Physics Produces Savings With Variable-Frequency Drives

Variable-frequency drives (VFDs) are very old technology—and everybody knows about savings from them, Peter Fischbach declares.

“It’s [through] applied physics,” declares this manager of components sales with Bosch Rexroth Corp.’s (www.boschrexroth-us.com) Electric Drives and Controls Group, Hoffman Estates, Ill.  

Running the drives at different speeds, rather than continuously at full load, produces those savings. VFDs automate the motors, explains Sharon James, application engineer with Rockwell Automation Inc.’s (www.rockwellautomation.com) Low Voltage Drives Group, in Mequeon, Minn. “So with a motor that used to run at 60 Hertz across the line at full speed, a VFD automates that to
run at a speed of your choice.”  

And that’s where applied physics—specifically, the Affinity Laws, which James says can be used with centrifugal fans and pumps used in about 80 percent to 85 percent of heating, ventilation and air conditioning (HVAC) systems—provides savings. The law describes how—based on speed—fans and pumps follow a variable torque-load profile: Flow is proportional to speed, torque is proportional to speed squared and horsepower is proportional to speed cubed. Thus, “bringing down speed from 100 percent to 80 percent actually cuts power in half,” James explains. “This [relationship] is going to be the case in any system.”  

The physics-based energy savings in pumps and motors has been the selling point for VFDs used in HVAC systems since their inception. But what about savings and benefits elsewhere, in applications such as conveyors used in materials handling? James mentions reduced maintenance and multi-motor capability. “You can run more than one motor on the VFD. You’re maximizing the system efficiency.”  

In some systems that run at very low speeds, variable-frequency drives can operate very smoothly, James points out. And those drives offer self-diagnosis and more reliability. They’re self-protective, too, he adds. “If there is a dip or surge in power, it protects itself through built-in software.”  

Through their intelligence and flexibility, VFDs also extend the life of the pumps and fans with which they’re associated. “The amount of power being output is the amount of power the motor is asking for,” James explains. “The [variable-speed] drive is intelligently monitoring the power demand.”  

But even more savings through variable-frequency drives may be possible with flux-vector control, which allows better torque control with motors. “Instead of changing frequency and voltage, the drive comes into the game,” Fischbach states. This control, which allows smart magnetization, or what some call the energy-saver or sleep mode, can be done with National Electrical Manufacturers Association or International Electrotechnical Commission induction motors, he says.  

State of flux  

“You control the torque and magnetic flux of a motor, independent of one another,” Fischbach explains about flux-vector control. Why is that an advantage? “You can reduce the flux in some applications, dependent on the load characteristics, when you don’t need a lot of torque on a motor shaft.” So, if an operator knows that a certain profile is going to be run, the operator can reduce the flux, based on that profile, he explains.  

How does having a flux-vector-control VFD vs. a standard VFD translate into money in a company’s pockets? If the motor always operates at full torque, there’s not much savings, Fisch-
bach indicates. “But if [there is] a lot of idle time or reduced load time, you might see 5 percent reduction in costs, but that’s application dependent.”  

C. Kenna Amos, ckamosjr@earthlink.net, is an Automation World Contributing Editor

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