DIRECT:- CURRENT DYNAMOS AND MOTORS. 4 
se ee 
using these figures, therefore, the armature resistance 
found by (2%) is that at ordinary room temperature, or 
is the cold resistance of the armature when the machine 
isnot running. In order to obtain the warm armature 
resistance, add J per cent. for every 23 degrees C. over 20° 
C., or J per cent. for every 44 degrees Fahr. over 68° F. 
Thus. if the temperature of the armature, when running 
at full load, is about 60° C. (= 140° F.), which is the 
usual limit given in dynamo specifications, the value 
60 —20 40 
ah. 2.5 
= 16 per cent., or must be multiplied by 1.16. Figuring 
140— 68 72 _ 
in Fahr pti TNR poo 
in Fahr. degrees, we have rel re 16 per cent., 
found by (2%) must be increased by 
the same as before. 
40. Example of Drum Armature.—A bipolar drum 
armature, 16 inches in diameter and 25 inches in 
length, contains 192 inductors, each consisting of 3 
No. 5 B. & S. wires. What is the armature resist- 
ance at 7O° C.? 
Solution. In this case N = 192, L, = 25, D, = 16, and 
k = 1.5 (the armature being of medium size); hence, by 
(25): 
192 _ 192 x 49 
b, = TS (25 + 1.5 x 16) = —— 5 — 
— ay is ‘4 feet. 
The resistivity of No. 5B. & S. wire, according to Table 
13, is .000313 ohm per foot; since 3 such wires are used 
in parallel, the resistivity of the armature conductor in 
the present example is .000313 + 3 = .000104 ohm per 
foot. The number of bifurcations in a bipolar machine 
isn, = 1. Therefore, the armature resistance at 20° C., 
from formula (27): 
__ %84 x .000104 
. ; = .0020 ohm. 
