Energy Management in Plastics Processing - Part 10 Motors and drives
A series of energy efficiency worksheets by Dr. Robin Kent for the Carbon Trust to help the plastics industry reduce costs through efficient use of energy. UK Government Environment and Energy Helpline 0800 585 794

Motors and Drives

Approximately 2/3 of the energy costs in polymer processing are the result of electric motor usage. Yet motors are often neglected when considering energy usage. The motors in the main processing equipment such as injection moulders and extruders are obvious but the majority of motors are ‘hidden’ in other equipment such as compressors, pumps and fans. When the energy cost of running a motor for 1000 hours can exceed the purchase cost and when the ‘whole life costs’ are often over 100 times the purchase cost then failing to take action with all motors in a factory is expensive!

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The energy cost of a motor can exceed the purchase cost in just 1000 hours of use

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The motor management policy

The greater importance of running costs over the initial purchase price means that companies need to change the way they look at motors. Decisions need to be made on the ‘whole life cost’ where all purchase, maintenance, repair and operating costs are considered. The changes with the development of Variable Speed Drives (VSD) and High Efficiency Motors (HEM) mean that, in order to reduce costs, companies must develop and implement a motor management policy for the purchase and operation of motors. This policy should include guidelines on:

• repair and replacement based on lifetime costing.
• the specification of HEMs for all new purchases.

When new motors are required, the benefits of opting for HEMs are obvious. However, the failure of an existing motor raises the question of whether the motor should be repaired or replaced. Repairing a failed motor may appear to be a cost-effective action but repair can reduce energy efficiency by up to 1% and may not be the most economical long term action. A motor management policy can provide the rules for making the best financial decision.

Motor sizing

Motors are most efficient when their load equals, or is slightly greater than, the rated capacity. Motors can even be overloaded for short periods provided that there is a later lower load to allow cooling. If machines larger than needed are purchased or used then the motor will not reach the design load and will never run at optimum efficiency. Oversized motors are inefficient and equipment needs to be carefully matched with demand. Even ‘steady’ loads from extruders, fans, compressors and pumps will fluctuate slightly and the basic operating load rarely matches a standard motor.

The demand graph (below) shows the instantaneous energy demand during an injection moulding cycle and illustrates the wide variations in load demand from a typical injection moulding machine.

Instantaneous energy use of an injection moulding machine

• Tip - It is strongly recommended that expert advice on motor sizing is sought to reduce costs.
• Tip - Where motors can be accurately predicted to run at less than 33% of the rated output it is possible to reconfigure the motor from Delta to Star connection. This simple low-cost action can produce savings of up to 10%.
• Tip - Devices, such as Variable Speed Drives (see below) will allow motors to run at the required speed to save energy.

High efficiency motors

The cost premium for High Efficiency Motors (HEMs) is small and easily offset by the energy cost savings that result form their use. HEMs achieve efficiency levels of up to 3% more than conventional motors and have a peak efficiency at 75% of load, thus reducing both energy costs and oversizing problems. A 3% efficiency gain may not sound much, but a £500 motor uses approximately £50,000 in energy over a 10 year life will be and a 3% saving is £1500 - this is equivalent to 3 free motors.

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The life cost of a motor is often over 100 times the purchase cost

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Variable speed drives

The speed of an AC motor is fixed by the number of poles and the supply frequency. As a result, the hydraulic pumps in many processing machines are driven at a constant speed, even though the demand varies considerably during the cycle. The flow demand changes from the hydraulic pump are controlled by a relief valve and re-circulation of the hydraulic fluid. .Another way of meeting the varying demands is to fit a variable speed drive (VSD) to the motor. A VSD allows the speed of an AC motor to be varied and the pump output can be matched to the variable demand. The demand graph shown would have considerably fewer peaks and troughs were a VSD to be used on the machine. The application of VSDs can significantly reduce energy costs. Other VSD benefits are:

• Reduced demand on the hydraulic system means that the hydraulic oil runs at a lower temperature and requires less cooling - an additional cost saving measure.
• Reduced noise.
• Lower maintenance costs.
• Better all-round performance.

VSDs can also be applied to fans, water pumps and air compressors where the load varies considerably. For constant loads, the use of a correctly sized motor is the best option.

Despite this the varying loads and the difficulty of matching the output to the need will inevitably lead to some energy losses.

Next steps

• Contact the EEBPP to get information on motors (GPG2) and motor management policies (GIL 56).
• Analyse the existing motors using ‘Quickstart’ from the EEBPP.
• Start to save real money by choosing the best motors and systems for your company.

The "Energy Management" series is designed to give plastics processors an insight into how to manage a valuable resource.

Download the complete series as an Adobe Acrobat file.

Last edited: 22/10/07  

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