30 n MOTORS May 2023 www.drivesncontrols.com Can repaired motors be more efficient? A study carried out jointly by the Association of Electrical & Mechanical Trades (AEMT) and the American Association, the Electrical Apparatus Service Association (EASA) found that motor efficiency can widely be maintained when a repair is carried out to a defined set of standards. The 2019 study (published in a paper called The Effect of Repair/Rewinding on Motor Efficiency) established that efficiency was maintained on repairs to current machines up to IE3 efficiency. This guide has been incorporated into the latest international repair standard IEC 60034:23:2019 and the American ANSI/EASA standard, AR100. There are, however, some circumstances under which the efficiency of a motor can actually be improved by refurbishment and rewinding. This generally applies to older, less efficient motors and the decision to repair rather than replace typically involves a broader range of factors than simply improving efficiency. Generally, the most effective way to improve a motor’s efficiency is to add more copper to its coils or, more specifically, to increase the copper crosssectional area. This is achieved by manufacturing coils to very tight tolerances and using CAD systems to optimise the coil design. This has several potential benefits, such as reducing the coil’s resistance, increasing its output, and reducing the operating temperature of the machine, which can increase its life. It is, however, important to understand that adding more copper will affect other characteristics of a motor which may require its wider system to be adapted. Adding more copper into a machine will affect not only its efficiency, but also other parameters. For example, you should expect an increased inrush current – which may conflict with existing protective measures. The ability to increase the amount of copper in a motor has been supported by advances in insulation materials which means that less insulation is needed, making more space available in the motor’s slots. The thickness of insulation needed for different voltage systems has decreased significantly over time, and modern insulation systems can achieve 15% reductions in thickness. Because this is on the outside of the coil, the effect on the copper crosssectional area can be greater still. Furthermore, these thinner insulations help with heat dissipation and can again lead to both longer lives and improved coil efficiencies. It is worth noting, however, that care is needed when comparing a motor’s efficiency levels before and after repairs. Efficiency should not be compared simply by looking at a motor’s datasheets. Before a 2007 update of the IEC / EN 60034-2-1 standard, which defines methods of determining rotating machine efficiencies, efficiency figures were more generous due to how certain effects could be calculated out. Efficiency levels could previously be calculated with smaller additional losses than today – a standard 0.5% of absorbed power, regardless of the motor output. Since the 2007 update, the methods have become much stricter. Now stray load losses have to be determined by a factor that reflects the motor’s output, ranging from 0.5% (≤1MW) to 2.5% (≥10MW). To avoid potentially misleading comparisons calculated from individual losses, efficiencies should be measured directly at nominal load. The decision whether to repair or replace a motor involves several factors. These can include cost, the availability of a suitable replacement, system compatibility and, of course, efficiency. So, while raised efficiency may be possible in certain circumstances, it is not often the only reason for repairing a motor. If you are looking to improve efficiency when refurbishing a motor, it may be possible, but you need to take care when calculating the actual benefits you are likely to gain. n Significant savings can be made by upgrading old motors to more efficient modern alternatives. However, simply swapping an old motor for a newer one is not always possible. Thomas Marks, general manager and secretary of the AEMT (Association for Electrical and Mechanical Trades), looks at how motor efficiency can be improved as part of a repair, and some of the factors you need to consider. It is sometimes possible to improve the efficiency of motors during repairs, but this usually applies to older, less efficient machines
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