THEORETICAL AND EXPiRlMENTAL INVESTIGATIONS ON FIELD HARMONICS OF INDUCTION MOTORS
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The airgap harmonics are generated due to windings and variations in the airgap. Stator-rotor slotting, effects of saturation and airgap eccentricity are identified as major sources of airgap asymmetry. In this thesis, the airgap MMF and permeance functions are expressed as cosine waves. Using a step by step procedure, the airgap field harmonic components are derived. The analysis is based on the permeance method. The method delivers detailed information about the harmonic frequencies and corresponding pole pairs. In order to verify the validity of the analysis, extensive experimental investigations were carried out on a medium size (10-hp) cage induction motor. A test-rig was designed and fabricated in such a manner that the airgap eccentricity can be set to the required value. The signals derived from the search-coils located in the narrow stator slot openings were analyzed with the help of a spectrum analyzer and oscilloscope. The harmonic frequencies were studied at different operating conditions. Further, the analysis provides information about the dominant airgap harmonics, the effect of field frequencies and the effect of saturation on the field waveforms. The thesis also presents an experimental analysis on the nature of airgap harmonic fields in the presence of stationary eccentricity. Of particular interest is the phenomenon of 'axial-shaft-flux', which is produced in the presence of an eccentricity. A brief description is given about the shaft-flux.