At a paper mill in the Midwest, every new motor controlled by a variable frequency drive (VFD) is tested, and if shaft voltages are found, the motor must be equipped with an AEGIS® Bearing Protection Ring to divert damaging currents to ground. The plant’s electrical reliability engineer established this money-saving specification after many years of frustration from recurring bearing failures. The maintenance history of one motor tells the story. The large 1,000 HP motor is part of a system that turns pulp into “parent rolls,” which are later cut into well-known brands of paper towels, napkins, and other products. The mill, one of many owned by a major paper company, employs more than 1,800 people in collecting and converting about 430,000 tons of wastepaper per year. The problem with the motor, which runs at up to 1,200 rpm, stemmed from its VFD, which induces stray currents that travel through the motor’s shaft. Despite the efforts of the plant’s in-house maintenance staff and the plant’s maintenance contractor, L&S Electric Inc., the shaft currents would destroy the motor’s bearings within two years. Seeking ground, the currents blasted the bearing balls and races with countless fusion craters. The arcing that created these pits released tiny particles of metal that contaminated bearing grease, causing friction and high temperatures that burned the grease.
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