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Variable Frequency Drives and Building Automation Systems
Part 1 Proper Selection
Paul Ehrlich, Ira
& Angela Lewis
The use of variable frequency drives (VFD’s) for controls of pumps,
fans, and even compressors have become common on both new construction
projects and retrofits. VFD’s are a valuable tool in providing
both improved control and energy efficiency. Over the last decade
the cost of these devices has dropped, and the reliability has
improved, making their use almost a “no brainer”. With that
said, careful consideration should still be given to how the drive will
be applied prior to selection. This months column will focus on
the best places to use drives and a future column will focus on how we
recommend connecting and integrating drives with the BAS.
Benefits of Drives:
The primary benefit of using a VFD is that it provides an efficient method to vary the capacity of rotating equipment such as fans and pumps. The fan laws show that reducing the flow in a system effectively can reduce the required power by the cube of the reduced flow. In other words reducing airflow by half can result in the fan energy being reduced to one eighth of the original power required. For example a fan, which used 100, kW at full flow, would drop to approximately 12.5 kW at 50% flow. Similar results can be expected with pumps and other rotating equipment. While there are other available technologies that provide effective methods to modulate flow, such as inlet guide vanes and variable pitch fans, the use of VFD’s is more effective in achieving performance that comes closest to matching the theoretical limits of the fan and pump laws.
Given the potential benefits of drives it isn’t surprising to see them being widely applied. However there are some cautions we urge in considering use of drives:
We consider the VFD to be a key part of the control system, providing
numerous benefits, at an affordable cost. The next portion of this
series will focus on how we recommend integrating drives, available
data, and added benefits including the ability to get “free
sub-metering” out of VFD’s.
About the Authors
Paul and Ira first worked together on a series of ASHRAE projects including the BACnet committee and Guideline 13 – Specifying DDC Controls. The formation of Building Intelligence Group provided them the ability to work together professionally providing assistance to owners with the planning, design and development of Intelligent Building Systems. Building Intelligence Group provides services for clients worldwide including leading Universities, Corporations, and Developers. More information can be found at www.buildingintelligencegroup.com We also invite you to contact us directly at Paul@buildingintelligencegroup.com or firstname.lastname@example.org
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