Water & Wastewater Treatment Magazine
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Correct installation wrings all the benefits Variable speed drives are helping to make borehole extraction more efficient and reliable – but there are technical challenges that must be considered in order to wring out the greatest benefits www.wwtonline.co.uk | WWT | DECEMBER 2018 | 27 MARTIN RICHARDSON, WATER FRAMEWORK MANAGER, DRIVES AND CONTROLS, ABB The Knowledge Pumps and drives how much particulate matter is suspended in the water. Starting pumps with direct-on- line (DOL) or star delta starters tends to stir up silt from the borehole, causing turbidity, whereas using a VSD achieves a gradual ramp up of speed, avoiding turbidity from occur- ring. Benefits stack up The benefits of using VSDs on borehole pumps don't stop at turbidity reduction. By run- ning the motor to match actual demand, energy use can be significantly reduced – by as much as 50 percent – com- pared to DOL driven pumps which would always run at full speed. Periods where the sys- tem is 'pumping to waste' can also be dramatically reduced, saving even more energy. Another advantage is the ability to vary output to comply with the terms of an extraction licence. Consider, for example, a licence that specifies an extraction rate of seven mega litres of water a day. A water company might have one pump that could extract five mega litres a day. Adding another pump of the same capacity would exceed the limit. By using a VSD, the speed of the second pump could be varied to ensure the pumps' combined output did W hen it comes to water sources, groundwater in the form of aqui- fers holds several attractions for water supply companies compared to surface sources such as rivers. Where aquifers exist, they are usually acces- sible over large areas simply by sinking a borehole where needed. Groundwater also not exceed the licence limit. Identifying challenges Depending on the extraction site, the pump motor may be located within the borehole pump itself, or at the head of the borehole connected to the pump by a shaŠ. Whichever method is used, there are tech- nical challenges that must be addressed to ensure efficient and reliable extraction, as well as to reduce equipment wear and tear thereby prolonging the life of both the pump and motor. Submersible and borehole pump motors present specific challenges because of their generally weaker insula- tion and typically long cable lengths. However, selection of the correct VSD, as well as pos- sibly an output choke or sine filter, careful EMC planning and good installation practices can avoid these problems. Variable speed operation puts a higher voltage stress on the motor insulation system than a DOL supply and it is recommended to have either a reinforced insulation system or a filter between the motor and the drive. For instance, motors in borehole pumps may have a cable longer than 100 metres making it necessary to fit du/ dt or sine filters. These are oŠen requires less treat- ment than river extraction, for example, as it is filtered naturally. When installing borehole pumps, there are a number of advantages of specifying and installing variable speed drives (VSDs) to run the pumps. One of these is the reduction in turbidity, a quality measure of