DIRECT CURRENT DYNAMOS AND MOTORS, 105 
the number of layers 210 + 48 = 5. The corresponding 
winding depth is 5 X .172 = 1 inch, allowance being 
made for core insulation ; and the mean turn, since the 
core diameter in this case is 10 inches, will be 
1,” = (10 + 1) X x = 34.56 inches, or 2.85 feet. 
The total resistance of the two series coils, since there are 
3 times 70 turns of No. 6 wire in parallel in each core, is: 
2X 70 X 2.85 X .000394 
vse = 
3 
or .0368 X 1.16 = .061 ohm at 60° C. 
The power loss in the magnet coils, therefore, is 
65° X .061 = 258 watts, 
while the cooling surface is : 
Su = (10 + 2) X z X 8§ + 2 X 34.56 X 1 = 380 square 
inches per coil, or ‘760 square inches total. 
This makes the specific cooling surface 
= .0524 ohm at 20° C., 
760 + 258 = 2.95 square inches per watt, 
which, according to Par. '%2, corresponds to a tempera- 
rise of 30° C. 
78. Determination of Shunt Winding for Bipolar 
Smooth-Armature Motor.—Let the given motor be 
the same as considered in Par. 76. For a shunt wind- 
ing, the required size of wire is obtained from formula 
(49), in which, for the case of a motor, H’ is usually the 
total supply voltage, a field rheostat being seldom em- 
ployed with the ordinary motor. Substituting in (49) 
the value AT = 17,630 ampere-turns (as determined in 
Par. 66), EZ’ = 500 volts, and 7," = 10 X 3.73 = 37.3 
inches (from Table 2'7), we obtain: 
9.2 — 17,630 X 37.3 
500 
The nearest size wire, according to Table 28, is No. 19 
B. & §., which has a diameter of .041 when insulated for 
use as magnet wire. 
= 1,310 circular mils. 
