Uncertainty Issues in Direct and Indirect Efficiency Determination for Three-Phase Induction Motors: Remarks About the IEC 60034-2-1 Standard

Electric motors and the systems they drive are the single largest electrical end-use; it is estimated that they consume between 43% and 46% of all global electricity consumption. Recent studies show that efficiency values of induction motors can be successfully increased, with improvements up to 20%-30%. In 2009, the European Commission published a new regulation (640/2009) concerning requirements for the eco-compatible design of electric motors. In such a scenario, the experimental determination of induction motor efficiency is getting more and more important, because of the need of placing these motors in the right energy efficiency levels defined by international regulations. The correct motor classification strictly depends on the uncertainty associated with the efficiency determination. According to the IEC 60034-2-1 standard, the efficiency of three-phase induction motors can be determined by applying the direct or indirect efficiency technique. The direct procedure is suggested for low uncertainty efficiency measurements of single-phase induction motors and three-phase motors with rated power ≤1 kW. In this paper, we propose a comparative analysis between direct and indirect efficiency determination for three-phase induction motors, according to the IEC 60034-2-1 standard. The contribution of this paper consists in the comparison between the uncertainty calculations in indirect and direct efficiency determination for induction motors, to investigate the advisability of adopting the direct efficiency determination as reference method, because it is easier to implement and it has a good accuracy. This task is carried out starting from the accuracy requirements for measurement equipment, as demanded by the Guide to the Expression of Uncertainty in Measurement GUM. Some experimental results obtained with the indirect and direct efficiency determinations on a 3-kW three-phase two-pole induction motor are presented and discussed. A final suggestion related to a future version of IEC 60034-2-1 is also reported.