Paper machine retrofit pioneers encoder-less drive operation

Encouraged by efficiencies gained following installation of ABB’s (New Berlin, Wis.) 100-, 150- and 350-hp drives, motors and controls on its presses several years ago, engineering managers at Georgia-Pacific (G-P) Gypsum paper mill (San Leandro, Calif.) recently retrofitted the mill’s finishing end too.

“The drives, synchronized to the existing line shaft via encoders that provided feedback of line speed, freed up horsepower to the steam turbine, so the whole paper machine could speed up,” says Chris Hall, president of Intec Solutions, a systems integrator that performed both upgrades. Also, the motor controllers now provide more precise draw control, which enables tighter draws and fewer paper breaks.

Built in 1952, and acquired by G-P in 1996 from Domtar Gypsum, the 400-ft mill and its steam-turbine-driven paper machine supplies eight-ply face- and back-side paper to other G-P Gypsum board plants at a maximum capacity of 750 fpm. Before the mid-1990s upgrade, heat and wear caused steam-driven turbine failures and downtime.

Common bus = less equipment

Fred Curcio, G-P’s plant manager, and Intec next discussed the complete retrofit of the finishing end, including its dryer section, calendar stack and reel. “We designed completion of the project for 1,000 fpm,” says Mr. Curcio.

Intec’s solution: build and install a multi-drive with seven ABB ac motor drives, ranging from 40-125 horsepower, all connected to a common 700-V dc bus supply. The common bus is the power supply to each drive, which eliminates the three-phase supply in a shared-bus configuration. Input power semiconductors and pre-charge hardware is eliminated from all drives.

“Common bus also eliminates the need for added reactors in front of each drive to provide filtration,” adds Mr. Hall. Using ABB’s ACS 600’s open-loop Direct Torque Control (DTC) technology also eliminated the need for encoders on the finishing end. ABB reports that DTC enables each machine’s motor drive to calculate the state—torque and flux—of the motor 40,000 times per sec, making the drive virtually trip-less.

Going encoder-less

Seamless operation of the finishing end enabled removal of encoders from the paper machine’s forming end. Mr. Curcio and Mr. Hall say encoders were the “weakest link” in the electrical chain. If the glass disks fail or short out, the processing line stopped until the specific encoder is found and replaced.

Also, Intec replaced the PWM drive on the third press with an ABB DTC drive, creating a mini multi-drive between the second and third press. “The second press is operated in torque, not speed, mode at a set-point, so the third press drive uses that generated energy,” says Mr. Hall.

“Encoder-less operation of a paper mill is a quantum leap in simplification,” adds Mike Giraudo, Intec’s vp. “Mechanically, the encoders and wiring to them cease to be necessary.”

Following an ambitious four-day installation of the multi-drives, stand-alone ACS 600 drives, and a motor control room, G-P’s paper machine was making paper within two hours of start-up on the fourth day of the retrofit in March, 1999.

Two years later, the mill is presently producing heavier paper, for use in G-P’s ToughRock wallboard. Increased throughput capacity has let the mill increase tonnage of this heavier paper from 175 to 225 tons per day. Machine delay is minimal. “We’re down to 2% during many months, which is 5% below the industry average, ” says Mr. Curcio, who adds the 13 DTC drives on the paper machine have operated without a hitch.

For more information, Circle 499 or visit www.us.abb.com .

Author Information

Jim Montague, news editor jmontague@cahners.com

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