More often these days, multiphase A.C. electric motors are being controlled by variable speed drives, commonly referred to as VSD’s or VFD’s. The benefits of these devices in reducing power usage, and thus running costs is beyond the question. An example is on building HVAC systems, where mechanical dampers traditionally are the main method of controlling airflows. VSD’s are now being retrofitted in many existing systems as a method of “greening” the building, with excellent energy and economic savings.
Now for the bad news..
Research has shown that within VFD controlled motors, significant voltages are being generated along the motor shaft. In many cases, especially where motors are driving belts, the current will discharge through the bearings, causing tiny recesses in critical surfaces within the bearings, which can eventually lead to costly failures.
There has been plenty of work undertaken to resolve this problem, and now there are some excellent devices that have been proven to alleviate this damaging effect.
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