Theoretical and experimental investigations on radial electromagnetic forces in relation to vibration problems of induction machines

Abstract

Accurate assessment of the electromagnetic excitation forces and the vibration behaviour of stators is essential to arrive at a suitable design for quiet operation of an electrical machine. These electromagnetic forces are produced on the surfaces of the stator and rotor along the air-gap periphery. The physical mechanisms underlying the production of the electromagnetic forces, and the characteristics of the ensuing vibrations are described in this thesis. In this thesis, comprehensive analytical methods are developed for the determination of the radial electromagnetic forces in squirrel-cage and wound-rotor induction machines. Various magnetic fields are generated due to the distribution of the windings in slots, the slotting of the stator and rotor surfaces, and the 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 the analyses, the mutual interactions between the stator and rotor are incorporated. Asthe load increases, these interactions become prominent and influence significantly the nature of the air-gap field. Their effects on the electromagnetic forces are discussed from the perspective of the production of vibrations. Extensive investigations were conducted on a 7.5 kW squirrel-cage induction motor and a 70 kW wound-rotor induction motor to verify the validity of the analyses. The comparisons made between the experimental and the analytical results prove the general validity of the analytical methods. 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 and the ensuing vibrations. In the course of these investigations, an experimental set-up which is particularly suited for the measurement of magnetic fields, magnetic forces, resonant frequencies, vibrations and noise was developed. A fundamental study on the vibration behaviour of electrical machine stators using an experimental modal-analysis is presented in the thesis. Modal-analysis is a process of forcing a structure to vibrate predominantly at a selected resonance. In order to achieve this, distributed electromagnetic forces are used.Detailed investigations were conducted on the stator models of a 120 hp induction motor to study their vibration behaviour, and to critically examine the damping present in them. The physical interpretations of the results given in the thesis would help in acquiring a better understanding of the vibration behaviour of stators in relation to the problem of electromagnetic acoustic noise in electrical machines. This work was supported by the National Sciences and Engineering Research Council of Canada through grant no. OGP0004324.