Regardless of what they’re called—variable frequency drives (VFDs), adjustable frequency drives (AFDs), adjustable speed drives (ASDs), speed drives, motor controllers, inverters or alternating current (AC) drives—VFDs vary speed and torque of AC electric motors by controlling AC frequency. “The most common name used is AFD, because adjustable implies control, whereas variable implies lack of control,” says Mark Kenyon, a product manager with ABB Inc. (www.abb.com) Automation Technologies, in New Berlin, Wis.
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
“It’s a result of the centrifugal fans and pumps operating along the Affinity Law,” Spronk declares. Through it, such fans and pumps follow a variable torque-load profile. Flow is proportional to speed, torque is proportional to the square of speed, and horsepower is proportional to the cube of speed, Kenyon explains. “That simply means if I need 80 percent flow, that generally equates to 80 percent speed. But if I run that motor at 80 percent speed, however, it only requires 50 percent of the power,” Spronk clarifies. And this cubed relationship, he says, makes VFDs energy savers.
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, firstname.lastname@example.org, is an Automation World Contributing Editor.