DETERMINATION OF RADIAL ELECTROMAGNETIC FORCES IN SQUIRREL-CAGE INDUCTION MOTORS
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The aim of this thesis is to present an unified qualitative and quantitative analysis for the determination of the radial electromagnetic forces in squirrel-cage induction machines. These electromagnetic forces are generated at the surfaces of the stator and rotor along the air-gap periphery. The physical mechanisms and the characteristics of the audio-frequency vibrations induced by the electromagnetic forces are discussed. Accurate determination of the electromagnetic excitation forces and the vibrational behaviour of the machine are essential to achieve quieter operation of electrical machines. An analysis of a genteral nature is developed for the determination of the elctromagnetic excitation forces in squirre-cage induction machines. The air-gap of most squirrel-cage induction machines contains an assortment of various harmonic fields. They are generated due to the distriabution of the windings in slots around the machine, slotting of the sator and rotor surfaces, and magnetic saturation of the iron. The air-gap fields are determined using the permeance method by expressing the MMF and the air-gap permeance as waves. In this analysis modifications are made to the permeance method to incorporate the effects of the mutual interactions between sator and rotor fields. As the speed of the rotor decreases with load, these interactions become prominent and determine the nature of the air-gap field. Their influences on the electromagnetic forces are discussed from the perspective of the production of acoustic noise. Relevant experimental investigations are conducted on a 10 hp squirrel-cage induction motor to verify the validity of the analysis. The magnetic fields are measured and analyzed with the use of search-coils located in the sator slot-openings. Vibration measurements are made on the machine to derive information on the electromagnetic forces. Conclusions are drawn with a view to determine the actual role played by the harmonic air-gap fields on the production of the electromagnetic forces. This work was supported by the National Sciences and Engineering Research Council of Canada through grant no. OGP 0004324.