Up Time Magazine- April/May 2008 - (Page 46) motor testing upload Anatomy of an Efficient Motor Repair Ensuring Efficiency and Reliability in Rewound Motors by Howard W. Penrose, PhD A motor repair customer must work closely with a motor repair center to ensure that the equipment sent out for rewind repair is handled in a manner that does not reduce efficiency or reliability. This two part article is an updated version of a 1997 paper based upon the author’s work on energy efficient electric motor repair with both the US Department of Energy and Canadian Electrical Association. At the time, the author was the Director of Field Service and R&D for Dreisilker Electric Motors, Inc. in the early to mid-1990’s and the Chair of the IEEE Chicago Section Chapters of Dielectrics and Electrical Insulation Society and Power Electronics Society as well as the Region 4 representative to the IEEE USAB Energy Committee. It was found that the average loss of efficiency was in the area of 1.1%, with the greatest reduction around 3.4%. The increase in core losses averaged 2.2%, with a maximum of 46%. Although the numbers may not appear to be large, when considered in an operating cost formula (Equation 1), they become significant. This may be termed as the post-repair cost of an electric motor. Cost = Introduction Until the mid-1990s, the subject of AC induction motor rewind had been carefully skirted. However, with the advent of the Energy Policy Act of 1992 (EPAct ‘92), the subject was thrust forward. By now, repair versus replace decisions have become business as usual within maintenance organizations, including the consideration of energy efficiency and the impact through electric motor rewind. In several studies, most notably one conducted by the Canadian Electrical Association (CEA) at the LTEE Hydro Quebec Laboratories, the effects of electric motor rewind were examined. Previous studies, completed by BC Hydro and Ontario Hydro, produced interesting results as well. It is important to note that in all of these Canadian studies, the standard used for efficiency testing was the CSA 390, which is similar to the IEEE Std. 112-1991 Test Method B, identified as the method for efficiency testing by EPAct ’92. In this article, we will first discuss the results, then review methods for properly rewinding and testing failed electric motors to retain original motor efficiency. The motors that will be focused on are those motors outlined as energy efficient in accordance with NEMA MG-1 Table 12-10, Design A and B, horizontal foot-mounted, ball-bearing, 230/460 VAC electric motors, as outlined by EPAct ’92. 0.746kW/HP * HP * L * $ * Hr * (100/Ef – 100/Ei) Where: HP = Horsepower L = Load $ = cost per kWhr Hr = hours of operation per year Ef = final motor efficiency after repair Ei = initial motor efficiency Equation 1 - Operating Cost BC Hydro Rewind Study The BC Hydro Rewind Study was published in April 1993. The significant difference between the two studies was that, while the Ontario Hydro study used standard efficient motors, the BC Hydro study used the ‘new’ energy efficient motors. In the same way as the first study, eleven 20 horsepower electric motors were used, with ten being failed and sent for repair. They were returned for analysis. Unlike the Ontario Hydro study, the average decrease in efficiency was 0.5%, with the most significant reduction being due to friction and windage. As it turns out, this was due to improper bearing replacement and not stator core and I2R stator winding losses, which differed from what was predicted. Ontario Hydro Rewind Study This study was published in November 1991, and consisted of an experiment in which nine of ten identical 20-horsepower, standard efficiency electric motors were rewound. The nine motors were identically failed and sent, blind, to nine separate electric motor repair facilities throughout Canada. When returned, they were analyzed for efficiency impacts. Hydro Quebec Rewind Study Of the three studies, the CEA commissioned the Hydro april/may 2008 46
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.