Up Time Magazine- April/May 2008 - (Page 47) Quebec Rewind Study, which was compiled by Demand Side Research of Vancouver, BC. The result was the booklet, “Evaluation of Electric Motor Repair Procedures Guidebook (CEA 9205 U 984),” which outlined the findings. In this study, the coils of a number of energy efficient motors were removed by several different methods (burnout oven and mechanical stripping) and were rewound. This process was repeated three times per motor, with a CSA 390 test performed after each rewind. It was shown that no significant loss in efficiency was detected through all three rewinds (less than 0.2%). However, in order to achieve this, a number of quality control steps were required to be followed. while the higher-grade core steels are hardier and more able to withstand higher temperatures. In all, the most significant result is that, if an electric motor is economical to rewind versus replace, the electric motor shop must have the appropriate equipment and some type of recognized quality control plan in place. It should also be noted that the owner of the electric motor should have repair versus replace plans and repair specifications in place. In this author’s opinion, the EASA/ANSI AR100, “Recommended Practice for the Repair of Rotating Electrical Apparatus” (available as a download from http://www.easa.com) is an excellent standard for both electric motor repair owners and repair centers, to be followed by the new IEEE Std 1068, “IEEE Recommended Practice for the Repair and Rewinding of Motors for the Petroleum, Chemical and Processing Industries,” which is presently in development. Motor Losses There are several types of AC induction motor losses that effect efficiency. Following is a brief description of each: • Core Losses (15-25%): consist of eddy-current and hysteresis losses. Eddy currents are circulating currents found in ferromagnetic materials as magnetic fields are induced into them. They are reduced by using thin core materials insulated from each other. Hysteresis is the energy necessary to change the direction of the magnetic fields in the steel. This is reduced by creating a core material that is low in carbon, or silicone-based, magnetic grade steels. • Friction and Windage (5 – 15%): caused by air density, fans, turbulence within the stator, bearings, and anything else that may cause a friction force on the shaft. • Stator Losses (25 - 40%): caused by current flow through the stator winding (I2R). • Rotor Losses (15 - 25%): caused by heating in the rotor bars (windings) (I2R). • Stray Load Losses (10-20%): leakage losses and other losses not previously accounted for. These losses are important when considering the effects of electric motor rewind: Stator Core After Burnout Oven While initially supported by the Electrical Apparatus Service Association (EASA), the trade association for electric motor repair, the findings were not well publicized. Instead, a UK-based study, fully financially supported by both EASA and a European trade association, was performed immediately following the CEA project. The result of this study included the ability to increase winding removal temperatures on motors with certain core steels. Several processes, including mechanical stripping, were not included in the study. Summary of Studies By using ‘identical’ motors in the Ontario and BC Hydro studies, it should be apparent that one manufacturer was used for each, whereas several motor manufacturers were used for the Hydro Quebec study. This may have had some bearing on the results. One main difference between Standard Efficient Motors (SEM) and Energy Efficient Motors (EEM) is the core material. SEMs tend to use lower cost annealed core steels while EEMs use higher-grade silicone steels. The lower cost core steels are more susceptible to temperature and environmental conditions, www.uptimemagazine.com %Eff = ((Input-Losses)/Input) Equation 2 - Simple Efficiency Formula 47 http://www.easa.com http://www.alignmentsupplies.com http://www.uptimemagazine.com
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