DC-bus connection makes energy saving easy

By changing from DC to three-phase AC technology, the world's largest provider of corrugated board plant BHS has considerably improved the energy efficiency of its machines. The company today uses a drive system created in close collaboration with Lenze who acted as its partner in the project. The core elements are Lenze 9400 servo drives and an effective DC-bus connection system. Peter Michler, Head of Development at BHS, says: "They enable us to lower the peak load and this is the key point".

A flat outer liner, fluted corrugated sheet and a flat inner liner - that's all there is to the most widespread packaging material in the world. Often an additional inner layer and a second fluted sheet are added to increase strength. Worldwide, the production of corrugated board is over 40 billion square metres a year, enough to cover the surface of Wales twice! On an international scale, the corrugated board industry is booming and BHS reports growth of around eight per cent per annum. As a result of this market development, competition in machine construction and systems engineering is increasing as well, especially from the Far East. Peter Michler says: "We are the technology leader and have to maintain this lead every day in terms of development. The energy-efficiency of corrugated board production systems has become an increasingly important aspect of the research and development activities undertaken by the company."

To construct the new medium-capacity SteadyLine corrugator, BHS changed its drive technology completely. AC servo drive technology from Lenze has replaced the previous DC system. Among other things, this opens up welcome opportunities to improve energy efficiency for the company, and, above all, to reduce the incoming power supply for the corrugator.

To produce corrugated board which usually consists of five layers of paper as previously described, a production line designed for this purpose needs five splicers which switch from an empty reel of paper to a full one without stopping the production process. The key elements of a splicer are two unwinders with an automatic roll changer (the actual splicer). The units, which are placed one after the other, feed a web of paper into the next processing stage without interruption - with exactly the right tension and at exactly the right speed. At the heart of this design BHS uses the Lenze servo drives series 9400 with integrated PLC for the main drive of the unwind and for the braking roller. This ensures reliable and high splicing speeds even for low-weight paper as a web tension regulating system has been integrated into the servo controller.

Because the braking roller after the unwind acts as a generator, BHS uses the generated energy to feedback power to the machine. The servo drive of the braking roller acts as the central source of power for a complete splicer. A DC-bus connection connects the 9400 servo drive to all other drive controllers, these include Lenze frequency inverters of the 8400 series. All the drives are located directly under the DC bus in the control cabinet. The bus is made of wide copper bars, primarily for reasons of electromagnetic compatibility (EMC).

Enhancing internal consumption

Re-using energy instead of returning it: when BHS had DC drives without any DC-bus connecting capability it used to feed the braking energy back into the utility company grid. Now that BHS is using AC technology, enhancing internal consumption has become the method of lowering energy costs. Summing up the issue, Michler says: "In this way, we are able to reduce the peak load." This approach is extremely attractive, especially from the viewpoint of business economics. In the case of power recovery (feeding power back into the grid), large amounts of power initially have to be purchased from the energy supplier but considerably less money is obtained for feeding the braking power back into the supplier's grid. Michler says: "This is simply not worthwhile, especially in view of the fact that we also had to invest in regenerative units that output the electricity in a clean sine-wave form."

By contrast, the DC-bus connection is a much simpler technology. In terms of energy efficiency, it gets to the root of the matter - specifically, it automatically limits the amount of power fed in to what is actually needed. This is a particular advantage at the moment for the BHS sales operation in China, where energy saving is a big issue. Customers expect that such technical systems are evaluated in advance, before any investment decisions are made. Michler says: "So we're working on ways to document energy savings by means of comparison and verification."

With Drive Solution Designer (DSD), Lenze provides a software tool that can be used for this purpose. It makes it easier to size the drives - and it also allows optimisation of the energy balance as well. DSD is part of the Lenze energy-saving BlueGreen Solutions and makes extensive design knowledge available in a simple, structured manner. This tool can be used to compare different drives with each other, not only in terms of performance but from an energy viewpoint as well. While the corrugated board market is certainly ready for ROI calculations where energy saving investments have to be made, companies need clear evidence of whether these investments are worthwhile, hence the value of such engineering tools. Michler says: "Green thinking is on the rise in China. With our new SteadyLine, we are pointing the way forward when it comes to the efficient use of resources".

BHS were able to take advantage of engineering design with the aid of the Lenze DSD software tool giving precise design and dimensioning without excessive power reserves. An additional output is an Energy Performance Certificate detailing the energy costs of running the machine. Using AC drive technology opened the way to connect drives with a DC-bus where re-generated energy can be re-used rather than fed back into the mains in a manner that is not commercially viable. These factors together give BHS a commercial advantage in energy-conscious markets.

To learn more about Lenze 9400 servo drives and DC-bus connection systems, lease go to www.lenze.com.

Lenze Ltd

6 Abbey Court Road
Priory Business Park
Bedford
MK44 3WH
UNITED KINGDOM

+44 (0)1234 753200

sales.uk@lenze.com

www.lenze.com

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