By Karin Bergkvist
• Energy-efficient decanter centrifuges from Noxon for sludge dewatering deployed at southern Swedish treatment plant are aided by Emotron variable speed drives
When operators of the municipal wastewater treatment plant in Falkenberg, Sweden, invested in new decanter centrifuges, units equipped with Emotron variable speed drives -- also known as variable frequency drives (VFDs) -- were not only chosen, they were requestedWhen Falkenberg treatment plant invested in new equipment for dewatering sludge, they chose decanter centrifuges from Noxon controlled by Emotron VFX variable speed drives. Operational electrician Tomas Bjurklint is happy with the cost-effective solution, with the energy generated by the braking motor being re-used by the drive motor.This was one of the first times centrifuge manufacturer Noxon AB, of Fjärås, Sweden, had used Emotron VFX drives in its solution, which reuses the energy generated by the braking motor for the driving motor.
Falkenberg municipality in Sweden has been using Emotron products as standard for several years now to protect and control operation of its electric motors in treatment plants, water purification plants and pumping stations.
Operational electrician Tomas Bjurklint sees major benefits with the company's shaft power monitors, soft starters and variable speed drives: "We value the user-friendliness above all. The control panel makes it easy for operators to monitor and adjust operating parameters. In addition, we always receive excellent support if any problems arise. That's essential in order to minimize downtime."
New decanter centrifuges 
Falkenberg Municipality uses Emotron's products as standard for operation of electric motors, including pumps and blowers. "Above all, we value the user-friendliness and the fact that we receive excellent support if any problems arise," says perational electrician Tomas Bjurklint.The decanter centrifuge from Noxon -- which has its own development, design and manufacturing resources and customers primarily in Europe and Asia -- consists of a drum and a screw conveyor that rotate independently of each other at up to 3,600 rpm, depending on machine and sludge type. The sludge is fed into a mixing chamber where polymer is added so that the sludge flocculates. Dewatering is achieved through the sludge being flung out through holes against the outer walls of the drum. The dewatered sludge is transported out by means of the screw conveyor.
Emotron VFDs requested
When placing its order, the city of Falkenberg requested that Emotron drives be used. This was a new experience for both parties. The municipality had previously only used the Emotron FDU Emotron FDU drive for regulation of pressure and flow, but an Emotron VFX, which was developed for this type of dynamic application, was now required. Noxon had always used a different make as standard for its centrifuges and had never tested Emotron. Since the process uses two electric motors, one driving and one braking, it required an efficient solution without brake resistors.
Braking energy re-used
The two Emotron VFX variable speed drives of 37kW and 11kW, respectively, control the two electric motors that power the drum and brake the screw conveyor so that it maintains a suitable speed. When braking, so-called generative operation, energy is generated that's normally converted into heat and dissipated via brake resistors. Noxon's solution re-uses the energy instead. Thanks to the two VFDs being interconnected via a common DC-link, the braking motor's energy is fed directly to the driving motor. The requirement for energy from the mains is reduced and the cost of brake resistors is saved.
Conclusion
Falkenberg treatment plant was one of the first installations where Emotron variable speed drives were used for such energy-efficient control of Noxon's decanter centrifuges. The positive experience here has resulted in the solution being implemented for other customers, and more new projects are planned. WWi
Author's Note: Karin Bergkvist works for Emotron AB, of Helsingborg, Sweden. With manufacturing and development resources in Helsingborg and Bladel, Holland, and sales offices also in Benelux, Germany, China and Latin America, Emotron has focused on solutions for starting, protecting, controlling and stopping machines and processes driven by electric motors for over 30 years, including applications in the water industry. Contact: +46 42 16 99 97, [email protected] or www.emotron.com
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