Ploughing savings back into production

A more arduous duty cycle for an inverter than that of the drive of an industrial centrifuge is hard to imagine. A plough which scrapes the solids from the walls of the basket of the centrifuge applies forces which attempts to slow it to a crawl and can play havoc with the performance of the drive.

At Rhone Poulenc's agricultural herbicides plant in Norwich, Hitachi J300 sensorless flux vector inverters have dramatically improved the effectiveness of the company's centrifuges in spite of the demands of the application. The Norwich facility manufactures Bromoxynil/Ioxynil (BX/IX) for which the final part of the manufacturing process involves the separation of solids from a liquid in two centrifuges.

The entire process and the productivity of the plant relies on the efficiency of the two centrifuges. Indeed, if either centrifuge drive failed, the effect would be felt immediately. Once the slurry has been pumped into the centrifuge and the solids separated, wash water is sprayed onto it and the drying cycle commences. The basket of the centrifuge is driven by a 22kW flameproof AC motor. The basket revolves at speeds of up to 1200 rev/min while the ploughing of the product occurs at 60rev/min. The basket is slowed to allow the mechanical plough to scrape off the remaining cake, without damaging either the product or the machine.

One of the major problems faced by Rhone Poulenc's engineers is the fact that the slurry must be distributed very evenly onto the walls of the basket. Failure to do this results in damage to the centrifuge, so the loading is controlled to the extent that even a deviation of a few revolution a minute results in the feed being stopped until the correct speed has been attained. Moreover, during the ploughing operation, the speed is very slow - typically 60 rev/min - yet the maximum torque is required.

What Rhone Poulenc's engineers needed was an inverter which had excellent speed holding under varying load conditions, high torque at low speeds (1Hz to 2Hz) and full four quadrant control to give smooth acceleration and braking. The Hitachi J300 adequately fitted the bill, but the installation of the drive was also to bring about savings in process time.

Cycle times through the two centrifuges were improved thanks to the fact that the Hitachi J300 enabled full torque even at low speeds under extreme load and thereby avoided stalling while ploughing.

While substituting the Hitachi J300 sensorless flux vector inverter was a relatively straightforward modification to the existing plant, the benefits to Rhone Poulenc have been substantial. Cycle time savings of about 20% have been achieved.

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