DC Drive Adds Efficiency To Legacy Motors

The USA uses approximately 4Mt of office paper annually. That’s enough to build a wall 3m high and 11,000km long. To keep up with demand, paper mills constantly are looking for ways to improve efficiency and quality. Although there are many different machines and processes involved with papermaking, a vital part of any paper mill is the slitter-rewinder. 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. As one of the final production points in a paper mill, any problem with the slitter-rewinder can halt production for the entire mill. Consisting of multiple motors and controls, the slitter-rewinder is one of the most advanced pieces of equipment in a mill. At the heart of this machine’s process is tension, which determines the quality of the paper rolls.

Accurate and consistent forces need to be applied throughout the process to ensure perfect web tracking, precise slitting and proper roll density profiles. Except for nip control, the quality of the roll is governed, largely, by the performance of the drives. The winder must run as a co-ordinated system and provide tight control of speed and torque through the continual cycles – from stop to full speed, and back down to zero speed. Even small inaccuracies in performance can cause problems, such as sheet breaks, rough edges, telescoping, offsets and tie-ups, resulting in downtime, rejected rolls and, ultimately, lost profit. To increase productivity and efficiency, one leading paper mill facility in the Southeastern USA decided to rebuild its slitter-rewinder, and incorporate the latest technology. The goal of the upgrade was to improve roll quality, increase machine speed to the highest possible level and eliminate costly shutdowns. The mill turned to SDS, a US-based systems integrator that specialises in control designs for paper and web handling industries.

SDS analysed the existing system and provided a cost-effective solution that used existing motors and components, integrated ABB’s state-of-the-art ACS800 Series AC Inverters and new DCS800 Series DC Inverters – the most advanced DC drive of its kind. The drive hardware was rounded out with SDS’s Intelli-Wind, a popular two-drum winder HMI that provides TNT control, recipes, numeric and graphical set points, permissives and diagnostics. System integrator personnel paid several visits to the facility in the spring of 2005, to survey the situation. They learned there were a variety of issues with the existing slitter-rewinder. ‘It’s imperative to get the perspective of everyone who is affected by that rewinder’, says John Parker, Senior Sales Engineer for SDS. ‘The maintenance crew had concerns regarding component failure and a drive room that required precise temperature control to keep analogue components from drifting’. ‘The production department wanted to increase up-time, improve roll quality and provide their customers with a cost-effective product, and they wanted to use technologically advanced equipment’. ‘Our partnership with ABB allowed us to address and achieve these advantages’. This paper mill, according to both ABB and system integrator experts, was among an increasing number of manufacturers that are still operating with drive systems that are 25 to 30 years old.

The drive system on the slitter-rewinder was getting increasingly expensive to repair and maintain. A number of components had become obsolete and could not be replaced. Some of the analogue equipment needed constant attention due to drift, and old wiring started showing signs of fracture and insulation breakdown at hinged points. The 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 structral of the power modules. Craig Tierno, Senior Application Engineer with SDS, explains that, with these multiple problems, a retrofit that would replace analogue regulators with digital counterparts, while maintaining their existing power bridge, was not practical. The SCRs inside the power modules would not be dependable. So the integrator recommended a complete 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 digital, high-performance front ends (DFE) to enhance the regulation performance while firing the existing SCRs’, says Tierno: ‘In this case, we could not do that’. In its search for a drive solution, the integrator found an answer in ABB.

‘Our biggest concern was that drive suppliers were not doing anything new with DC. Most of the suppliers were sitting on products that were near 10 to 20 years old’, says Tierno ABB, he learned, was developing a new DC drive, designed with some of the same software tools and communication modules as its AC products. That made it feasible to use and integrate both DC and AC products in the project. There is still a large installed base of DC motors throughout North America with a wide variety of horsepower and speed ranges. ‘These motors have a long life with proper maintenance and occasional repairs, so customers are not anxious, necessarily, to replace them’, notes Tierno ‘Finding a supplier such as ABB, which continues to invent and introduce new technology to control these motors, gave us an advantage from the beginning’. For this system, four ABB DCS800 drives were installed. The first drive controls the 500hp unwind motor that provides tension regulation for the jumbo parent roll. The second DCS800 drive was installed on the 50hp lead-in paper roll which is used to transport the paper to the slitter section. The third and fourth drives were installed on the 250hp front and rear drums – components that are responsible for providing machine speed reference and profiling torque to the rewound roll. Due to the size of the rolls and the demanding torque required to accelerate and decelerate, all the DC drives were sized to handle 200% current limit for one minute.

Two ACS800 AC drives and motors rated at 15HP were connected to the two ends of the rider roll to provide vertical force for acceleration and deceleration torque. From a mechanical viewpoint, the motor frame size was based on the application horsepower and ventilation style since the motors are mounted within a moving rider-roll frame. The load cells, which measure web tension on the unwind section of the machine, were replaced with ABB PillowBlock style Pressductor transducers. The PFTL units selected where based on system requirements and the fact that they were a direct physical replacement for the old load cells. The new technology load cells from ABB, which came precalibrated for the application, enhance tension control and reduced installation time and cost. Integrator personnel felt the DCS800 was a perfect fit for the paper mill’s engineering and IT staff, who were already familiar with the operating and commissioning functions of DC drives. ABB’s complementary ACS800, with a look and feel similar to the DC product, made it easy for the mill to transition to AC. ‘Making sure that the DC drives were fully complementary – hand-in-hand, step-in-step – with what we were going to do with the AC product, that was really important; having those two on the same platform’, says Tierno. He explains that the new DC drives enable retrofitters to provide up-to-date technology and outstanding regulation at a fraction of the cost of replacing the entire power system. ‘A lot of our customers aren’t looking to change out motors and go through a huge installation process’.

‘If there is a more cost-effective method, they want to hear about it’. In co-ordination with the drives, the system integrator installed their own winder-operating software, called iWind, which is designed to optimise roll profile and quality while offering automatic stopping, product recipes, numeric and graphical set-points, permissives and full diagnostics. An elegant and easy-to-use interface, iWind merges the various control equipment directly with the drive hardware, resulting in a less complex system that provides increased performance. It is easier to troubleshoot and maintain as well. Primarily focused on tension, nip, and torque (TNT), 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 of product length and diameter. Permissive and diagnostic pages provided operators quick help in keeping the winder running, and reduced support calls to maintenance. The recipe system allowed operations to set up orders quickly and maintain product consistency. The retrofit used existing construction – 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 enabled the integrator to prebuild a separate subpanel for each drive, allowing time before demolition of the old equipment to test the drives individually and as a group. This made it possible for all communications between the drives and much of the application software and drive functionality to be tested thoroughly before installing and commissioning. The setup and commissioning of the application program was a smooth process. It took the system integrator team approximately four weeks to engineer the schematics, create the drive and controller software and build the panels. After a thorough two-week testing process, the team was able to install the new equipment within the five-day contingency the mill required. Engineers remained on site for a few days after startup to ensure a seamless transition for the plant operators. Since the installation of the DCS800 and ACS800 drives, in complement with iWind software, the mill has been able to realise its goals of increased production, top-speed efficiency, better roll quality, reduced maintenance and minimal shutdowns. In fact, it has been able to virtually eliminate shutdowns that happened previously because of component malfunction. ‘We are pleased with the ABB products and support, and plan to continue using ABB products for future projects’, Parker said.

‘And, at the moment, we have eight paper manufacturing projects in front of us for which we are recommending ABB, either ACS800, DCR- or DCS-800 for various applications’. Craig Tierno adds: ‘Our relationship with ABB and the people in New Berlin has been very rewarding’. ‘They have been most helpful with product delivery and with training on the new products, especially the DCS800’. ABB products offer the system integrator the resources and flexibility they need to help satisfy each customer according to their specific needs and budget. ABB and Southeast Drive Systems, Inc. (SDS) have another reason to celebrate: the paper mill installation marks the first time that a DCS800 has been integrated into a total systems package in the USA. With the almost unlimited scalability of the DCS800, the entire team is looking forward to supplying customers with drives and software for many years to come.

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