TechArchive

Original article date: November 1996

Energy saving motors cost a lot less over their lifetime but you still have to present a case for the extra initial expense. There’s now some useful software to help out.

Power losses in electrical equipment are due to the electrical resistance in conductors and losses in the magnetic material and occur primarily in motors transformers and in all cabling. The conductor losses are proportional to the resistance and the square of the current) and can be minimised by using the optimum size of conductor for the application.

The lowest overall life cycle cost is achieved by specifying larger conductors than the safe thermal minimum and a detailed design methodology is presented. Magnetic losses can be reduced by the use of better materials and production methods.

The electric motor has a long history of development since its invention in 1887 with most early effort aimed at improving power and torque and reducing cost. The need for higher efficiency became apparent during the late 1970s. By the early 1980s at least one British manufacturer had started to market a premium range of motors with improved efficiency. Now the trend is towards marketing all motors with improved efficiency at little or no premium.

However because improved efficiency requires more careful manufacture only the higher quality manufacturers are supplying high-efficiency units. There is therefore still a price premium but one which applies between manufacturers rather than between ranges from the same manufacturer. There is still a choice to be made and paying the premium is likely to be an excellent investment.

The UK industrial motor population is estimated at about 10 million units while the new market is about 3 0 units per day mostly rated at less than 150 kW. Of the electricity supplied to power industrial motors one third is consumed by motors rated at 1.1 to 15 kW and a further third by motors rated from 15 to 150 kW suggesting that there are very large numbers of small motors among the installed base. Clearly it is important to consider energy efficiency for all sizes.

Most motors operate at less than their design loading. Safety margin selection of preferred sizes and starting torque requirements mean that most motors are operating at between 60% and 80% of full load and many will run at very low load for a substantial part of their working life. It is important that high-efficiency motors retain their energy efficiency at these typical load factors and the leading manufacturers typically optimise efficiency at about 75% full load.

An electric motor can consume electricity to the equivalent of its capital cost within the first 500 hours of operation ? a mere three weeks of continuous use or three months of single shift working. Every year the running cost of the motor will be from four to sixteen times its capital cost. Over its working life an average of thirteen years it may consume over 200 times its capital cost in energy. Clearly the lowest overall cost will not be achieved unless both capital and running costs are considered together.

The standard electric motor is already a very efficient device with efficiencies above 80% over most of the working range rising to over 90% at full load. However because of the high energy consumption and the very large number of installed units even a small increase in efficiency can have a major impact on costs. The efficiency of an electric motor depends on the choice of materials used for the core and windings their physical arrangement and the care and precision with which they are handled and assembled.

Losses can be categorised into two groups; those which are relatively independent of load (constant losses) and those which increase with load (load dependent losses). The factors which affect efficiency are:

• Conductor content (load dependent)
• Magnetic steel (mainly constant)
• Thermal design (mainly load dependent)
• Aerodynamic design (constant)
• Manufacture and quality control (constant)

Because many motors spend considerable time running at low loading or idling designers of high-efficiency units have paid great attention to reduction of the constant losses. The result is a halving of losses at loadings less than 25% load and an efficiency improvement of 3 to 5% at full load a reduction in losses of about 28%.

Justifying a capital purchase is probably one of the most difficult tasks faced by managers; in part this is because there are so many methods of calculation and even more opinions about which is right! There is enormous pressure to minimise the cost of projects and this means that decision makers tend to be looking for lowest first cost. However this initial cost is only part of the story – as mentioned above a motor may consume up to 200 times its capital cost in electricity so a proper examination must include running costs.

Starting from the premise that the need for and cost justification of the purchase of a new motor has been made how can the selection of a premium quality motor be justified? As with any project the capital outlay required in this case the difference in cost between the high-efficiency motor and a standard unit must be judged against the future cash in this case the savings due to reduced energy consumption generated in future years. The criteria by which the results are assessed will depend on the culture of the organisation and may often involve comparison with other potential uses for the capital available. Under a wide range of circumstances the payback periods are typically around two years ? short enough to be considered a good investment by most organisations. In order to assist managers to explore the savings available in their own circumstances CDA has made available a software package which enables users to enter motor utilisation characteristics day and night electricity tariffs and demand charges and calculate the relevant costs. The program is easy to use interactive and produces prints of the results for distribution and easy future reference.

The economics of the installation of high-efficiency motors re best when new plant is being built. However in certain circumstances the cost of replacing an existing motor before the end of its serviceable life can be justified but the economic considerations are complex. Consideration should be given either to comparing the additional cost of early replacement (for example the lost value of the residual life of the existing unit the higher cost of immediate rather than future capital) with the future savings or taking account of future energy savings to avoid or delay the expense of increasing the capacity of local supply transformers and circuits.

Another good time to consider the selection of high-efficiency motors is when an existing unit is being considered for rewinding. Approximately 300 0 motors are rewound in the UK every year with an average rating of about 12 kW so the efficiency of rewound motors is extremely important. The loss in efficiency on rewinding depends on the techniques processes and skill used to perform the rewind and is usually between 1 and 2%. If the choice is between rewinding a standard efficiency motor of purchasing a new HE motor the difference in efficiency will be 4 to 5% at full load in favour of the HE motor which will also have a much longer service life. It will be found more cost effective in most cases to choose a high-efficiency unit. The rewinding of HE Motors has been studied with the objective of defining rewinding techniques which will limit the reduction in efficiency to 0.5% so that the advantage of the HE motor can be preserved after rewinding.

Whenever a motor is to be newly installed or replaced it should be standard practice to examine the cost benefits of selecting a high-efficiency type. The cost of running the plant can be estimated for both types of motors. If the equipment is going to be running for a significant proportion of each day then it is very likely that it is worth paying a premium for a high-efficiency design. There is a need for a management policy commitment towards potential cost savings at the design and specification stages.

• Copper Development Association
• Tel: 01707 650711
• Fax: 01707 642769

November 1996