Influence of aluminum die-cast rotor porosity on the efficiency of induction machines

A number of induction motor manufacturers are replacing fabricated copper rotors with the aluminum die-cast rotor design since they can be produced at lower cost and allow flexibility in the design with no restrictions in rotor bar shape. Despite the advantages, porosity is inevitably introduced during the die-casting process, and causes degradation in the starting and operating performance. The porosity level and distribution varies from rotor to rotor, and can cause motors of identical design to exhibit different characteristics. This makes it difficult for motor designers to predict the performance of motors accurately to guarantee that they meet the minimum efficiency specified in international standards; however, the influence of porosity has not been properly studied in the literature. In this paper, a method based on a combined 2-D and 3-D finite-element analysis that takes the porosity level and distribution into account is proposed for accurate prediction of motor performance. Experimental test results on a 440 V, 15 kW induction motor prototype with rotors that have 93% and 67% aluminum fill factor show that the proposed method provides reliable prediction of motor efficiency for rotors with porosity.