A Georgia Pacific Gypsum Paper mill in San Leandro, Calif., sources its own corrugated and flyleaf wastepaper and supplies eightply face- and back-side paper to G-P gypsum board plants. A steam turbine, part of the original construction, powered the paper machine through a traditional single line shaft, giant cone pulleys, belt drives, and gearboxes.
The 1950s technology was replaced with direct torque control (DTC) ACS 600 VFDs and ac motors from ABB Inc., New Berlin, Wis. The result was increased capacity from 750 to 1,000 fpm and the first encoderless electrical operation of a paper machine.
Adding 100- and 150-hp drives and motors to the second and third presses, respectively, and a 350-hp drive and helper drive on the main press, freed up horsepower to the steam turbine so the paper machine could speed up. The drives also provided more precise draw control, giving the mill the ability to run tighter draws with fewer paper breaks.
Seven ac motor drives, ranging from 40 to 125 hp and connected to a common 700-V dc bus supply, are built in a multi-drive configuration to make it easy for the high-inertia dryers close to the calendar stack to share and return their regenerated energy within the true common dc bus systems back to the dryers near the presses.
Encoders are often the weakest link in the electrical chain. If they fail or short out, the processing line stops until the specific encoder is identified and replaced. The ACS 600 Series VFDs eliminated the need for encoders on the finishing end equipment, which includes the dryer section, calendar stack, and reel, as well as from the forming end. DTC drives also create a mini multidrive between the second and third press. All total, 13 DTC drives are on the paper machine.
The retrofit has enabled the mill to increase tonnage of its heavier paper — for use in its ToughRock wallboard — from 175 to 225 tons per day, and the machine runs with little delay - down to 2% many months, 5% below industry average.
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