Measurement and interpretation of power losses in electrical sheet steel

Abstract

The paper describes work that was undertaken to try to explain the anomalies that exist in the interpretation of the power-loss measurements made on electrical sheet steel. Wattmeter and bridge methods of making power-loss measurements are considered; the bridge method, which was judged to be the most suitable means of making the measurements recorded subsequently in the paper, is described, and it is shown that power-loss measurements made by this bridge circuit and a wattmeter agree to within 0.2%. A series of measurements taken by both Epstein and Lloyd-Fisher squares are presented, and the ways by which the results of these measurements can be varied are indicated; these observations lead to the conclusion that the Lloyd-Fisher/Epstein anomaly is part of a much wider problem.The flux waveforms that occur in the various parts of a stack of laminations are shown, and the many forms of distortion and flux flow between strips are considered; they are discounted as the main sources of the anomalies. The values of the power loss due to eddy-current formation, when electrical sheet steel is subjected to an alternating flux normal to its plane, are given, and these show that the performance of the steel in this respect is similar to that of any other metal. Considerations of this result lead to the view that the Lloyd-Fisher/Epstein anomaly and the anomaly factor may be due to a lateral flow of flux, and experimental results which support this view are presented. Finally it is pointed out that the Polivanov theory, and extensions of this theory, are inadequate in their present form because they do not enable an explanation to be given of the experimental results recorded in the paper.