D. C. COMMUTATING MACHINES 213 proportionally to the load, gives curves C, D, and E, which are higher at light load, but fall off faster at high load. A still further shift of brushes near the maximum current value even overturns the curve as shown in F. Curves E and F correspond to a very great shift of brushes, and an armature demagnetizing effect of the same magnitude as the field excitation, as realized in arc-light machines, in which the last part of the curve is used to secure inherent regulation for constant current. The resistance characteristic, that is, the dependence of the current and of the terminal voltage of the series generator upon 6000 6000 1 23 i 5 6 7 8 9 10 11 12 13 U 15 16 17 18 19 FIG. 116. — Series generator saturation curve and load characteristic. the external resistance, is constructed from Fig. 116 and plotted in Fig. 117. BI and Bz in Fig. 117 are terminal volts and amperes corre- sponding to curve B in Fig. 116, #1, Ez, and F% volts and amperes corresponding to curves E and F in Fig. 116. Above a certain external resistance the series generator loses its excitation, while the shunt generator loses its excitation below a certain external resistance. Compound Generator 73. The saturation curve or magnetic characteristic A, and the load saturation curves D and G of the compound generator, are shown in Fig. 118 with the ampere-turns of the shunt field 214 ELEMENTS OF ELECTRICAL ENGINEERING as abscissas. A is the same curve as in Fig. 109, while D and G in Fig. 118 are the corresponding curves of Fig. 109 shifted to 7000 50001 8000 1000 \ RE ISTA.NCE ANCE \ 200 400 600 800 1000 1200 1400 1600 180 FIG. 117. — Series generator resistance characteristic. ;>OQOOMMS 400 3000 1000 5000 6000 7000 8000 9000 FIG. 118. — Compound generator saturation curve. the left by the distance iqQ, the m.m.f. of ampere-turns of the series field. At constant position of brushes the compound generator, when