B. MOTORS Shunt Motor 74. Three speed characteristics of the shunt motor at con- stant impressed e.m.f. e are shown in Fig. 116 as A, P, Q, corre- sponding to the points d, p, q of the motor load saturation curve, Fig. 110. Their derivation is as follows: At constant impressed ir,0 1100 WOO £00 (•00 w 50 100 150 200250 300 350 400 450 500 FIG. 119. — Shunt motor speed curves, constant impressed e.m.f. e.m.f. e the field excitation is constant and equals FQ, and at current i the generated e.m.f. must be e — ir. The resultant field excitation is F0 — iqt and corresponding hereto at constant speed the generated e.m.f. taken from saturation curve A in Fig. 110 is e\. Since it must be e — ir, the speed is changed in , . e - ir the proportion • At a certain voltage the speed is very nearly constant, the demagnetizing effect of armature reaction counteracting the effect of armature resistance. At higher voltage the speed falls, at lower voltage it rises with increasing current. In Fig. 120 is shown the speed characteristic of the shunt 216 ELEMENTS OF ELECTRICAL ENGINEERING motor as function of the impressed voltage at constant output, that is, constant product, current times generated e.m.f. If i = current and P = constant output, the generated e.m.f. p must be approximately e\ = — , and thus the terminal voltage e = e\ + ir. Proportional hereto is the field excitation FQ. The resultant m.m.f. of the field is thus F = FQ — iq, and corre- sponding thereto from curve A in Fig. Ill is derived the e.m.f. eQ which would be generated at constant speed by the m.m.f. F. Since, however, the generated e.m.f. must be e\} the speed is changed in the proportion — • 00 REV. PER 2000 1800 1600 1400 1200 1000 800 COO [400 200 / * / / / s / / / 7 nt / / 1 / / 1 / ^r S vor -s* 20 10 60 80 100 120 110 160 ISO 200 FIG. 120. — Shunt motor speed curve, variable impressed e.m.f. The speed rises with increasing and falls with decreasing im- pressed e.m.f. Still further decreasing the impressed e.m.f., the speed reaches a minimum and then increases again, but the conditions become unstable. Series Motor 75. The speed characteristic of the series motor is shown in Fig. 121 at constant impressed e.m.f. e. A is the saturation curve of the series machine, with the current as abscissas and at constant speed. At current i, the generated e.m.f. must be /? A f e — ir, and the speed is thus — '- times that, for which curve A Q\ is plotted, where e\ = e.m.f. taken from saturation curve A.