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Viewing report
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Magnetically Coupled Adjustable-Speed Drives: Going Where No VFD Has Gone Before
E SOURCE, Nov 2003, Pages: 18
Magnetically coupled adjustable-speed drives (MC-ASDs) transmit torque between a motor and its load across an air gap, with no physical connection between the two. By varying the strength of the magnetic field in the air gap, these devices can vary the amount of torque transmitted and, thus, the speed at which the load is driven while the motor operates continuously at full speed. Thanks to recent improvements in MC-ASD technology, two products are now available that compete well with variable-frequency drives (VFDs) in limited applications.
Though not as efficient as VFDs, these MC-ASDs can not only save energy in pump and fan applications but also provide a number of non-energy benefits, such as vibration isolation, spike-load tolerance, maintenance savings, and extended equipment life in a variety of applications. MC-ASDs also avoid problems that are sometimes caused by VFDs, such as voltage distortion, accelerated degradation of motor winding insulation and motor bearings, excessive voltage applied to motor terminals, nuisance dropouts due to voltage sags, limitations on low-speed operation, and the expense of stocking parts for VFDs that may not be available in the future.
There are niches where these MC-ASDs have a first-cost advantage over their VFD competitors. In some medium and high-voltage applications, the MC-ASD hardware is simply cheaper. In others, high VFD installation costs can make the MC-ASD an attractive alternative despite its higher capital cost. Plus, MC-ASDs can sometimes yield lower life-cycle costs than VFDs, despite lower energy savings, in applications where their non-energy benefits are particularly valuable. However, the value of these non-energy benefits is highly site-specific and will need to be evaluated on a case-by-case basis.
We recommend the use of MC-ASDs where VFDs would be unusually expensive due to medium- or high-voltage operation or to high installation costs, where sensitive electronic equipment or high outage costs make VFDs a risky proposition, in processes where prolonged operation at low speed would make a VFD solution unusually expensive, or in cases where non-energy benefits of the MC-ASD will be particularly beneficial.
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