An investigation of iron losses due to rotating flux in three phase induction motor cores

Abstract

<p>In the past, the discrepancy between predicted and measured core loss values has led to the use of empirical scaling factors to improve correlation. Accurate prediction of core loss is important in the design of electrical machinery. Machine customers often demand guaranteed efficiency values at the quotation stage. If the guaranteed value is missed on delivery, they may impose penalties of as much as $5000 for every additional kW of loss. This thesis addresses one possible source of this discrepancy, that is, the failure to account for rotational iron losses. A significant portion of an induction motor stator is exposed to flux that rotates in the plane of the machine laminations. Iron losses in rotating magnetic fields differ from those obtained under alternating flux conditions and are not accurately estimated from alternating loss measurements. In recent years, a great deal of research has occurred for the explicit purpose of developing a standardized test for measuring rotational iron losses. Work in this field has been motivated by the opinion that rotational iron loss data will be used by machine designers to refine the accuracy of core loss predictions in rotating machines. The hypothesis of this dissertation is that if iron losses due to rotational flux in the stator are calculated rigorously, some portion of the discrepancy between tested and calculated values of no-load iron loss will be accounted for. To test this hypothesis, the iron losses due to the fundamental frequency variation of the flux density have been calculated in several ways. The stator losses were calculated using alternating loss data available from standard tests and by rigorously accounting for the losses caused by rotational flux. While some differences have been noted, they are too small to account for any major portion of the discrepancy between tested and calculated values of no-load core loss. The results of our investigation show the hypothesis to be false. If a standardized test apparatus for making rotational iron loss measurements were realized in the near future, it would benefit producers of electrical sheet and provide useful information to machine designers. However, as this investigation will show, the refinement to core loss calculation methodologies will not likely improve the correlation of tested to calculated values of core loss.</p>