Stator flux distributions in Lorenz-type medium-frequency inductor alternators

This theoretical and experimental analysis of the magnitudes and distribution of stator-flux harmonic components in a Lorenz-type inductor alternator is part of a broad investigation into this class of machine. The test alternator is a specially designed 30kW model of the standard 300kW industrial unit. Current designs employ field damping to reduce second-harmonic flux modulations expected by analogy with the Guy-type inductor alternator on load. The paper shows that no modulation of the field flux could exist in an ideal Lorenz-type alternator. However, the practical machine, with a.c.-slot openings interrupting the stator surface between field slots, will have harmonic components of flux, both on open circuit and on load, whose magnitudes depend on the width of the a.c.-slot opening. Damping is shown to be sucessful in reducing second- (and other even-) harmonic components, whilst being totally ineffective against fundamental variations in field flux and odd-order-harmonic fluxes. A theory is given that explains these effects. Measurements on the experimental model verify this theory. The paper introduces a technique for relating fluxes to the geometry of the airgap surfaces, and has shown the distribution for inductor alternators to be somewhat more complex than was presumed. It is expected that this analysis will solve the problem of accurate loss calculation in these machines.