Paper mill combines ac and dc drives to reach upgrade goals
Advanced technology in dc drives allows paper manufacturer to save expensive high-power dc motors during machinery upgrade.
Jeremy Nighan, ABB, Low Voltage Drives -- Control Engineering, 8/1/2007
According to The Federal Network for Sustainability, a voluntary, collaborative network of Federal agencies in the Western United States focused on fostering and furthering the concept of sustainability within the government through individual programs and group initiatives, the United States uses approximately 4 million tons of office paper annually, enough to build a wall 10 ft high and 6,815 mi. long. To keep up with demand, paper mills constantly seek ways to improve efficiency and quality. A highly productive strategy is to identify individual process machines whose efficiency might be improved by retrofitting with updated technology.
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| Schematic of dc and ac drive retrofit at large southeastern paper mill. Courtesy ABB |
In the critical path
One leading paper mill facility in the Southeastern United States identified its slitter-rewinder machine as one such candidate. To increase productivity and efficiency, the company decided to rebuild its slitter-rewinder, and incorporate the latest technology. The goal was to improve roll quality, increase machine speed, and eliminate costly shutdowns.
A slitter-rewinder machine is responsible for unwinding huge, newly made rolls from the main papermaking machine, cutting them into smaller sections, and rewinding them as more manageable rolls for distribution. This machine accomplishes one of the final production tasks, so any problem can halt production.
Consisting of multiple motors and controls, the slitter-rewinder is one of the most advanced pieces of equipment in a mill. Tension in this process determines paper roll quality. Accurate and consistent forces need to be applied throughout the process to ensure perfect web tracking, precise slitting, and proper roll density profiles.
Drive performance largely determines roll quality. The winder must run as a coordinated system, tightly controlling speed and torque through continual cycles from full-stop to full-speed, then back to zero speed. Even small inaccuracies can cause sheet breaks, rough edges, telescoping, offsets and tie-ups, resulting in downtime, rejected rolls, and lost profit.
Cry for help
The mill’s 25-to-30-year-old drives were growing increasingly expensive to repair and maintain. A number of components had become obsolete and could not be replaced. Some analog equipment needed constant attention due to drift, and old wiring started showing signs of fracture and insulation breakdown at hinged points. Mechanical assemblies that held the thyristor assemblies and fuses in place were beyond repair; and, on top of everything else, there were issues with the power modules’ structural integrity.
The mill turned to Southeast Drive Systems, Inc. (SDS), a systems integrator that specializes in control designs for paper and other web industries. SDS engineers visited the facility in spring 2005 to survey the situation.
“It was imperative to get the perspective of everyone who is affected by that rewinder,” says John Parker, a senior engineer for SDS. “The maintenance crew had concerns regarding component failure and a drive room that required precise temperature control to keep analog components from drifting. The production department wanted to increase up-time, improve roll quality, and provide customers with a cost-effective product, and they wanted to use technologically advanced equipment.”
Craig Tierno, senior application engineer with SDS, explains that, with multiple existing problems, a retrofit that would replace analog regulators with digital counterparts, while maintaining the existing power bridge, was impractical. The silicon controlled rectifiers (SCRs) inside the power modules would not be dependable, so the integrator recommended a drive and control system upgrade to improve reliability and performance.
“Often, if a company’s dc power modules are still within their life cycle, we will recommend retrofitting them with new high-performance digital front ends (DFEs) to enhance the regulation performance while firing the existing SCRs,” Tierno points out, “In this case, however, we could not.”
SDS provided a solution that utilized many existing high-power dc motors and components while integrating state-of-the-art ac and dc motor drives. The drive hardware ran SDS’s Intelli-Wind two-drum winder human machine interface (HMI) software that provides tension, nip, and torque control, recipes, numerical and graphical set points, and diagnostics.
A new dc drive designed with some of the same software tools and communication modules as ABB ac products, made it feasible to combine dc and ac equipment in the project and keep the existing high-power dc motors while updating the drive technology.
SDS installed four ABB DCS800 drives. The first drive controls a 500 hp unwind motor that provides tension regulation for the jumbo parent roll. The second was installed on the 50 hp lead-in paper roll, which transports paper to the slitter section. They installed the third and fourth drives on the 250 hp front and rear drums, which provide machine speed reference and profiling torque to the re-wound roll.
Due to the size of the rolls and the large torque required to accelerate and decelerate them, the dc drives were sized to handle 200% current limit for 1 minute.
The integrator connected two ACS800 ac drives and motors rated at 15 hp to the two ends of the rider roll, which provides vertical force to transmit acceleration and deceleration torque via friction.
Load cells that measure web tension in the unwind section were replaced with ABB PillowBlock-style Pressductor transducers that were direct physical replacements for the old load cells.
SDS engineers felt the DCS800 helped the paper mill’s engineering and IT staff, who were already familiar with the dc drives’ operating and commissioning functions. The complementary ACS800, with a look and feel similar to the dc unit, made it easy to transition to ac for the relatively low-power part of the system, while retaining the existing high-power dc motors.
By focusing on tension, nip, and torque control, SDS was able to create an architecture that met the customer’s need for improved roll quality.
The automatic stopping feature solved the mill’s problem with product length and diameter. Permissive and diagnostic pages in the HMI provided operators with quick help for keeping the winder running, which reduced the number of maintenance calls. The recipe system allowed operations to set up orders quickly and maintain product consistency.
Pre-built panels speed installation
The retrofit utilized existing construction, including an open-panel design that required minimal real estate and clearance in the control room. Instead of using floor-mounted cabinets, all of the power modules, circuit breakers and other components were laid out on elevated panels. This strategy enabled the integrator to pre-build a separate sub-panel for each drive, allowing time before demolition of the old equipment to test the drives individually and as a group. This made it possible to test communications between drives as well as much of the application software and drive functionality before installing and commissioning the system.
As a result, application-program set-up and commissioning became a smooth process. The integration team took approximately four weeks to engineer schematics, create drive and controller software, and build the panels. After a two-week testing process, the team was able to install the new equipment within the five-day contingency period that the mill required. Engineers remained on site to ensure a seamless transition for the plant operators.
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