GREEN PRODUCTION SUPPLEMENT ENERGY CONSUMPTION ANALYSIS


must be reduced; or make the machine more productive, so less idle,” Dr Kummetz offers, adding that: “Improving the energy efficiency of CNC components would contribute just a small percentage improvement – so it doesn’t make sense to develop feed drive motors or main spindle motors with better efficiency. Would that give a real improvement in energy per part? The answer is ‘not really’.” And turning to the matter of electrical


efficiency and the potential to improve CNC systems here, he develops this argument, running through the system and feed drive/spindle elements. “The energy efficiency of the CNC power module components is 96-99% and all CNC manufacturers use the same parts. So claims made by others regarding efficient power modules can be claimed by us, too.” After that come inverters, which also are 96-99% efficient; then the feed/spindle motors, where electrical losses of 7-21% can be seen; finally, losses in ballscrews are between 5 and 21%, with this including friction, too. So the electrical efficiency of the system is 50-86% at the ballscrew. “We can affect this by making use of better components available on the market,” Dr Kummetz adds.


MOTOR EFFICIENCY Turning to feed and spindle motors specifically, as these are the most obvious candidates for improvement, he offers an analysis here. “We undertook two extreme operations; the first, with respect to the main spindle, used an 18 kW unit at full power to produce chips, while the feed drives drew about 200 W. So, if we could get a 5% improvement in efficiency, what would be the energy saving over one year? For this extreme operation undertaken continuously across the year – 24 hours/day, 365 days/year, which you wouldn’t do, of course – the saving is around €1,000/year for the main spindle. The reality is maybe 20% of this, as the motor would not run at full power all the time. For the feed drives, forget it; nobody will pay €100 more for a more expensive motor to save €9.” The second experiment involved employing maximum acceleration, so driving the feed drives hardest, during circular pocket milling. Here main spindle energy savings were


www.machinery.co.uk May 2013


Power


Energy efficiency of the CNC and drive components Drive


module inverters Feed drives


Power measurement Power Joule effect 1-4% 1-4% measurement 7-21% Power measurement 5-21%


Input power (electrical)


Main spindle


Output power (mechanical)


€89/year, while feed drive savings were €11/year. Clearly, again the justification for higher efficiency motors is not made on money grounds. But Dr Kummetz does agree that for motors on such things as hydraulic pumps that do operate continuously, there is an argument for higher efficiency motors and/or gearboxes. Turning to a technology that is being applied to machine tools right now, regenerative power systems, Heidenhain has also looked at these. Regenerative power is gained when spindle or feed drives decelerate. But to achieve this requires more electronics, specifically filtering, to make sure that the electricity fed back conforms to specification. “Those filtering components will consume power, even if there is no regeneration and also when power is being fed back. So regenerative power supplies draw more energy than non- regenerative supplies,” Dr Kummetz explains. (Regenerative power supplies are, effectively, 2-4 % less efficient.) This extra power can be offset only by a


certain rate of toolchanges for machining centres and workpiece changes, in the case of lathes, he further adds. And for the system under consideration, the break-even


point is a toolchange every 0.6 minutes. It is also the case, it is offered, that to recover a reasonable amount of energy, spindle and feed motors must decelerate from high speeds; decelerating from low speeds will “see most of the energy eaten up by friction in bearings”. Okay, so the most power can be saved by reducing auxiliary component draw and standby consumption when the machine is not productive. In its CNC systems, Heidenhain offers the capability to control power drains. Machine tool builders can configure certain conditions, with these offered to end users through the control interface. “We think this is the best way to increase efficiency of machine tools; although the better way is to have no idle time, of course,” adds Dr Kummetz. But, if many heat sources are turned off, this will impact on the machine structure and mean that, when they are turned back on again, the structure will take some time to become stable. The use of linear encoders can help, as this can eliminate warm-up procedures. And by eliminating warm-up, production can start immediately, which contributes to energy efficiency, by reducing energy per part produced. ■


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Electrical input power 50.86%


50.86%


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