induced bearing damage? Consider the following:

Most motor bearings are designed to last for 100,000 hours, but motors controlled by VFDs can fail in as few as 720 hours of operation. Motor failures caused by VFD-induced shaft voltage result in hundreds of thousands of hours of unplanned downtime each year in the US alone.  In addition, these failures reduce the performance and mean time between failure of the OEM systems in which they are used.

An HVAC contractor recently reported that, of the VFD-controlled 30-60 HP vane axial fan motors he installed in a large building project, all failed within a year (two within 6 months).  Repair costs totaled more than $110,000. After testing AEGIS® Rings on a VFD-controlled 50 HP "problem" fan motor — a motor that had suffered bearing failure three times — the engineering firm hired to test and tune the VFDs at the Time & Life Building in New York recommended installing AEGIS® Rings on all 240 HVAC system motors in the building.

To protect VFD-driven motors from electrical bearing damage, effective long-term bearing protection is needed.  Proven in millions of installations worldwide, AEGIS® Shaft Grounding Rings channel harmful voltage discharges away from bearings and safely to ground, protecting motor bearings for their full L-10 life.

AEGIS® SGR Bearing Protection Rings are available for low-voltage motors with NEMA frame sizes 56 to 449T and shaft diameters to 4.875-in.

AEGIS® uKITs (universal mounting kits) are available in versions that include either a solid or split AEGIS® Ring and universal mounting brackets for fast, easy installation on any 
motor, even those with shaft shoulders, slingers, or other end bell protrusions.

Solid Ring uKITs are ideal for installation before motors are put into service. Split Ring uKITs speed and simplify field installation of AEGIS® Rings on coupled or in-service motors. 

 Click the download link below, to obtain a white paper on VFD-induced bearing currents.