Pumps & Systems, November 2012
All major manufacturers of three-phase, AC induction motors offer inverter-duty or inverter-ready models. While these motors have inverter-rated insulation to protect the windings, the bearings—their most vulnerable parts—are too often ignored. Some customers who purchase these motors may not understand this. The National Electrical Manufacturers Association (NEMA) has yet to recommend that new motors have bearing protection against damaging electrical discharges.
It is now common knowledge that inverters—also known as variable frequency drives (VFDs) and adjustable speed drives—can induce unwanted motor shaft voltages that, without effective mitigation, can destroy bearings, causing premature motor failure.
Although current NEMA standards highlight the possible need for extra bearing protection for VFD-driven motors, the language is often not specific enough to guide motor manufacturers and has not been updated to include new research results and developments in shaft grounding technology. Stronger, reworked standards, calling for effective mitigation in the form of reliable long-term shaft grounding technology, would go a long way toward cautioning motor users of the need for such mitigation and would help clear up common misconceptions.
VFDs can save 30 percent or more in energy costs. Because of this, they have been cited as a key technology for those wishing to make their commercial HVAC systems, automated assembly lines and other processes more energy-efficient. Whether used to control a motor’s speed or torque, VFDs often induce voltages and currents that can damage bearings. In fact, the costly repair or replacement of failed motor bearings can wipe out the savings that a VFD yields and severely diminish the reliability of the system. When a VFD-controlled motor fails, warranty claims against the motor and VFD manufacturers may not be possible. Because systems that use VFDs are so varied, the liability question is a sensitive issue.
Bearing Grounding Technology Advances
In virtually every case, the most reliable and cost-effective way to minimize electrical bearing damage and make these systems sustainable is a motor shaft grounding ring (combined with insulation for motors greater than 100 horsepower).Image 1. Viewed under a scanning electron microscope, a new bearing race wall is a relatively smooth surface, marked by nothing but mechanical wear.
|Image 1. Viewed under a scanning electron microscope, a new bearing race wall is a relatively smooth surface, marked by nothing but mechanical wear.
Unlike older, single-point contact brushes, new grounding rings encircle a motor’s shaft with contact points for greater effectiveness. One model has continuous circumferential rows of specially engineered microfibers to boost electron transfer rates and provide low impedance from the shaft to the frame. These rings safely bleed damaging currents to ground, bypassing a motor’s bearings entirely. Because the microfiber brushes work with little or no contact, they do not wear out like conventional brushes.