The “black band” method of commutation observation

Good commutation is essential for the reliable performance of d-c machines with low maintenance costs. As an aid to the commutation adjustment of machines in the factory, or in the field, a systematic test procedure has been found useful on large machines to indicate certain features of the machine's adjustment, notably the commutating field strength and brush position. This information is obtained directly from curves known as “black bands” or “buck and boost curves” plotted from test observations. The significance and interpretation of these curves in terms of machine adjustments, differential between commutation-reactance voltage and voltage induced by interpole flux are presented and results of observations on machines for special service (variable-speed motors and variable-voltage generators) are also given in this paper. Included are typically shaped “black bands,” obtained in practice. The black-band method has proved to be an extremely powerful and useful tool in commutation adjustment and it is hoped that this discussion of it will lead to an increasing use of the method with a corresponding furtherance of knowledge about it, thus enlarging its scope of usefulness.