How looking for new islands can improve pump motor efficiency
Around 80 per cent of maximal torque at 90 per cent speed is an efficient duty point for a motor and drive system. However, it is often possible to find a more efficient duty point by exploring a wider range of options. According to ABB, this offers opportunities for a designer tasked with improving the motor efficiency of a pump system.
By plotting motor ISO efficiency curves for various duty points, you will discover an “island” of improved energy efficiency. This may stretch from about 70 to 120 per cent of nominal speed at 70 to 80 per cent of maximum torque. This is due to the way iron losses and copper losses in the motor combine. Particularly in the region above base speed, where maximum torque reduces as the speed increases.
At some point in this region, copper losses start to go up with higher current, increasing with the speed. Meanwhile, the iron losses, which are due to the magnetising current, begin to come down. The best efficiency comes when the reduction in iron losses is greater than the increase in copper losses. The result is lower losses against operation with the same power output in the region below the base speed.
Operating the motor at less than full torque produces small but valuable motor efficiency gains in the region of 1-2 per cent. This helps designers meet the motor efficiency criteria required by the Ecodesign Directive and may offer more scope for flexibility. For example, it could be more efficient to operate a fan with a smaller impeller and compensate with a higher motor speed.
When tasked to improve the motor efficiency of a particular system, options may be few. Most parts of the system will have already been optimised for peak efficiency over years of successive product generations. Selecting a different motor may be one of only a few choices available to the designer.
The key to overall efficiency is to minimise the losses through the system. Higher motor losses need more input power, increasing the losses in the variable speed drive and the supply system. A main aim behind the Ecodesign Directive is to minimise such losses and improve energy savings.
By looking at alternative duty points for the system and selecting a different motor, may achieve motor efficiency gains to get the system over the compliance threshold. A Best Practice Guide for Variable Speed Driven Pumps is available for download from UK Trade Association GAMBICA.
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