Advances in AC drive technology

It seems that AC drives and motors are becoming serious alternatives to DC and even hydraulic-powered control solutions. Contributing Editor Wayne Labs investigates the implications for today's machine builder.

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By Wayne Labs, Contributing Editor

WASTE NOT, want not. Industrial machine builders see more data supporting this old idea every day, and many hear it from potential customers. According to the Consortium for Energy Efficiency (CEE), electrically-driven motor systems in industrial applications use about 679 billion kWh annually, or about 23% of all electricity consumed in the U.S. Using more energy-efficient motor systems reportedly could reduce this demand by 11-18%, saving 62-104 billion kWH per year, which translates to $3-5 billion annually. These savings can be obtained by using mature technologies and practices, which also could reduce carbon emissions by 15-26 million metric tons per year.

And, besides saving energy and maintaining the environment, manufacturers can improve their processes by more tightly controlling motors in their process loops to reduce variability and scrap and improve product quality.

So, it’s no surprise that today’s machine specifiers want to know how machine builders are reacting to the need to provide more energy-efficient systems, particularly if the previous solution relied on hydraulics or DC power. In many cases, designing machines with AC drives is the answer.

What To Do With Hydraulics?
Hydraulic power can’t be beat for mobile vehicle applications where there’s adequate power on board to run the hydraulic pumps. It’s efficient, powerful, takes up little space, and is cost-effective. However, maintenance costs make alternate technologies attractive when a hydraulics-powered application is on the factory floor. At Ford Motor Co.’s St. Paul, Minn., truck assembly plant, hydraulic motors, cylinders and valves have been used on its transfer line (See Figure 1) for more than 20 years. Al Boik, Ford’s maintenance planning specialist, says increased maintenance time for oil leaks, spills and similar high-cost events was required to keep the motors running, and the outdated technology was unable to keep pace with growing production demands.

     FIGURE 1: CHANGING OUT HYDRAULICS
 
 

Ford's increasingly more complex front-body transfer line uses AC drives with accompanying control software to replace aging hydraulics.

The plant’s engineering team looked at retrofitting or replacing the hydraulic drives with new ones. Compared to AC drives, hydraulic drives are noisy and more expensive to buy and maintain. Hydraulic systems aren’t as flexible when programming changes are needed to accommodate product changeovers.

For Ford, the solution was to select an AC drive system, which was installed during the plant’s two-week shutdown. Boik replaced the old hydraulic systems with refurbished AC motors and gearboxes and PowerFlex 700S AC drives with DriveLogix from Rockwell Automation. “The drives provide precise motor torque, speed and position control necessary for the transfer line,” reports Boik. “As a result, Ford has minimized operating expenses and reduced maintenance. Engineers justified the improvements by comparing current production with previous levels, and the numbers have proven favorable.”

With some high-powered applications, however, hydraulics still have a place. In addition, some applications don’t need to be running at maximum pressure continuously. Typically, hydraulic power units run flat out all the time. When hydraulic power isn’t needed, the oil runs in a loop at high pressure, causing wasted energy in heat. “We thought this was pretty wasteful,” says Chuck Hollis, manager of system integrators sales for ABB. “We have some applications where we’re running the AC motor on a hydraulic power unit, but only when hydraulic power needs to be released. Injection molding machines are a good example. When the hydraulic motor on the molding machine needs the power, you then crank up the electric motor driving the hydraulic pump.”

DC Motors’ Support Fading
DC motors have a long history that can’t be brushed aside. “DC motors were first used because they provided adjustable speed for constant-torque applications,” says John Malinowski, motors product manager at Baldor. “DC traditionally has been the easiest and lowest initial-cost technology to use, particularly on applications requiring control with regenerative capabilities.”

The downside, he points out, is a lot of maintenance—primarily brush and commutator care. One application that’s particularly hard on brushes is extruding PVC. While DC motors were used with analog controls for years, digital controls were late in being applied to DC systems, and often the control wasn’t as good as with its AC counterparts. Malinowski adds the SCRs fire around 360 Hz in most DC controls, but they can’t react as fast as AC controls that work in the 3,000-5,000 Hz range.

DC drives have lost popularity due to a few basic issues. “In the early days of vector drives, the computing horsepower just wasn’t there to make AC drives a real contender to DC drives, but now the performance of AC drives is as good as DC or better,” states Peter Fischbach, component sales manager, Bosch Rexroth Corp.

Though they may be declining in deployment, DC drives aren’t finished just yet. As recently as the latter half of 2003, project manager Doug McConnachie, of BPB Paperboard in Purfleet, U.K., saw no reason to retire the paper mill’s DC motor-based winding equipment. Replacing old drives with ABB’s DCS 600 Multidrives let him control the tension of the winders with an accuracy of ±0.01%, providing reliable winding and minimizing breakage (See Figure 2).

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