Axial flux distribution in laminated steel cores, with particular reference to the end region magnetic field of large turbo-generators

Abstract

This thesis describes a new approach to the consideration of leakage fields in the end regions of large rotating electrical machines. In particular the approach is applied to the axial field developed at the ends of stator cores both inside and outside their laminated steel structure. The method considers individually the magnetic sources of axial flux. The sources involved are the winding currents, surface polarities of the magnetised core and the eddy currents and volume polarities within the core. Mathematical expressions are derived for the sources and their geometrical behaviour is then investigated as the design and dimensions of both windings and core are altered. Measurements are made of axial flux inside and outside cylindrical, slotless, laminated cores excited with a rotating field. The experiments are scaled to show the effects on axial flux in a fullsize machine of winding design, core geometry, core s:..:iration and eddy currents. The measurements are in good agreement rn th predictions made in air and on the core end using the source expressions. Axial flux is reduced as winding overhang and radius are reduced. The penetration of axial flux inside the core is shown to be independent of winding design but for a given frequency that penetration increases as the core is saturated. The behaviour of eddy currents in the core is shown to be controlled by the homogenised axial permeability of the core.

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