The Theory of Commutation

Abstract

IN this pamphlet Mr. Hawkins enters in to a complete mathematical investigation of the reactions occurring during the process of commutation in continuous current dynamos. The author first examines the case in which the contact resistance of the brushes is neglected, and then proceeds to give a complete solution of the equation for the current in the short-circuited coil, taking into account this resistance. This solution is due to Prof. Arnold and Dr. G. Mie, but our thanks are due to Mr. Hawkins for introducing it into England and for pointing out its practical bearings. Mr. Hawkins shows that the contact resistance is of the greatest importance in preventing sparking; the employment of carbon as the material of the brushes is consequently desirable; since the contact resistance of, carbon is about fifteen times that of copper. Allowing for the fact that the surface needed to collect the same current must be about five times as great with carbon as with copper, the carbon brushes are still, approximately, three times as good as copper. The author also points out the other considerations affecting sparking, and goes fully into the question how it may be best avoided, both in dynamos and motors. The mathematical investigation is made clearer by the application of the results to a practical case, and by a careful explanation of the physical interpretation of the equations.