An instrument for measuring commutation

“BLACK BANDS” have been used for many years JD to compare the commutating ability of d-c machines and to set the brushes in the proper place. A black band, as used here, is defined as the number of amperes by which the current through the commutating winding can be changed without producing sparking at the brush faces. This band width usually is taken at 25, 50, 75, 100, 125, and 150 per cent of rated load. This is plotted as shown in Figure 1, the black band being the shaded portion.