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.”
Many practitioners put in their first drives to save energy but end up adding more for improved pressure, flow and motion control. “If you are controlling speed or flow or pressure, you can control speed or torque or both, for example, on a conveyor where you want one section to run faster than another,” Offik says. “You can do dynamic control, where you control the process with the speed, or adjustable control, where you change speeds for different conditions.”
Drives are available with flexible, built-in control capabilities that phase equipment in and out to match varying demands. Some have a bypass mode that can switch the motor to line power to give maximum efficiency for extended, full-speed runs, then do a flying restart to catch the motor when speed control is needed.
These capabilities are widely applied in compressed air applications. SeaQuest Perfect Dispensing’s Cary, Ill., plant handles a load that varies from 200 cfm to 3,000 cfm with three 300-hp, fixed, no-load compressors and one 250-hp variable-speed compressor from Atlas Copco. The compressors run at 100 psi into five 1,000-gal. storage tanks, and the plant air is held constant and low at 80 psi to minimize leak losses and smooth the operation of more than 200 assembly machines.
The fine speed control a VSD offers can reduce shock loads on equipment and provide flexibility for product handling. “On our plating lines and automated processes, variable-speed drives control the hoists both horizontally and vertically to move the parts gently at the right speeds,” says Barry Brusso, principal facilities engineer, S&C Electric, Chicago. “This way, the parts don’t fall off the racks and we don’t have large, swinging loads.”
Torque can be controlled directly. “Our Direct Torque Control (DTC) can deliver a surprising amount of torque and fine speed control,” says Cliff Cole, director of marketing, low-voltage drives, ABB. “Conventional AC induction motors can rival early servo systems in high-torque applications like extruders.”
For example, this torque-controlling capability is used in a simplified centrifuge called CentraSep. Conventional centrifuges use two motors, one for high-speed extraction and a gearmotor for the low-speed scraping cycle. “The drive let us eliminate the gearmotor by using a vector drive motor,” says Jeff Beattey, president, Midwest Engineered Products, Indianapolis. “We get low torque and high speed for the extraction cycle, then low speed and high torque for the scrape cycle. When we demand a lot of torque — 300% of the motor rating for a limited time — we need it right now, and we can get that kind of response for good control.”
Putting a drive on a 400-hp pulper allows Georgia Pacific’s paper mill in San Leandro, Calif., to change paper grades without shutting down. “We have a paper mill full of drives,” says Fred Curcio, mill manager. “A lot were put in for energy savings, but we found they improve control. They can lower maintenance costs by reducing wear and tear — we’ve seen that on pumps by running them at lower speeds. And they give us a window into horsepower, amperes and temperature. But the main advantage is much better control. We can vary start speeds, boost torque for extra power when needed, and control the drop to off. They let you do things you just can’t do with a control valve.”
Get more information
Perhaps the most dynamic area of drive development and use is as a medium for obtaining information about equipment and operating conditions. Drives are measuring and storing an astounding amount of data. “Our drives can read currents, voltages and temperatures, both ambient and of the heat sink,” says Hauser. “We can read the output current by phase, and the motor thermostat can be read over a link.”
Hauser is modest. Siemens lists more than 4,000 registers for its MicroMaster 440 drive. A few of the more interesting ones for equipment, load and energy monitoring include:
- Output frequency: set point, actual, deviation.
- Output current: set point, actual, phase currents.
- Output voltage: actual, maximum.
- Torque: set point, actual.
- Inverter temperatures: heat sink, chip, rectifier, control board, ambient.
- Motor: temperatures, rotor speed, slip, air gap flux, active current, rotor angle.
- Power factor.
- Energy consumption.
This data can be used to detect motor, drive and wiring problems, deterioration of driven equipment, changes in process conditions and more. Drives can monitor energy consumption, running hours or motor revolutions, and they can annunciate when limits are reached, flagging the drive, motor or driven equipment for attention. This allows for more efficient maintenance activities and provides early warnings of incipient failures to avoid unscheduled shutdowns.
Taylor says drives can even use this kind of information to adjust operating conditions. For instance, if a drive senses a high temperature on a transistor, it can adjust parameters to shift some of the heat load from the drive to the motor.
Deal with reliability issues
While self-diagnostic and early-warning capabilities help, incorporating drives still raises legitimate concerns about reliability, reparability and negative effects such as motor damage, harmonics and interference. Drives are no longer considered exotic — they’re moving from specialized to commodity items — but despite low cost and plug-and-play installation, applications must be engineered to be sure they’ll be trouble-free.
“I had one customer tell me he expected the drive to be easy to install and last forever so he could just forget about it,” Cole says. “I told him we weren’t there yet.”
Causes and cures for line interference, harmonics and motor damage must be considered in almost any VSD application. They are discussed in detail in the IEEE 519 and NEMA MG1 Part 31 specifications, but the best defense is an experienced application engineer who is familiar with your plant.
“It all comes down to a fear factor due to lack of education,” says Cherney. “Reliability is not an issue anymore. You can always find a piece of junk, but if you avoid the bottom-feeders, they’re reliable and simple. You can plug it in and it will work. But you have to apply them properly to avoid creating a weak link. As drives become more of a commodity, sales engineers are being replaced by salesmen, and misapplications are increasing.”
A drive’s main enemy is heat, and even something as simple as the load rating can trip you up. For example, “Drive sizing for refrigeration can be tricky,” Clark says. Manufacturers will size evaporators for the BTU rating and use a fixed-speed motor at the lowest possible cost. “The evaporator does fine moving 80Â°F air, but at 20Â°F or 0Â°F or -10Â°F, the air is a lot more dense. The motor goes beyond its full-load amp rating, and the cold air usually keeps it alive,” he says. “But the drive has to handle the higher amps. You need a 10 A drive for a 7.5 A motor to handle the load or the drive will keep tripping.”
In an air compressor application where the plant switched from steam to compressed air to atomized oil for combustion, a drive had to be installed inside the boiler room. “The ambient temperature was 110Â°F, so it was hard to keep cool, but we made sure the drive was robust by choosing one rated at the lower end of its size class, not a stretch from a smaller size range,” Spitzer says. “You have to know when to spend just a little more money.”
If asked, most manufacturers will offer a mean-time-between-failure (MTBF) specification somewhere between 20 and 100 years. “Ask how the numbers were derived,” Cole says. “All drive manufacturers use the same types of components with roughly the same life spans.”
MTBF is an indicator, but life span mainly depends on operating conditions — temperature, line voltage conditions, airborne matter, etc. — and critically on maintenance. “The weakest components are the ones that move, such as fans and relays, or contain fluid, such as electrolytic capacitors,” Cole says. “Fans can have tremendous lives if someone cleans them.”
Electrolytic capacitor life depends on power conditions and can be shortened by low line voltage, buss ripple, power surges or, especially, a hot environment. They also can be damaged by sitting unused for an extended length of time.
“If you operate them in the specified temperature range, clean the fans, and don’t contaminate the electronics — put them in a cabinet if needed — drives should last at least 10 years,” Cole says. “I’ve seen them last 20, but if the application is critical, consider replacing them after eight or nine years.”
During that life, if a complex drive has a problem, what will you do? With today’s high levels of integration, compactness and close tolerances, manufacturers say it doesn’t make sense to repair a smaller drive in the field. “Some customers want to, but it generally makes better sense to swap it out and send it in for repair,” Cole says. Depending on who you’re talking to, the threshold varies from 20 hp to 50 hp. Cole says, “Have one available on the shelf or at your distributor or in a redundant system.”
On larger drives in critical applications, line up a local solution. The drive can be serviced on-site by a service center or swapped out with a certified unit from the vendor’s repair pool.
Manufacturers are making the drives as reliable as possible. New drives are burned in and stress-tested. “If it passes the factory tests, it will work in the plant,” Cole says. “We can’t 100% prevent any problems, but we can be sure the benefits of using the drive will outweigh the problems.”
Curcio at Georgia Pacific says, “We were concerned about reliability, but it turns out to be a non-issue. You want to have a spare, but they’re very reliable.”