The virtues of variable-speed drives (VSD) are great in theory: Save energy by running motors at less than 100% output when full power isn’t needed; gain fine speed and torque control; and nowadays, extract equipment condition and operating information for predictive maintenance and plant optimization.
But in practice, the same electron-flinging marvels that promise so many benefits also have confounded plants and their maintenance personnel with reliability issues, leaving many practitioners willing to forgo the advantages to avoid the hassles.
Drive benefits appear to be solid, but they can also seem academic compared to the consequences of an equipment outage. David W. Spitzer, principal of engineering and consulting firm Spitzer and Boyes LLC, tells the tale of a utility engineer who wanted to put a variable-speed drive on a 500-hp compressor for a critical plant air application. With a process change, the project would deliver a six-month simple payback, according to the compressor manufacturer. But the maintenance and operations managers were against it because if the drive quit, it would shut the plant down.
The utility engineer ultimately prevailed, and the drive was installed. It performed as planned and held plant air pressure constant at 95 psi instead of cycling between 110 and 125 psi. Within a month, maintenance personnel came back with suggestions for additional drive applications.
Still, “There are many applications that appear wise, but when customers do them they find unintended consequences — negative consequences upstream or downstream,” says John Pfeiffer, program manager for energy-efficient products and services at Baldor. “They don’t even always save energy.”
Getting the energy savings you expect starts with making sure the application runs a significant amount of time at a low enough percentage of full output. “About 80% of those applications are pumps or fans,” says Mike Offik, P.E., general product manager, Rockwell Automation.
Spitzer re-engineered a brine recirculation application where three pumps ran constantly.
“The pumps were sized for the maximum flow requirement and generally ran throttled to 60% output,” he says. “Putting in a VSD and opening the valves reduced power consumption from 36 kW to 12-15 kW.”
For Tim Clark, director of controls, refrigeration and warehouse for design-build company The Stellar Group, the most effective use is refrigeration condensers, where VFDs allow the fans to be run at the minimum speed to control the compressor discharge pressure. But he warns, “Many companies put VFDs on only one or two of, say, 10 fans and use them for pressure trim, which improves control but doesn’t give significant energy savings.”
They’re also used on evaporator fans to de-stratify the air in freezers. On weekends, for example, when the freezers aren’t opened, single-speed fans might run only 20 minutes per hour. “Then stratification can lead to melting and spoilage,” Clark says. VFDs let the fans run constantly at lower speeds where they use less energy. He adds, “It’s a significant process improvement — you should see the difference in the temperature trends.”
In motion applications, a lot of starting and stopping, changing speeds or lifting and letting down may mean, “You can save a lot of energy,” says Brian Taylor, business manager, standard drives, Rockwell Automation. Instead of using resisters to dissipate the energy, a large capacitor bank stores power for reuse. “One regeneration supply can handle multiple drives,” Taylor says. “As the price of energy continues to rise, it’s something to consider.”
Drive efficiencies run from 95% to 97% or more. At full speed, the motor is more efficient on its own, so many drives can be bypassed. “Under those conditions, our Smart Bypass dynamically switches to line power to eliminate that loss,” says John Cherney, product manager, Saftronics. When demand is reduced, it automatically switches back and reduces speed.
Offik points out, “If you have to run at full speed all the time, you may just want to get the most efficient motor you can and put it in there.”
But another energy cost consideration is a drive’s ability to improve the plant’s power factor. “You pay for reactive power, but you get nothing for it, says Cherney. “A motor alone will have a power factor of about 0.8; with a drive it’s 1.0.”
Before going too far, you might want to check your assumptions using a simulation package. “Modeling software lets users see the impact of using drives on a system,” says Rudy Hauser, product manager, drives, Siemens. “You enter the parameters and see the savings. It’s free on our Web site.”
Energy is always increasing in cost, and drive prices keep coming down. And you may be able to factor in a kickback from your local power company. Utilities now often offer rebates, especially in the power-strapped Northeast and West Coast. “Some do instant rebates — you do the job and install the drive and they give you the money,” Offik says.
“They used to just look at motors; now they’re recognizing drives in their programs.”