The magnetic field of the end-windings of turbo-generators

摘要

An investigation of the magnetic field generated by the currents in the stator end-windings of a 3-phase 2-pole turbo-generator is described. It includes an examination of the influence of winding design, namely the effects of cone angle, straight projection and coil pitch, of the influence of the air-gap, and also of the permeability, or eddy-current screening, of the core end-plate. The simple and accurate method which is used to calculate the field of the stator currents, and which can be used for a circuit of any shape, is described. The effect of the air-gap is represented by a fictitious conductor, and the influence of the end-plate is accounted for by use of the method of images. Results are presented for the magnitude and phase of the fundamental and harmonic waves of the rotating fields on the end-plate and rotor-coil retaining-ring surfaces. From the results the following points appear: {a) The predominant loss-producing component of the field at the end-plate surface arises from the fringing at the end of the air-gap. (b) In order to reduce loss-producing components of the field at the end-plate due to the winding the cone angle should be made small and the straight projection large. (c) The use of a non-magnetic end-plate reduces the axial component of the field. (d) The use of eddy-current screening, whilst eliminating the axial components, increases the radial components. {e) Near the winding and the air-gap, the field contains a high proportion of harmonic components which induce loss in the rotorcoil retaining ring. They increase in magnitude with increases in the straight projection of the winding and the air-gap magnetomotive force. (f) Changes in coil pitch do not significantly influence either the fundamental or the harmonics of the end field.