ALTERNATING GENERATORS AND SYNCHRONOUS “MOTORS. 143 
density, especially with such a high peripheral speed as 
specified for this machine; but as the resulting efficiency 
is improved, due to the smaller density employed, namely, 
1,740 amperes per square inch, there is no need for any 
alteration. 
250. Length of Field Winding.—tThe length of mean field 
turn is equal to 36 inches; therefore the total length of 
the 12 field coils which are connected in series is 12 X 35 
X 36 = 15,120 inches. 
251. Weight of Field Copper.—tThe sectional area of a 
turn being equal to .125 X 1.125 = 0.1405 of a square inch 
and the specific weight of the copper assumed as 0.32 lb. 
per cubic inch, the weight of the field copper will amount 
to 15,120 x 0.1405 X 0.32 = 680 pounds. 
252. Resistance of Field Winding.—If the hot copper 
are ‘ ; 8 
winding has a specific resistance of i0" ohms for a length 
of one inch with a section of one square inch, the resistance 
a : : 15,120 g 2 
of the 12 field coils in series will be equal to 04408 xX ip 
= 0.0865 ohm. 
253. Voltage of Exciting Current.—From Par. 247, the 
Maximum excitation current required is 244.5 amperes, 
which implies a maximum voltage on the field terminals 
of 244.5 x 0.0865 = 21.5 volts. 
The exciter should be capable of supplying a higher voltage 
than above mentioned, because there are additional losses 
in the connecting cables and between the slip rings and 
brushes. A certain margin should be allowed for the 
regulating resistance. 
An exciter with a terminal pressure of 28 volts will be 
found ample to supply the field system of this alternator. 
