Precision control from variable speed drives

10 July 2012

Variable speed drives supplied by Control Techniques have been chosen for use on drying and coating lines supplied by Phoenix Dryers.

A typical line designed by Phoenix could be for rubber-coating a cotton web as the first process in the production of industrial textiles. Such a line will employ a total of 14 Unidrive SP and Commander SK AC drives supplied by Control Techniques.

The coating line comprises an unwind section with an air brake controlling tension, followed by a 100ft accumulator section, a spray/dipping coating unit, a dryer/curer and a take-off roll with dancer to control tension. Unidrive SP AC drives are used throughout, with additional Commander SK AC drives providing exhaust and recirculating fan control. The master for the line is a water-cooled pull roll drive, positioned just before the take-off section, to provide a reference master signal via DeviceNet to all the drives and the controlling PLC to keep the line synchronised.

The first of the Unidrive SP drives on the line is a 0.75 kW unit controlling the 8in diameter nip roll feeding the accumulator with the 60in fabric web. An identical drive controls the S-wrap pull rolls, followed by a 1.1 kW reverse dancer backup roll for tension control. The web then either goes through applicator rolls, for direct coating, again with a 1.1 kW drive, or is diverted through a dipping process, both followed by a nine inch diameter squeeze roll controlled by a 1.1 kW drive. Also on this section a 0.75 kW Unidrive SP controls a 7in diameter metering roll.

Throughout this process and the subsequent drying/curing, web tension is critical. The tolerance band is adjustable up to 136 kg and controllable to better than ±0.5%, which is crucial to eliminate tension lines in the web as well as to prevent web breaks. “We had found problems with maintaining tension with some other drives,” said John Beardsworth, Phoenix managing director. “However, drives supplied by Control Techniques always give accurate tension control”.

The drying/curing section has a graduated temperature from 62°C to 140°C and the web exits the tunnel into a web-guider, where load cells monitor the tension and provide a feedback signal to the reversing drive. The hot web is taken through a water bath by water cooler pull rolls controlled by a 0.75 kW drive.

All of the Unidrive SP drives have encoder feedback and SM-DeviceNet communication modules, as well as an LED keypad. Additionally, all of the drives are heavy duty rated to give 150% overload.

The four 7.5 kW recirculating fans and the 11 kW exhaust fans are all controlled by Commander SK heavy duty rated AC drives and all have flameproof motors. “This plant can use either water-based or solvent-based coating materials,” said Beardsworth. “In the case of the latter, the safety and economy of the plant is totally dependent on the accurate and reliable operation of the fan system,” he says. Sensors monitor the levels of solvent in the exhaust and feedback to the drives. Levels in the recirculating air are kept well below the lower explosive limit by controlling the amount of flow to the exhaust. The exhaust air is then fed to an abatement plant to prevent release of solvents into the atmosphere. “It is important to recirculate as much of the hot air as possible to keep energy costs down,” adds Beardsworth.“Even more important is the need to maintain a good safety margin.”

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