Advances in AC drive technology

By Wayne Labs, Contributing EditorWASTE 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maintenance time for oil leaks, spills and similar high-cost events was required to keep the motors running, and the outdated technology was unable to keep pace with growing production demands.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).

FIGURE 2: KEEP THE DC MOTORS  

Maintaing correct tension demands high accuracy at high speed in paper mill winding applications. Rather than switch to AC drives, BPB Paperboard updated the drives for its existing DC motor system and achieved +/-0.01% accuracy.

     Push and Pull Prefer AC
Ivan Spronk, AC drives product line manager at Schneider Electric, reports that AC motors/drives are taking over in extruders and hoisting applications. “These applications once required DC motors and drives to deliver the needed motor performance,” says Spronk. “The shift to AC is largely due to improvements in AC drive hardware and software. For example, AC motors/drives can match the torque performance delivered by a DC motor/drive and provide better speed regulation and torque performance at lower cost.” Hollis thinks DC drives still make sense where the horsepower requirement is big and regeneration is required, though DC drives don’t have a corner on the market when it comes to regeneration. “With the advent of open/sensorless or closed-loop vector control, AC drives (See Figure 3 below) can control an induction motor, matching the dynamic performance of its DC counterpart, argues Bill Conway, product manager for Siemens’ 6SE70 and enclosed drives. “Regen front-ends and common bus configurations for multiple motor applications also are available to meet tough requirements.”AC Up To The Task
The fact that AC motors can do tasks once relegated to DC motors becomes more obvious to industrial OEMs as they see how motor manufacturers adjust product lines. Emerson Industrial Motors Division dropped its DC business a couple of years ago due to declining interest and sales of DC motors. “With NEMA-standardized AC motor products, the customer buys off-the-shelf and plugs into the drive to get the performance the application requires,” says Rob Boteler, Emerson’s industrial motors marketing director.As shown above, running a hydraulic pump with a VFD can save energy, and the same principle can be applied to industrial air compressors, pumps and industrial ventilation system fans. A centrifugal pump or fan running at half-speed consumes only one-eighth of the energy used during full-speed operation. This is because the torque needed to run a pump or fan is the square of the volume, and power consumption of the motor is proportional to the cube root of the shaft speed. For instance, in a perfect world (100% efficiencies), reducing the pump speed to 80% only requires 64% of the torque (0.8x0.8); the motor power consumption will be 0.8³ (cubed), or about 51%.Lou Lambruschi, product manager at Danaher Motion, says applications that benefit from AC motors and drives include wire drawing (single and multi-stage processes), yarn manufacturing and plastic and rubber extrusion.      FIGURE 3: THE CHOICE WAS AC

 

Voestalpine Stahl’s hot-dip galvanizing line will use around 300 drives, with outputs ranging 2.2-1200 kW for a total installed drive power of 15 MW.

For example, a sugar centrifuge operated by the Sugar Cane Growers Cooperative of Florida had experienced one or two failures per year with an old SCR-based VFD drive that powered a 450-hp motor. Jose Mena, the cooperative’s chie