VFD Spells Energy Savings

Regardless of what they’re called—variable frequency drives (VFDs), adjustable frequency drives (AFDs), adjustable speed drives (ASDs),速度驱动器，电机控制器，逆变器或交流电流（AC）驱动器 - VFD通过控制交流频率改变AC电动机的速度和扭矩。“使用最常见的名称是AFD，因为可调意味着控制，而变量意味着缺乏控制，” ABB Inc.的产品经理Mark Kenyon说（www.abb.com）自动化技术，威斯康星州新柏林。

And no matter how they’re described and defined, VFDs save energy. They do so by reducing motor speed as demand decreases, Kenyon explains. “Some AFDs have, however, a feature called energy savings, or flux optimization, that provides them the ability to further reduce energy consumption during steady-state conditions,” he adds. These functions decrease voltage supplied to the motor by several percent when speed remains constant for a specified interval. “Since power is proportional to current times voltage, the power consumed will naturally decrease,” he notes.

Estimating approximately 22,000 different applications for AFDs, Kenyon recalls how the U.S. Department of Energy reported that 63 percent of the power produced in the United States is used to operate motors. And of those motors, Kenyon believes 60 percent operate pumps and fans. Approximately 50 percent of those pumps and fans could be controlled by AFDs, he thinks, noting that AFD is a family of electrical, mechanical and hydraulic products.

Now there’s an even more acute reason compelling the use of VFDs. “Increasing energy costs,” declares Ivan Spronk, the Raleigh, N.C.-based manager of Schneider Electric’s (www.schneider-electric.com) AC drives and starts. Another reason he gives for usage of VFDs in the United States is the government’s Energy Policy Act (EPAct). Passed originally in 2002 and amended in 2005, it offers tax incentives for certain energy-savings measures. “EPAct requires the ability to monitor and measure energy usage. This is what’s partially driving the increase in network connectivity into asset management software,” Spronk adds.

To explain the most significant way in which VFDs can save power, Spronk uses the example of a building having a variable air-volume system. In the application, at conditions other than worst-case cooling or heating, motors run slower than full speed but still provide optimum comfort. VFDs find use in the chilled/hot-water and air-flow components of the building’s heating/cooling system.“You’re using the AC drives to set the air flow or liquid flow to match heating/cooling demands,” Spronk explains. “Depending on how much heat I need to change out of the water, I’m going to change flow.” The VFD would be on the closed-loop system between the chiller and the cooling tower, he notes.

How, then, do VFDs save energy in this and similar applications?

It’s the law

Spronk宣称：“这是离心粉丝和泵沿着亲和力法运行的结果。”通过它，此类风扇和泵遵循可变的扭矩负载配置文件。肯尼恩解释说，流动与速度成正比，扭矩与速度的平方成正比，马力与速度立方体成正比。“这仅意味着如果我需要80％的流量，通常等于80％的速度。但是，如果我以80％的速度运行该电动机，则只需要50％的功率。”他说，这种立方体的关系使VFDS能量避免。

Savings similar to those in Spronk’s building application example also exist in manufacturing. Besides heating and cooling, VFDs can be used to maintain correct air pressure and number of air changes per hour, he says. “Low-power AC drives provide a distinct energy-saving advantage when deployed for the control of motors used in manufacturing industries consuming significant power,” adds Kenyon. “This trend will continue in the foreseeable future.”

C. Kenna Amos,ckamosjr@earthlink.net,is an自动化世界Contributing Editor.