A Novel Technique for Refurbished Induction Motors’ Efficiency Estimation Based on No-Load Tests

摘要

Induction motors are the most widely used in the industry. They use two-thirds of the total energy utilized by industry in the industrialized countries, and hence, contribute to the global environmental problem which is represented by the emission of greenhouse gases. Several Canadian and U.S. utilities took serious steps in implementing demand side management programs to reduce both greenhouse gas effects and the cost of power that feeds this tremendous population of electric motors.The precise estimation of efficiency of refurbished motors or any existing motor is crucial in industries for energy savings, auditing and management. Full-load and partial load efficiency can be determined by using the dynamometer procedure which is expensive and available only in well-equipped laboratories. An inexpensive and easily applied procedure for efficiency estimation is of importance in the field. This research presents a proposed method for estimating refurbished induction motors’ full-load efficiency that could be applied successfully in electric motor service centers. The method requires only a DC test (including stator winding cold temperature), nameplate details, and RMS readings of no-load input power, voltage, and current.This research aims to develop an algorithm for induction motor’s efficiency estimation that can be applied easily in any electric motor service center. To do so, the research team decided to make technical visits to a few of those centers in North America to determine the facilities available in such workshops and to design an algorithm that matches those workshops’ technical environment.The proposed efficiency estimation algorithm starts with the DC test to obtain the cold DC resistance of the stator winding. The temperature of the cold stator winding should be measured and recorded as well. The stator winding DC resistance and the corresponding temperature are essential values to be used in the algorithm.The next stage will be reading and recording the nameplate data.Then, the machine should be run with no load coupled to the shaft and the following RMS values of the input voltage, input current, and input power should be read and recorded.The above mentioned values will be fed to a user-friendly software based on a spreadsheet which is designed to handle the necessary calculations and provide values for the estimated efficiency with illustrative graphs. Visual Basic programming was used in designing the software.An important assumption was implemented within the calculation. The assumption is that mechanical rotational losses which are assumed to have the same value during both no-load and the full-load conditions [4]. The assumption is made because there is no way to separate core losses with friction and windage losses as the proposed technique works with only one no-load operating voltage.Stray load loss and full-load temperature are assumed based on IEEE Std. 112™-2004 [5] and IEC 60034-2-1 [6] formulas. Seven induction motors of different power ratings (3-150 hp) were tested for efficiency estimation using the proposed technique. By comparing the estimated efficiencies with the measured values of the seven motors a maximum deviation of 1.679% and minimum deviation of 0.361% were noticed. The problem of having 1.679% deviation from the measured value was investigated and it was found that the two assumed values of stray load loss and full-load temperature have a large impact on the accuracy of the technique.The measured values of the stray load loss and full-load temperature were used instead of the assumed ones and a big difference in the estimated efficiencies accuracy occurred. By using the measured values, the maximum deviation became 0.49% and the minimum deviation was 0.09% for the seven motors under test.Hydro-Québec which is one of the largest power companies in North America offered to contribute to the research by providing an extremely valuable database of a large number of induction machines with power rating range 1-500 hp [7]. Another set of data is received from the Canadian BC Hydro [8]. The data has been implemented in the software. Experimental and field tests of 192 induction motors showed very promising results.The software is under assessment by several power companies in Canada.