Soft starters help to ensure reliability of recycling plant

Axion Polymers in Salford, Manchester, prepares discarded plastic for reuse. Its control systems were designed and built by Sean Lamy of integrator 3-Ohms Ltd, who says: "Parts of the process are very demanding, so we had to design-in a level of sophistication and robustness to guarantee reliable operation - which often runs on a 24x7 continuous cycle."

The first stage in the process is shredding the infeed materials using a single-shaft shredder with a rotating drum-cutter. This represents a large mass, which would draw a lot of power at start-up and feed voltage spikes into the supply. It is also in the nature of this axis to jam.

Lamy explains: "We have a main drive motor of 60kW and therefore fitted a soft start with a current monitor. If the current rises to 250 per cent, the soft start reverses the motors to clear the obstruction, then goes back into forward drive to resume shredding."

Outfeed and compactor

Another soft start is used on the outfeed screw conveyor, and a third is fitted to the motor on the power pack of the hydraulic compactor that feeds the shredder.

After shredding, the plastic goes through a novel separation stage to classify different type of plastic. These are melted, extruded and granulated to form new plastic stock for sale to moulders, extruders and manufacturers.

A final soft start is fitted to the 110kW drive motor of the granulator, which incorporates a heavy, precision-engineered cutting rotor. This rotating cutter needs a very high but controlled starting torque - provided through the soft start - for when it is starting under partial or full load.

An Hitachi EH150 PLC is used to manage the whole system, including control of the hydraulics and collection of throughput data for productivity analysis.

Full control from one PLC and HMI

Lamy says: "The EH150 accepts both analogue and digital inputs, using the former to monitor the current to the 250 per cent threshold. Despite being a fairly complex machine, we were able to achieve a very neat control architecture based on a single PLC."

An HMI provides the operator with clear, concise information on machine status. This includes operation information, production information, diagnostics for easy faultfinding and emergency stop activities.