DIRECT-CURRENT DYNAMOS AND MOTORS. 117 
abs 3000 e 3000 
Neg ia 3000 +(25° X.32)-+ (1 X125) 3000+ 200-+125 
_ 3000 
= 3905 = .902, or 90.22. 
(3) A compound-wound 50 K. W. generator gives a ter- 
minal FE. M. F. of 500 volts. Its armature resist- 
ance (warm) is .12 ohm; its series field resistance is 
-04 ohm; and its shunt resistance is 2'%O0 ohms. 
Compute its electrical efficvency. 
50,000 
500 
Solution.—The current output is = 100 amperes; 
the shunt current is ees = 1.85amperes. Hence by (60): 
270 
50,000 
Hl, Eff. = 50,000 + [101.85* x (.12 + .04)] + (1.85? x 270) 
50,000 __ 50,000 000 
= 50,000 + 1660 | 925 ~ 52,585 "907 95%. 
(4) Find the electrical efficiency of a series motor which 
takes a current of 100 amperes when supplied at 
220 volts, if its armature has a resistance of .11 
ohm, and tts field a resistance of .015 ohm. 
Solution.—By (61): 
(220 x 100) —[100?x( 11+.015)] __ 22,000—1250 
; 220 X 100 St 22,000 
a 750 _ 
(5) What is the electrical efficiency of the machine con- 
sidered under (2), if it is runas a motor from a 125- 
volt circuit, being supplied with 26 amperes? 
Solution.—By (62): 
Bl. Beg, = (125X26)=[(20"x..82)+ ("x 125)] _ 3260— 325 
125 X 26 , 8250 
= .90, or 90%. 
El. Eff. = 
__ 2925 
~ 3250 
