Influence of Die-Cast Rotor Fill Factor on the Starting Performance of Induction Machines

One of the most important requirements for low-voltage induction motor designers is to meet the customer-specified starting characteristics and international regulations on efficiency. Although porosity in die-cast rotors degrade motor performance, the fill factor (FF) of the rotor bar and end ring are usually assumed to be 100 in the design stage. If the influence of FF is not properly taken into account in the design, it results in an inaccurate prediction of the starting performance. In this paper, the influence of die-cast rotor FF on the starting torque is investigated based on the FF and porosity distributions measured from actual aluminum die-cast rotors. A simplified 3-D finite-element analysis model that accounts for the porosity distribution in bars and end rings for different FFs is presented for predicting the starting characteristics. The effectiveness of the proposed method is verified on a 440 V, 15 kW induction motor prototype with rotors with FF of 93% and 67%. The results show that porosity has a significant influence on the starting torque, and must be taken into account for reliable prediction of motor performance for design optimization.