processing feature | Improving energy effi ciency
Table 1: Characteristics of DC and AC motor systems DC motors
For Against
Accurate, fast, direct torque control. Lower motor reliability. Good speed response. Simple control systems.
Accurate speed and torque control. Flat speed versus torque profi le.
For
Small, light and robust. Simple design.
Low initial motor cost.
Low maintenance cost. Reduced operating cost. Improves site power factor.
Good gearbox maintenance is essential for effi cient
extruder operation
VSDs option on new extruders. The AC motors should be IE3 rated.
Tip - One of the neglect- ed savings in using AC motors + VSDs is the greatly reduced maintenance load of AC motors.
Retrofi tting AC motors + VSDs AC motors + VSDs can easily be retrofi tted to most extruders and this is a recommended option. The energy savings will generally be in the region of 5-20%, but reductions in maintenance costs are very signifi cant. The payback should be approxi- mately 1-2 years. Tip - Sites should strongly consider retrofi tting AC
motors + VSD for existing DC motors. The AC motors should be IE3 rated.
About the author
Dr Robin Kent is an independent expert specialising in plastics processing technology and business management. Founder and managing director of plastics engineering consultancy fi rm Tangram Technology (
www.tangram.co.uk), he is also the author of Energy Management in Plastics Processing: Strategies, Targets, Techniques and Tools, which has just been published in its second edition. This practical workbook takes readers through the complete set of options for saving energy at the plastics processing level and across the entire plant. It explores benchmarking and site surveys, understanding energy supplies and bills, measuring and managing energy usage and carbon footprinting. This revised and expanded edition is available now from the Plastics Information Direct bookstore:
http://bit.ly/EMIPP2
72 COMPOUNDING WORLD | March 2014
www.compoundingworld.com
AC motors + VSD Against
More complex control system. High motor controller cost.
Variable torque versus speed profi le.
High initial motor cost. High maintenance costs.
Getting the power to the screw Gearboxes used to connect the motor to the load must be treated with care - they can give excellent service, but they do need regular maintenance. Tip - Do not exceed the maximum torque allowance of the gearbox. Tip - Make sure that gearbox oil is the correct
temperature. If it is too hot then there is a problem with the gearbox or the lubrication system. If it is too cold then lubrication will be ineffective. Tip - Excessive vibration is always a good indicator of
potential problems. For more information on gearbox maintenance for twin-screw extruders, see the article ‘Learn to love your gearbox’ by Tim Perry of Century Extrusion in the November 2011 edition of Compounding World:
http://bit.ly/CWgears.
What do we do next? Improving the energy effi ciency of an existing site or extruder can be as simple as running the existing equipment harder to minimise the effect of the base load and this is certainly recommended if the orders are there. There are many other actions that can be taken in the area of
services and machines, but one of the most effective actions in the
long-term is to ensure that the extruders
are as energy effi cient as possible by choosing the right technology for the extruder motor. The average extruder will be in service for
over 15 years and the cost of the energy consumed over this long period of time will add up to be far greater than the capital cost of the extruder. Getting the right motor is an important decision for the future and selecting an extruder should be based on more than simply the initial purchase price.
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