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By Jim Montague, Executive Editor
Why do overnight successes always take at least 10 years? For instance, it’s been just over a decade since Waterford Township Public Works (twp.waterford.mi.us/publicworks) installed variable-frequency drives (VFDs) on the high-service, constant-speed motors at its 11 drinking water plants in Michigan (Figure 1). Terry Biederman, Waterford’s public works director, says, “Opto 22’s PLCs and RTUs control these VFDs from Danfoss, Square D and Robicon. The VFDs use flow-matching to eliminate transients that were contributing to about 80 water main breaks per year, and reduced the breaks to 15 to 20 per year. This saves hundred of thousands of dollars in repair costs each year.” However, there was more good news to come.
In the following 10 years, the township’s physical system grew by 25%, and its water treatment and delivery capacity grew from 14 million gallons per day in 1996 to 25 million gallons per day at present. “So, our annual energy costs probably should have doubled, but they only increased from $450,000 in 1996 to $512,000 now,” adds Biederman. “A lot of the thanks goes to the VFDs we installed and the RTUs we programmed to run only when necessary. On the wastewater side, we went from 49 sewer pumping stations in 1996 to 62 now, and our demand has increased by 25% from 1996 to 5.5 million gallons per day right now. However, this annual energy bill only increased from $68,000 in 1996 to $71,000 in 2007.”
As with so many other control and automation disciplines, ever faster, cheaper and more powerful computing has enabled VFDs to be applied in more and smaller settings. So where oil and gas, pulp and paper, and a few other big-ticket processes used to be the only applications able to afford widespread use of VFDs and controls, they’ve more recently been popping up in water/wastewater, food and beverage, pharmaceuticals, consumer products and many lower-profile facilities.
“Using drives and motor controls, especially AC VFDs, gives us better control of each drive, so we can handle glass with a softer touch, for example, or do a better job applying labels. VFDs also allow us to better manage our energy consumption with supply, instead of running our pumps at full speed all the time and having to put in recirculation loops,” says Scott Richards, project manager and electrical engineer at Polytron, an Atlanta-based process-and-packaging system integrator (SI). “In the mid-1990s, the large suppliers focused on large, stand-alone, 5 hp to 10 hp VFDs with custom HMIs and mass-produced drives. Now we can call our distributor and get 5 hp to 10 hp VFDs that we can plug in and program on-site.”
In the past five to seven years, Richards adds, the biggest leaps in drives and motor controls have been networking and intelligence. “So when we put in a linear distribution system, all the separate devices that we used to have are now linked by Ethernet,” he says. “And having Ethernet connectivity in 50 networked drives means we can program them in advance, look at current draws in real time and do more preventive maintenance. Added intelligence means VFDs can report when there’s a problem, such as when they’re likely to fail soon. We also can program VFDs to do periodic diagnostic checks. It’s like having a room of college graduates instead of a room of third graders.
“More powerful, less costly computing also makes it more economically feasible to have intelligence at smaller points along a production line or process. For example, a continuous oil-and-gas process can monitor its in-line flow and density, provide the data about what’s going through its pipe via DeviceNet and Ethernet, and if its running slow, take back instructions to check the upstream filter. We can even set up a web page, so remote users can do real-time monitoring at one-tenth of the previous cost, and use this shop-to-top connectivity to squeeze 3% to 5% more production out of their process.”
Though drives and motor controls don’t occupy quite the starring role in process settings as they do in discrete manufacturing, they still support essential functions, such as pumps, fans and other rotating devices that all processes require. Many motors used to run at or close to full power, so flows were controlled by damping or valves. Consequently, just as production lines benefited from increasing motor efficiency and drive/control intelligence, many process applications came to use variable-speed control and PI loops in drives for better continuous control.
“In the past 10 years, drives and motor controls have become more sophisticated and less costly. Low-voltage drives went into all kinds of applications and were connected to integrated architectures, communications systems and added safety and security features,” says Sean Gaffney, Rockwell Automation’s AC drives marketing manager. “These days motor-control hardware is tied into communication networks, and drives are integrated for better diagnostics and control. Our Premier Integrator software makes it easier to integrate drives with our controller. Lockout programs are available for added security, and many drives now respond to safety inputs.
“In addition, VFDs and controls can save energy because the energy consumed by a motor is proportional to the cube of its speed. So in centrifugal pumps and fans, a small change in speed can give you a big energy savings (see sidebar) because of the drag that occurs as the pump or fan pushes through whatever fluid it’s processing. Some U.S. states require VFDs for motors over certain voltages, while some users, especially in water/wastewater applications, requiring harmonic migration, which involves sending less electrical noise to the grid.”
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