204 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
Therefore, the constant of the short circuit characteristic 
29 § ‘ 
will be &, = 1+ Ces 1.6; that is, the normal armature 
eurrent will be obtained in the short circuited armature, 
when the field excitation is equal to 1.6 X 3,800 = 6,100 
ampere turns. 
394. Particulars of Field Coil.—The application of the 
graphical method shown in Fig. 90 indicates that the 
specified regulation is complied with. 
The maximum excitation required is 13,000 ampere turns, 
which is supplied by the central field coil. 
The terminal voltage of the field coil is fixed at 55 volts, 
and the approximate length of mean turn is 80 inches; 
from which the conductor section qg, is equal to .0152 
square inch, when 55 = 13,000 <F x =. 
The nearest wire with a section almost equal to .0152 square 
inch, is a No. 9 §.W.G. with a section of 0.01629 square 
inch. 
When such a deep central coil is used the current density 
should not exceed 1,000 amperes per square inch, as the 
cooling conditions are very unfavorable. 
The maximum excitation current will be 0.01629 x 1,000 
13.000 
16.3 = 800. 
=16.3 amperes and the number of turns, 
395. Temperature Rise.—The maximum energy utilized in 
the field winding is equal to 16.3? X 3.15 = 840 watts and 
the cooling surface is 1,440 square inches; consequently 
: 84) 
the temperature rise will be equal to 7, = 120 x —— 
1,440 
mf ge 
The high factor of 120 has been used in this ease because the 
winding depth and height is comparatively large. 
396. Armature Copper Loss.—The-armature copper loss 
