Investigation of high-frequency no-load losses in induction motors with open stator slots

High-frequency components form a very important part of the no-load losses of induction motors having open stator slots. The paper presents the results of an experimental investigation which was designed to assess the effects on the losses of a variety of rotor design features. Twelve experimental rotors included three squirrel-cage types having, respectively, unskewed slots, skewed slots with uninsulated bars and skewed slots with insulated bars. A 3-phase wound rotor was tested for comparison purposes. The main conclusion is that, in rotors having skewed insulated bars or phase windings, the high-frequency core losses can be as large as the high-frequency I2R losses in unskewed rotors. The effect of variations in the bar-to-iron contact resistance Rq is large. The nature of this resistance was investigated separately, and it is shown that it is by no means a constant parameter. High-frequency stator core losses were also investigated, using search coils, and these are shown to be of importance only with unskewed cage rotors, and also possibly with phase-wound rotors with wide slot openings. The experimental results are compared with theoretical calculations, including the effect of bar-to-iron resistance.