Energy re-use at a Swedish water plant

When a Swedish wastewater treatment plant invested in new decanter centrifuges, Emotron variable speed drives were chosen. This was one of the first times that manufacturer Noxon had used Emotron VFX in its solution, with the energy generated by the braking motor being re-used by the driving motor.

Falkenberg Municipality in Sweden uses Emotron’s products as standard for operation of electric motors.
Falkenberg Municipality in Sweden uses Emotron’s products as standard for operation of electric motors.

The Falkenberg Municipality in Sweden has been using Emotron products as standard for several years now to protect and control the operation of its electric motors in treatment plants, water purification plants and pumping stations. Demands and expectations are high and operational electrician Tomas Bjurklint sees major benefits with the ease of use in Emotron's equipment. He commented: “We value the userfriendliness 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 is essential in order to minimize downtime.”

Decanter centrifuges

When Falkenberg treatment plant invested in a new solution for dewatering sludge, they chose two decanter centrifuges from Noxon, a Swedish company with its own development, design and manufacturing resources and customers primarily in Europe and Asia.

A decanter centrifuge consists of a drum and a screw conveyor that rotates 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.

When placing their order, Falkenberg Municipality requested that Emotron variable speed drives be used. This was a new experience for both parties. The municipality had previously only used the Emotron FDU for regulation of pressure and flow, but an Emotron VFX was now required, which has been developed for this type of dynamic application. Noxon had always used a different make as standard for its centrifuges and had never tested Emotron. Because the process uses two electric motors, one driving and one braking, it required an eÿcient solution without brake resistors.

Braking energy re-use

Two Emotron VFX variable speed drives of 37 kW and 11 kW respectively control the two electric motors that power the drum and brake the screw conveyor so that it maintains a suitable speed. When the motor brakes, so-called ‘generative operation energy’ is created. While this is normally converted into heat and dissipated via brake resistors, Noxon’s solution re-uses the energy instead. Thanks to the two variable speed drives 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.

The 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.