Application Corner: The fine art of variable speed drive operation

Applying variable speed drives is a combination of art and science. Many applications are "academic" in the sense that a control strategy utilizing a control valve is replaced with a control strategy utilizing a variable speed drive. In other words, instead of installing a pump or fan operating at full speed and using a control valve to throttle to the desired flow, the control valve is eliminated by operating the pump or fan motor with a variable speed drive to meet demand. This generally reduces operating costs by only generating the hydraulic energy necessary to meet system demand. This strategy reduces energy and is a "good thing" on many levels.

However, "academic" applications typically yield energy savings of 20 to 30 percent. Simple payback is often approximately three years. This is nothing to sneeze at, especially when large motors are involved.

The art behind variable speed drive applications is to make process changes in conjunction with "academic" changes. This is not always possible, but when it is, the savings can be significantly higher. How significant? Some years ago, I installed a variable speed drive on a plant air compressor that was operated by the largest motor in the plant. In the end, the project actually increased electrical energy consumption while decreasing fuel costs much more. The project had a six-month simple payback. In a matter of days, the project politics went from "You want to do what?" to "How fast can you get it done?"

Projects such as this are typically conceived by users at the operating companies. Manufacturers and vendors are knowledgeable of their variable speed drive equipment and the "academic" applications in which it can be utilized. However, the end user and appropriate consultants are in the best positions to apply art and find the gems.

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes LLC offering engineering; seminars; strategic marketing consulting; distribution consulting; and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. Spitzer has more than 40 years of experience and has written more than 10 books and 300 articles about flow measurement, instrumentation and process control. He may be reached at 845-623-1830 or at spitzerandboyes.com.