ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 61 
quired for excitation, on account of the watts lost in the 
field regulating resistance R, Fig. 1. 
The necessary variation of the exciting current is generally 
obtained by regulating the resistance in the field circuit of 
the alternator, but the application of a regulating resist- 
ance in the shunt coils of the dynamo will be found not 
only economical but very convenient, as it permits a more 
close regulation than the alternator field regulator alone. 
105. Exciting Current.—The watts required in the magnet 
coils are equal to the square of the excitation current mul- 
tiplied by the warm resistance of the field winding, thus: 
Mee ge be ea eh he heey (13) 
where C, = the current in the coils, and 
R, =the resistance of the coils. 
\ 
N 
» wG Ss 
ed 
pel 
Pe 
tI 
ii 
700 200 300 Wo 500 600 oo 800 900 7000 
WIPET LW KA. 
Fie. 32.—AVERAGE ExcrrATION LOSSES FOR DIFFERENT SIZES OF 
ALTERNATORS 
LOSSES IN Yo OF KVA. 
Ss 
106. Resistance of Field Coils.—The ohmic resistance of 
the winding can be calculated by determining the total 
length of the coils in series, then multiplying the figure ob- 
tained by the specific resistance of the copper and divid- 
ing the result by the value of the sectional area. 
The field coils, having a comparatively great winding depth, 
do not allow the same economical use of copper as the 
armature coils, because the cooling conditions for the inner 
layers are very unfavorable. 
