Pole-face losses due to open slots

Abstract

The measurement of pole-face losses in laminated and solid materials has been made by a novel method employing a homopolar machine in which the ordinary core losses do not occur. The ?retardation method? has been employed to measure the losses, using a stroboscope to measure the speeds and a tuning-fork-controlled siphon recorder to measure the time-intervals. Using this method, the hysteresis loss can be separated from the eddy-current loss with considerable accuracy since the observations can be continued to very low speeds. The investigation covers a large range of variables, and empirical formulae have been developed for the hysteresis and eddy-current losses separately. It has been found that the hysteresis loss is independent of the thickness of the laminations, and is the same in both solid and laminated poles, while the eddy-current loss is proportional to the first power of the thickness. The results show that the slot width and air gap should be treated as separate variables. The exponent of B (the average magnetic induction in the air gap) has been found to vary considerably with the air gap and the slot pitch; the loss formulae therefore contain a variable exponent for B. Since the pole-face loss is, apart from a very small friction loss, the only loss involved, the accuracy of measurement should be of a high order. The results are based on a large number of observations, but only a few of the curves obtained are included in the paper.