Most older plastic extrusion machines were driven by DC drives, but few new ones nowadays do. What has driven this change are new generations of high performing AC motors and VSDs. Using AC drives to update plastic extrusion machines improves performance, lowers energy consumption and reduces maintenance costs.
AC motors and variable speed drives are now standard in modern plastic extruders due to the wide range of advantages they offer the user. The reason for replacing DC with AC are mainly due to financial savings. Even if your existing machine is still working, changing over to AC motors and VSDs can still make economic sense.
The three main costs for any plastics extrusion business are raw materials, labour and energy. Changing plastic extrusion machines 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 mean more costs.
Take the case of UK plastic pipe manufacturer Radius Systems in Derbyshire. They estimate that 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 when updating plastic extruders
We realise that we’re in danger of repeating ourselves, but 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 that DC motors draw a higher current, increasing the energy and requiring larger cables and switchgear. Moreover your electricity supplier may add a further charge for increased kVA or peak current usage.
Energy efficient 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.
AC Drive 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 1000mm diameter and associated fittings used by gas and water utilities. An existing extrusion machine underwent a conversion from DC by installing a new AC drive and an ABB IE4 SynRM motor. The system now operates with higher power factor and its efficiency has also increased. The result was system energy savings estimated to be up to 15 percent and giving an amazing 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 them easier to work with 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.
To discuss how we can support you with your switch over to an AC drive on your machine or machine, please get in touch with us using the contact form below, and we’ll respond to you as soon as we possibly can.
Plastic Extrusion Machines benefit from AC Drive Update
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