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Variable frequency drive for motors

VFD

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Description

The ideal solution would be a low-cost, maintenance-free device that safely redirects shaft currents along a very-low-impedance path from shaft to ground (protecting connected equipment as well as bearings). This device could be installed by the motor manufacturer or retrofitted in the field in virtually any variable frequency drive application. One device that meets all these criteria is the AEGIS? SGR Bearing Protection Ring from Electro Static Technology, a relatively recent invention that overcomes the problems of conventional grounding brushes. The ring works with virtually no friction or wear and is unaffected by dirt, grease, or other contaminants. A few manufacturers have introduced motors with bearing protection rings already installed, but at this writing such motors are the exception, not the rule.

The use of nonconductive ceramic balls prevents the discharge of shaft current through this type of bearing. As with other isolation measures, shaft current will seek an alternate path to ground. This technology is very costly, and in most cases motors with ceramic bearings must be special ordered and have long lead times. In addition, because ceramic bearings and steel bearings differ in compressive strength, ceramic bearings must be resized in most cases to handle mechanical static and dynamic loadings.

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