Heat Losses in D-C. Armature Conductors

Abstract

The pioneer paper on this subject of copper losses in armature conductors was presented by A. B. Field.⁴ Since then Field's fundamental premises have been adopted by practically all who have contributed to this subject, both in this country and abroad, although the method of presentation has been modified as the individual thought best. Thus one of the present authors has favored a vector method of analysis that seems to him to have marked advantages. In this paper the vector method is extended to the case of the d-c. machine. Without doubt, it is generally believed that there are extra losses in the armature coil of a d-c. machine when it is undergoing commutation, but that during the major portion of the time, since the current is steady, the loss in the coil is correctly computed by squaring the value of the current and multiplying by the true or ohmic resistance of the coil. This view, however, is not correct. The disturbance in the coil current produced by the process of commutation persists throughout the cycle so that at no time is the current uniformly distributed over the cross-section of the conductor. Although the disturbance and resulting extra loss factor are greatest during commutation, the extra losses are present at all times, even while the current in the coil is steady. The procedure for finding the extra losses is simple. First, the time variation of the coil current must be known and analyzed for its harmonic components.