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This paper is as to DC machines which have been widely applied to steel mill industries and paper mill industries, due to its advantages for adjustable speed drive. As is well known, the DC machines have its long history. However theme of ldquoCommutationrdquo for DC Machines is still ldquoNEWrdquo. It is a brush exit spark which determines commutation reliability, or which cause commutation failure. In this paper, commutation circuit analysis and the experimental research including spark measurement are described, where equivalent commutation circuit is used, which is usually applied to commutation spark study or brush grade selection. As a result of the experiments, measured results are agreed well with the analysis results.