Energy Management in Rubber Processing - Part 6 - Motors and drives
The sixth in a series of energy efficiency worksheets by Dr. Robin Kent for
the Carbon Trust to help the
rubber industry reduce costs through efficient
use of energy.
UK Government Environment and Energy Helpline 0800 585 794
Approximately two-thirds of the energy costs in rubber processing are the result of electric motor usage. Yet motors are often overlooked when considering energy usage. The motors in the main processing equipment such as compounders, 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 the motors in a factory is expensive!
The energy cost of a motor can exceed the purchase cost in just 1000 hours of use
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 energy efficiency improvements available with the development of 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 HEM for all new purchases
When new motors are required, the benefits of opting for HEM are obvious. However, the failure of an existing motor needs a decision on 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.
Instantaneous energy use of a moulding machine
Motors are most efficient when their load equals, or is slightly greater than, the rated capacity. Motors can 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 a typical moulding cycle and illustrates the wide variations in load demand from a typical 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: VSD will allow motors to run at the required speed to save energy.
The cost premium for HEM is now very small and easily offset by the energy cost savings that result from their use. HEM 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 ten year life and a 3% saving is £1500 - this is equivalent to three free motors.
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 recirculation of the hydraulic fluid. Another way of meeting the varying demands is to fit a 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 energy demand graph shown would have considerably fewer peaks and troughs if a VSD were to be used on the machine. The application of VSD 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
VSD 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.
- Contact the Carbon Trust for information on motors (GPG002) and motor management policies (GIL056).
- Start to save real money by choosing the best motors and systems.
The life cost of a motor is often over 100 times the purchase cost
The "Energy Management" series is designed to give plastics processors an insight into how to manage a valuable resource.
Part 1 - Reducing energy costs - the first steps
Part 2 - The rewards
Part 3 - Compounding
Part 4 - Moulding
Part 5 - Extrusion
Part 6 - Motors and drives (This Section)
Part 7 - Compressed air
Part 8 - Buildings