MV starter elbows VSD in hammer mill control contest
29 July 2011
When EdF needed a starter for a medium voltage hammer mill drive at its Cottam Power site in Nottinghamshire, most suppliers approached by the energy supplier claimed that a variable speed drive was the only device capable of meeting the demands of this application. Softstart UK, however, had other ideas.
At a site adjacent to Cottam Power Station in Nottinghamshire, 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 comprising a cylindrical ‘squirrel cage’ with six bars. Hammers are attached to these bars and, as the cylinder rotates they flail outwards, reducing the olive cake to a consistent size suitable for mixing with the coke. To ensure that the hammers wear evenly, the direction of rotation of the mill reverses each time it is started.
The hammer mill is no small machine. Driven by a 350kW, 3.3kV motor, it receives the olive cake via a triple screw feeder at a rate of up to 30 tonnes per hour. 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 3,000rpm. 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 team 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 team was warned against the use of a soft starter as, according to supplier claims, this would not provide consistently accurate starting control.
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 trim thousands of pounds from the overall project cost.
After careful consideration, EdF decided to follow Softstart’s advice and check out the market for a suitable product. In fact, a unit from Softstart UK’s new MV-DS range was ultimately selected, largely on the strength of the company’s proven experience of heavy-duty industrial applications, the technical specifications 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 are based on the standard European-style motor control centre, and are available with ratings of up to 700A in versions for use on ac supplies from 3.3kV to 6kV. All starters in the range feature PLC control for maximum 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, including 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, thanks to a fibre optic link used for gate firing control. MV-DS soft starters are also equipped with line contactors based on maintenance-free vacuum switching technology, which combines high reliability with long life. Graham Wolsey, an engineer on the EdF project team, takes up the story:
“These characteristics seemed to make the MV-DS an excellent match for our needs, 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 reported to be 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. Mr Wolsey again:
“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, 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.”
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