VFDs Reduce Wear, Save Energy

Oct. 1, 2008
They come with many names: variable frequency drives (VFDs), adjustable frequency drives (AFDs), adjustable speed drives (ASDs), speed drives, motor controllers, inverters or alternating current (AC) drives.
But “probably the most widely accepted [name] would be VFDs,” observes Brian Geoffrey, senior software engineer with the Embedded Software Development Group within the Drives Division of motion control products vendor Yaskawa Electric America Inc. (www.yaskawa.com), New Berlin, Wisc.End-users know or should know that VFDs produce energy savings. One of the biggest ways comes from optimizing motor speed and demand, Geoffrey explains. “Applications that benefit the most from this would be fans, blowers and pumps.” Why? “Without a VFD, the motor would run at full speed all the time. If a certain pressure or flow less than this maximum is required, an inlet, throttling or bypass valve would be installed. The energy lost due to these kinds of devices can be significant.”But by obtaining data with a sensing device, such as a pressure transducer, the VFD can automatically adjust its speed to maintain a certain pressure or flow. “If fan or pump speed can be reduced to 75 percent of rated speed, power consumption can be reduced by as much as 42 percent of rated power,” Geoffrey states. The Affinity Law describes that speed-power consumption relationship.Other ways VFDs save energy? Improved power factor. “A motor draws 25 percent to 50 percent of its full-load current at no load. The result is that a motor presents a very low power factor at light mechanical loads,” Geoffrey comments. A second way is inrush current. “Starting an induction motor across the line causes the motor to draw high starting current—up to 10 times nominal or full-load current—to get the motor turning,” explains Rich Mintz, electronics products manager with drives technology supplier SEW-Eurodrive Inc. (www.seweurodrive.com), Lyman, S.C. When that VFD is connected to a motor, inrush current is eliminated when the motor starts, Geoffrey adds.The VFD also greatly reduces the heating effects of high inrush currents, Mintz notes. Those currents in across-the-line starts produce a lot of heat in the windings of the motor, he says. “And heat is bad. It increases the resistance of the motor windings, which in turn causes the motor to draw higher current, which produces more heat.” That heat is everywhere, Mintz remarks. “When the motor is started, it heats fast. While it runs at nominal current, it cools back down, generally due to air from the motor fan being blown across the motor’s body.”In cycling applications, the motor is subjected to high starting current and heating, and then it sits still and can’t cool itself, Mintz says. This reduces the rate at which the motor can be cycled. But, he asserts, “A VFD greatly reduces the heating effects of high inrush currents.”Improved stopping
VFDs can improve a motor’s brake life in cyclic applications, Mintz points out. “Engaging the brake at full load and speed causes the brake to wear, no different than [what happens with] the brakes on your car. But, if you use a VFD to stop the motor, in a reverse operation, the brake can be controlled to set once the motor has already stopped.” That means much less heat and wear on the brake disc, he states.For material-handling applications, such as conveyors and machine-tool applications, VFDs also provide soft starts, Geoffrey says. Those reduce shock and increase the life of bearings and other components, Mintz says. “The ramping effect on start-up means a lot less mechanical shock to connected load. And shock kills mechanical things,” he states. Smoother is better, he suggests. “But just because it’s smooth, doesn’t mean it’s slow.” It will, however, be more energy efficient—and that’s one huge reason to use VFDs, or whatever you wish to call them.C. Kenna Amos, [email protected], is an Automation World Contributing Editor.

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