AC drives to update plastic extruders

ABB AC drives update plastic extruders.

Most older plastic extrusion machines used DC drives, but few new ones do. What has driven this change are new generations of high performing  AC motors and VSDs. Using AC drives to update plastic extruders improves performance, lowers energy consumption and lowers maintenance costs.

AC motors and variable speed drives are now standard in modern extruders due to the wide range of advantages they offer the user. The reason for replacing DC with AC are the mainly financial savings. Even if your existing machine is still working, changing over to AC motors and VSDs can still make sense.

The three main costs in any plastics extrusion business are raw materials, labour and energy. Changing extruders from DC to AC drives significantly reduces maintenance and energy costs. As DC motors age, their maintenance costs increase. If the brushes need annual replacement, the material cost is likely to be over £800 for one motor. Furthermore users risk lost production due to unplanned maintenance down-time. And more motors also means more cost.

Take the case of UK plastic pipe manufacturer Radius Systems in Derbyshire. They estimate their outsourcing costs for checking and changing brushes to be about £2,000 a year for each motor. However, AC motors do not use brushes. They are also smaller, lighter, more rugged and generally less expensive. Moreover, they are almost maintenance-free.

Using AC drives to update plastic extruders

The main reason for replacing a DC drive with an AC drive is reducing energy costs. To provide variable speed, DC motors use a semi-conductor controlled rectifier, but this lowers the power factor. This means it draws a higher current,  increasing the energy and requiring larger cables and switchgear. Moreover the electricity supplier may add a further charge for increased kVA or peak current usage.

AC drives however, have an excellent power factor across their speed range and make them a practical alternative to DC control. Energy savings come from the lower running current and minimising kVA charges.

Application example

Unlike fan and pump loads, the extrusion process needs a constant motor torque. This means energy savings are smaller, but still significant due to the larger power ratings of the motors used.

Radius Systems in Derbyshire makes polyethylene pipes from 16 to 1000 mm diameter and associated fittings used by gas and water utilities. An existing extruder underwent conversion from DC by installing a new AC drive and ABB IE4 SynRM motor. The system now operates with higher power factor and its efficiency has also increased. The result was a system energy savings estimated to be up to 15 percent and giving a two-year payback period.

SynRM motors offer up to 40 percent more power density than conventional induction motors and are therefore smaller. They also run cooler, making them more reliable and reducing maintenance levels. They are also quieter, making it easier for operators.

Control accuracy is further enhanced by the ABB drive’s use of direct torque control (DTC). This enables them to provide better control of the process than DC drives. With an AC motor, the drives can measure the open loop speed. This achieves the constant speed and torque control accuracy needed to extrude plastic. The