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20 June 2011

When EdF (Cottam Power) needed a starter for the MV drive fitted to a hammer mill at its Nottingham site, most suppliers claimed that only a costly variable speed drive would be suitable. Softstart UK, however, with its wide experience of similarly demanding applications, was confident in offering instead a much more cost-effective MV soft starter. This has proved entirely satisfactory in service, and its adoption has allowed EdF to make substantial savings on the overall budget for the project.

At its Nottingham site adjacent to Cottam Power Station, EdF pulverises olive cake, which is made from the husk residue that remains after olives have been processed to extract their oil. The company then mixes the pulverised cake with coke to form an environmentally friendly biofuel that is burned in the power station.

The pulverising process is carried out using a hammer mill that essentially comprises a cylindrical “squirrel cage” with six bars. Hammers are attached to these bars and, as the cylinder rotates they flail out, pulverising the olive cake. To ensure that the hammers wear evenly, the direction of rotation of the mill reverses each time it starts.

The hammer mill is no small machine. It receives olive cake via a triple screw feeder at a rate of up to 30 tonnes per hour, and it is driven by a 350 kW 3.3 kV motor. The drum with its hammers attached has very high inertia and takes up to 15 seconds at start up to reach its full operating speed of 3,000 rpm. These characteristics mean that conventional direct-on-line starting is not possible because the very high torque that would be generated during the starting process would be likely to damage the couplings in the drive system.

For help in addressing this problem, the engineers at EdF approached a number of suppliers of MV drive systems and, in almost every case, were told that the only feasible solution would be to install a variable speed drive, as this would allow the most flexible control of the motor during start up. In particular, the engineers were warned against the use of a soft starter as, it was claimed, this would be unable to consistently provide the accurate starting control needed.

Softstart UK disagreed with these claims, and advised EdF that, on the basis of its previous experience with high-power hammer mills, a soft starter would, in fact, be entirely adequate and that using one instead of a variable speed drive would reduce the overall project cost by thousands of pounds. After carefully considering this suggestion, the EdF engineer decided that a soft starter would be used and surveyed the market for a suitable product.

Ultimately a unit from Softstart UK’s new MV-DS range was selected. Key factors that influenced this decision were Softstart UK’s proven experience in heavy duty industrial applications, the excellent technical characteristics of its products, their value for money, and their ready availability – MV-DS soft starters are usually delivered within eight weeks of an order being placed.

MV-DS digital soft starters from Softstart UK are designed around the standard European-style motor control centre, and are available with ratings up to 700 A in versions for use on ac supplies from 3.3 kV to 6 kV. All starters in the range incorporate PLC control to maximise their versatility and a touch-screen user interface to facilitate programming, diagnostics and data recording. The user interface is configured to provide instant access to statistical data that includes the last start, current, power and more.

Interconnections in the MV section of the soft starters use MV cable, thereby making them lighter and eliminating the need for costly partial discharge testing. In addition, the MV section is fully isolated from the control section by using a fibre optic link for the gate firing control. MV-DS soft starters also incorporate line contactors that use the latest vacuum switching technology which combines high reliability and long life with complete freedom from maintenance.

“These characteristics seemed to make the MV-DS an excellent match for our needs,” said Graham Wolsey, an engineer on the EdF project team, “but after the dire warnings we’d received about using soft starters, there was still a certain amount of tension when we hit the start button for the first time! We needn’t have worried, however, as the advice provided by Softstart UK was every bit as good as the company’s products, and the mill performed exactly as we wanted.”

The mill is now in daily use and the soft starter is performing faultlessly. Currently it is only called upon to perform one start per day, but it has been demonstrated as being fully capable of handling up to four starts per hour.

“There is no doubt that Softstart UK’s expertise and confidence in its own products has saved us many thousands of pounds on this project,” said Graham Wolsey, “as well as providing us with a compact and dependable starting system that is an exact match for our needs. I would, therefore, have absolutely no hesitation in recommending the company to anyone who has unusual and challenging starting requirements.”

For further information, please contact Stuart Harvey:

Softstart UK Limited, Unit 14 Brinell Way, Great Yarmouth, Norfolk, NR31 0LU, UK Tel: +44 (0)1493 660510 Fax: +44 (0)1493 660511 Email:  Web: are not responsible for the content of submitted or externally produced articles and images.
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