It is shown, both theoretically and experimentally, that the reaction of eddy currents on the inducing tooth-ripple field, previously considered to be negligible, can be very large. This has an important effect on the subsequent calculation of loss in both turbogenerators and salient-pole machines. A comprehensive set of results, giving the pole-face field for the possible ranges of design values, is presented. These show that the two main parameters are the permeability of the pole material and the length of the air gap. The analysis on which the computation of results is based is quite general, and previous analyses of the tooth-ripple and related problems, apparently dissimilar, are correlated in the light of it.

Tooth-ripple losses in solid poles

The various losses produced by flux oscillations in the unwound pole-face are considered for three main types of shoe, namely the solid ungrooved, the solid grooved and the laminated. The treatment is theoretical and includes in particular an examination of the evaluation of the amplitude of flux oscillation, the quantitative effect of harmonics, and the method of dealing with the permeability, which is a variable quantity. The losses considered are those due to eddy currents, to hysteresis and to pulsations; it is found that in normal designs only eddy-current loss has importance. The resulting equations are applied to independently published experimental results, and the calculation of a typical alternator is included showing the effect of grooving both by theory and by test. These applications confirm the general validity of the equations.

Tooth-ripple losses in unwound pole-shoes

Classical linear electromagnetic theory is used to investigate the mode of penetration of tooth-ripple flux into smooth laminated poleshoes, without making any assumption as to the thickness of the laminations employed. It is shown that, apart from the mode corresponding to the classical ‘depth of penetration’ concept, there also exists a completely different mode which penetrates to a much larger radial depth but has a non-uniform distribution across the thickness of the laminations, the maximum values occurring at the interfaces. With thicknesses usually employed in practice, at least 60% of toothripple eddy-current loss is found to arise from this latter mode of penetration, so that only a fraction of the total loss could properly be called a ‘pole-face’ loss.

An Analysis of Tooth-Ripple Phenomena in Smooth Laminated Pole-Shoes

Eddy current is a very important and intriguing phenomenon. A direct result of Lenz's law, it has been an integral part of research on electrical machines. The applications and associated research field of eddy currents are very widespread and virtually impossible to include in a singlestudy. Therefore this study tends to capture research trends in eddy-current loss analysis pertaining to the field of rotating electrical machines only. The research period taken up is from 1892 to 2010. The trends in research and application have been brought out. This study aims to serve as a reference guide to all those interested in the field of eddy-current loss analysis in electrical machines.

Literature survey of eddy-current loss analysis in rotating electrical machines

A loading-back test has been used for the determination of the stray load losss in polyphase induction machines. This test, which was found to have a high degree of accuracy, enables the various losses to be separated. The application of the test is simple and convenient.

Stray Load Loss Test on Induction Machines

The object of the experiments recounted below was to establish a reliable and if possible a rational quantitative relation between the pole-face losses and the principal variables involved therein. The work was originally undertaken as a supposedly small part of a larger investigation without any realizing sense of the amount of work involved or of the difficulty of obtaining reliable results. Another time the method employed would be modified in the light of this experience, but although the results are not altogether satisfactory from the standpoint of the investigator, they are sufficiently accurate for most practical purposes, and it is believed more reliable than those heretofore published.

A. C. Lanier

Papers

Pole-Face Losses