D. C. COMMUTATING MACHINES 211 constant, curve B to varying armature reaction. It is seen that at a certain definite resistance the voltage becomes zero, and for lower resistance the machine cannot generate but loses its excitation. The variation of the terminal voltage of the shunt generator with the speed at constant field resistance is shown in Fig. 115, at no load as A, and at constant current i as B. These curves are derived from the preceding ones. They show that below a certain speed, which is much higher at load than at no load, the r 50 100 150 200 250 300 350 FIG. 113. — Shunt generator load characteristic. machine cannot generate, and cannot be realized. The lower part of curve B is unstable Series Generator 72. In the series generator the field excitation is proportional to the current i, and the saturation curve A in Fig. 116 can thus be plotted with the current i as abscissas. Subtracting ab = ir, the resistance drop, from the voltage, and adding bd = iq, the armature reaction, gives a load saturation curve or external characteristic B of the series generator. The terminal voltage is zero at no load or open circuit, increases with the load, reaches a maximum value at a certain current, and then decreases again and reaches zero at a certain maximum current, the current of short circuit. Curve B is plotted with constant coefficient of armature reac- tion q. Assuming the brushes to be shifted with the load and 212 ELEMENTS OF ELECTRICAL ENGINEERING 0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.1 1.6 1.8 2.0 FIG. 114. — Shunt generator resistance characteristic. / 1RO / 170 / / A 160 150 140 130 / / / / / ' / / / / / no / t / 100 / ' 90 80 70 60 50 40 30 A / 4 I / / / v / •*- — — • A / / SO ( 10 / SPEE ->• I 0.2 0.4 0.6 0.8 1.0 1.2 1.4 FIG. 115. — Shunt generator speed characteristic at constant field circuit resistance.