Plastic recycler goes greener, swaps to ac drives 03 April 2013

Maintenance costs have apparently been slashed at an unnamed plastic recycling plant by replacing the dc drive on its compounder with a variable speed ac unit.

The recycling plant, opened in 2004, converts discarded plastic into HDPE (high density polyethylene) pellets, used for manufacturing a range of heavy duty products.

Like all dc motors, the unit driving the main compounder required regular maintenance for brush wear and refurbishment of the armature. This was a major task and disruptive to production, so the site engineers decided to explore possibilities for switching to a brushless ac motor.

George Newton, managing director of Warrington-based drives engineering firm Newton Tesla recalls being asked to provide specialist support. That led to a proposal for an ac drive that would provide the necessary low speed torque operating characteristic, while also reducing maintenance.

"We included a Mitsubishi inverter in the design, as this would contribute to a substantial reduction in power consumption and would further enhance the environmental credentials of the recycling plant," states Newton.

"And of course there were significantly reduced energy bills as well as the maintenance savings," he continues. "The ac drive package we proposed would require no ongoing maintenance other than routine bearing lubrication."

Newton paired a 400kW six-pole motor with an 866A inverter, the latter being a 740 Series Mitsubishi Electric variable frequency unit. "A compounder or extruder runs way below its maximum speed the vast majority of the time, so to the energy savings [of speed control] are considerable," he explains.

Additional savings are made possible by Mitsubishi Electric's OEC (optimum excitation control) technology, which ensures that magnetic flux is always optimised for maximum efficiency.

Brian Tinham

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