Consulting-Specifying Engineer - December 2007 - (Page S16) Variable frequency drives are great for energy efficiency, and new technologies can be deployed to limit the negative effects on electrical motors. Protecting Motors Against Drive-Induced Damage ue in large part to an increased focus on energy savings, the use of pulse-width-modulated (PWM) variable-frequency drives (VFDs) to control AC motors has grown dramatically in recent years. But this increased use of these drives is not without its downside when it comes to power quality. While they offer low operating costs and high performance, VFDs can cause motor problems. Shaft currents induced by VFDs can lead to motor failures. Without some form of mitigation, shaft currents travel to ground through bearings, causing pitting, fusion craters, fluting, excessive bearing noise, eventual bearing failure, and subsequent motor failure. BY WILLIAM OH, General Manager Electro Static Technology, Mechanic Falls, Maine D This is not a small problem, and one must consider several factors. The following are some of the issues that must be addressed: >> Most motor bearings are designed to last for 100,000 hours, yet motors controlled by VFDs can fail within one month—which translates into 720 hours. >> An HVAC contractor recently reported that, of the VFD-controlled, 30- to 60-hp vane axial fan motors he installed in a large building project, all failed within a year—two within six months. Repair costs totaled more than $110,000. >> Several large pulp and paper companies surveyed noted that the VFD-controlled AC motors used in their plants typically fail due to bearing damage within six months. >> One of the largest motor manufacturers in the United States has cited eliminating drive-related motor failures as its number-one engineering challenge. >> Today, there are almost a dozen blogs on the Internet focused on discussing the problems presented by VFD-induced shaft currents, sharing information and experiences, and suggesting a number of solutions to the problem. >> Motor failures caused by VFDinduced shaft currents result in hundreds of thousands of hours 16 | PURE POWER // WINTER 2007
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