Title:

A study of magnetic nonlinearity and finite length effects in solid iron subjected to a travelling MMF wave

This thesis describes an experimental and analytic study of the effects of magnetic nonlinearity and finite length on the loss and field distribution in solid iron due to a travelling mmf wave. In the first half of the thesis, a twodimensional solution is developed which accounts for the effects of both magnetic nonlinearity and eddycurrent reaction; this solution is extended, in the second half, to a threedimensional model. In the twodimensional solution, new equations for loss and flux/pole are given; these equations contain the primary excitation, the machine parameters and factors describing the shape of the normal BH curve. The solution applies to machines of any airgap length. The conditions for maximum loss are defined, and generalised torque/frequency curves are obtained. A relationship between the peripheral component of magnetic field on the surface of the iron and the primary excitation is given. The effects of magnetic nonlinearity and finite length are combined analytically by introducing an equivalent constant permeability into a linear threedimensional analysis. The equivalent constant permeability is defined from the nonlinear solution for the twodimensional magnetic field at the axial centre of the machine to avoid iterative solutions. In the linear threedimensional analysis, the primary excitation in the passive endregions of the machine is set equal to zero and the secondary end faces are developed onto the airgap surface. The analyses, and the assumptions on which they are based, were verified on an experimental machine which consists of a threephase rotor and alternative solid iron stators, one with copper end rings, and one without copper end rings j the main dimensions of the two stators are identical. Measurements of torque, flux /pole, surface current density and radial power flow were obtained for both stators over a range of frequencies and excitations. Comparison of the measurements on the two stators enabled the individual effects of finite length and saturation to be identified, and the definition of constant equivalent permeability to be verified. The penetration of the peripheral flux into the stator with copper end rings was measured and compared with theoretical penetration curves. Agreement between measured and theoretical results was generally good.
