116 DIRECT-CURRENT DYNAMOS AND MOTORS. 
= armature resistance, warm; 
Ys. = series field resistance, warm; 
rn" = resistance of shunt circuit, including field regu- 
lator. - 
The electrical efficiency of modern dynamos is very high, 
ranging from .8, or 80 per cent., for small machines, to 
as high as .995, or 994 per cent., for very large gene- 
rators. Since the electrical efficiency does not include 
waste by hysteresis, eddy currents, and friction, but is 
dependent upon the power losses due to heating by 
the current only, it may be adjusted to any desired value 
by properly proportioning the resistances of the machine. 
853. Examples of Calculating Electrical Efficiency. 
—The following examples show the application of formu- 
las (58) to (63): 
(1) What 7s the electrical efficiency of a series-wound 
generator, producing 2,500 volts and 10 amperes, 
af its warm armature resistance 1s 9 ohms and its 
magnet resistance is 11 ohms? 
Solution.—By formula (58) we have in this case: 
2500 X 10 25,000 
BN ED = a500 X10) + [10 X (+ 11)] ~ 25,000 + 2000 
ROS. ge 
= ne 925, or 9242. 
(2) Find the electrical efficiency of a shunt-wound 3 K. W. 
125-volt generator, knowing tts armature resist- 
ance (warm) to be .32 ohm, and tts shunt resistance, 
including field regulator, 125 ohms. 
3000 
Leb 
= 24 amperes, and the shunt current is found, by Ohm’s 
Law, a = lampere. Hence the total current flowing 
in the armature, being the sum of current output and 
shunt current, is 24 + 1 = 25 amperes. By (59) we 
therefore obtain: Priteecert 
Solution.—The current output of this machine is 
