AC replaces DC drive technology to cut energy costs for recycling plant

05 April 2013

Maintenance costs have been reduced at a plastic recycling plant through the replacement of the DC drive on the compounder with a variable speed AC system.

The recycling plant in question converts discarded plastic into high-density polyethylene (HDPE) pellets.

In common with most DC motors, the motor driving the main compounder required regular maintenance for brush wear and refurbishment of the armature.
This proved to be a major task that was also disruptive to production, so the site engineers began to look for an alternative solution – switching to a brushless AC motor. Newton Tesla (Electric Drives) was asked for some specialist support. This company specialises in supplying large drive packages, usually up to 1,000kW and based on an AC motor and inverter to give variable speed capability.

Newton Tesla engineers developed a proposal that would provide the necessary low speed torque operating characteristic, while the robust reliability reduced the regular maintenance requirement.

“We included a Mitsubishi inverter in the design, as this would contribute to a substantial reduction in power consumption and would further enhance the environ-mental credentials of the recycling plant,” explained George Newton, managing di-rector of the drives engineering company. “And of course there were significantly reduced energy bills as well as the maintenance savings.”

Newton explained further, that traditionally DC solutions were used for low speed applications, such as compounders and extruders, but as inverter technology has developed AC has moved more and more into favour. “The AC drive package we proposed would require no ongoing maintenance work other than routine bearing lubrication.”

Newton paired a 400kW six-pole motor with an 866Amp inverter as the core of the drive system, the latter being a 740 series Mitsubishi Electric variable frequency unit.
These 740 inverters are often referred to as ‘pump and fan drives’, as this is a major application area for them. However they are said to be equally appropriate for other duties, where their ability to enhance the energy balance of the driven machine can generate energy savings of 60% or more.

“If there is a volumetric flow involved – of air, gas, liquid or molten plastic – energy savings are proportional to the cube of the difference between operating speed and maximum speed,” explained Newton. “A compounder or extruder runs way below its maximum speed the vast majority of the time, so to the energy savings are considerable.”

Additional savings are made possible by Mitsubishi Electric's innovative OEC (Optimum Excitation Control) technology. This system ensures that the optimum magnetic flux is always applied to the motor, thus reducing losses. The result is maximum utilisation of motor capacity at maximum efficiency.

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