Load Losses in Synchronous Machines:a study of the pole face loss caused by armature reaction M.M.F. harmonics

Abstract

The investigation divides into two main parts, the first relating to the power loss in the pole face of synchronous machines caused by the rotating space harmonics of armature reaction, the second to the surface distribution across the pole face. The harmonic loss, important in modern machines with high specific current loadings, is calculated from established eddy-current coupling theory derived analytically from the diffusion equation. The theory accommodates the demagnetising effects of the surface currents and has been extended in generalised form by the author. The theory contains an analytical substitution for permeability and leads to simple formulae to predict the effect of changes in machine parameters. A comprehensive series of examples, using a digital computer, emphasises the importance of the slot openings, the gap to wavelength ratio, and the pole profile. Tests on a range of production machines and an experimental model support the proposed formulae. The model machine, with a synchronously rotating laminated primary carrying a three-phase winding and a solid unwound secondary supported on a dynamometer frame, is designed to accentuate the harmonic losses. Difficulties experienced in separating the losses therein include: the accurate assessment of primary iron loss and secondary slot ripple loss. The primary iron loss is measured by considering the distribution of power supplied from mechanical and electrical sources. The slot ripple loss is calculated using constants derived from conformal-transformation theory together with established formulae. The second part of this investigation, the distribution of surface effects, is limited to the first pair of harmonic terms. It stresses the fundamental nature of the fluctuation at any point on the surface and the factors controlling its magnitude. Experimental evidence lends support to the author’s mathematical treatment of this periodic non-sinusoidal fluctuation superposed on a polarising wave.