Braking energy reuse boosts corrugator efficiency
11 January 2015
German corrugated board production machinery manufacturer, BHS improves its machine's energy efficiency by upgrading from dc control to three-phase ac servo control.
By upgrading from an original dc control system to one comprising three-phase ac servo drives connected over a dc-bus, the German corrugated board production machinery manufacturer, BHS has not only improved the energy efficiency of its machines, but has also secured its lead position as a global supplier of corrugated board production systems.
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 - corrugated board. Sometimes 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 meters a year, enough to cover the surface of Wales twice over!
Located in Weiherhammer in Bavaria, BHS is seeing sales growth for its machines of around eight per cent per annum. While this is clearly good news for the company, competition in machine construction and systems engineering is increasing as well, especially from the Far East. "We are the technology leader and have to maintain this lead every day in terms of development," says BHS head of development, Peter Michler. “The energy-efficiency of corrugated board production systems has become an increasingly important aspect of the research and development activities undertaken by our company.”
Peak load reduction
For its new medium-capacity SteadyLine corrugator, BHS changed the drive technology completely. Now, ac servo drives from Lenze have replaced the previous dc system. The core elements are Lenze 9400 servo drives featuring a dc-bus connection system. According to Michler, this arrangement enables the peak load to be reduced, which was a key aim of the control system upgrade.
To produce corrugated board, which usually consists of five layers of paper, the production line requires 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 9400 series servo drives with integral PLC for the main unwind drive and for the braking roller. This ensures reliable and high splicing speeds even for low-weight paper, particularly as a web tension regulating system has now been integrated into the servo controller.
As the braking roller following the unwind section acts as a generator, BHS uses the generated energy and returns it to the machine. In this way, the braking roller servo drive acts as the central source of power for a complete splicer. The 9400 servo drive is connected to all other drive controllers in the system – including some Lenze8400 series frequency inverters – via a dc bus. All the drives are located in a control cabinet beneath the dc-bus, which comprises wide copper bars, primarily for reasons of electromagnetic compatibility.
Reuse, rather than return
When BHS used dc drives without any dc-bus connecting capability, the braking energy was fed back into the utility company grid. Now, with ac technology installed on its machines, enhancing the way energy is consumed internally has become BHS’s preferred method of reducing its customers’ energy costs.
Reusing energy internally rather than exporting it to the grid enables operators to reduce their peak load – an extremely attractive option 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. According to Michler, this is simply not worthwhile, especially in view of the fact that a machine operator would also have to invest in regenerative units that output the electricity in a clean sine-wave form.
Furthermore, the dc-bus connection is a much simpler technology. In terms of energy efficiency, it gets to the heart of the matter – specifically, it automatically limits the amount of power delivered according to what is actually needed. This is a particular advantage at the moment for the BHS’s sales operation in China where energy saving is a big issue.
Customers now expect that technical solutions to energy related issues are evaluated in advance, before any investment decisions are made, and BHS is now working on ways to document the energy savings potential of its machines by means of comparison and verification. Fortunately, Lenze can provide the software that will help its client to meet these objectives.
Lenze’s Drive Solution Designer (DSD) is a software tool that simplifies drive sizing – and it also enables the energy balance to be optimised. DSD is a product within Lenze’s energy-saving BlueGreen Solutions portfolio and it makes extensive design knowledge available in a simple, structured manner.
The tool can be used to compare different drives with one another, not only in terms of performance but from an energy viewpoint as well. While the corrugated board market is certainly poised to make its RoI calculations where energy saving investments are concerned, companies need clear evidence of whether these investments are, in the end, worthwhile; hence the value of engineering tools like DSD. With the huge market for these machines in China, where the pursuit of energy efficiency is on the rise, BHS believes its new SteadyLine corrugator is particularly well positioned to meet the aspirations of its customers.
BHS was able to take advantage of the engineering design capabilities of Lenze’s DSD software tool which provided a level of precision in terms of system sizing that avoided over-engineering of power reserves. The software also generates an ‘Energy Performance Certificate’ detailing the energy costs associated with the running of the machine.
The switch to ac drive technology opened the way to dc-bus connection, enabling regenerated energy to be reused by the machine, rather than being fed back to the grid in a manner that does not necessarily make economic sense for the machine operator. Taken together, these factors have given BHS a commercial advantage in increasingly energy-conscious markets.
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