The Loss in Solid Poles of Synchronous Machines On Load

Abstract

The loss in solid poles of synchronous machines on load may be caused by several travelling waves and the thesis is directed towards a discussion of the problems arising from the superposition of these waves and of the effect of the superposed main field, which is stationary relative to the poles. The major part of the work is concerned with the effect of a superposed parallel steady field on the permeability and losses for an alternating field in mild steel. Experiments show that, for a given surface alternating field strength, the steady field decreases the loss, which may be accurately calculated by a linear theory if the steady field strength is greater than the alternating field strength. Otherwise a non-linear theory which allows for unsymmetrical hysteresis loops is required for strong alternating fields, and a limiting non-linear theory is developed, but not tested. A consideration of the usual linear theory for the loss induced by a travelling wave shows that it may be written in terms of an eddy-current reaction parameter leading to simple normalised curves, and that the tangential field strength of significant travelling waves is likely to be fairly strong in typical large machines. The superposition of waves which induce different frequencies in the rotor may therefore cause an oscillation between linear and non-linear behaviour, with variations around the rotor periphery caused by interference between pairs of winding harmonics and by the variation of the steady main field. It is emphasised, however, that the main field is perpendicular to the travelling fields in a smooth rotor, and it is believed that its effect will only be significant in regions where it is very strong, where it will both reduce the permeability and makee the behaviour more linear.