DIRECT CURRENT DYNAMOS AND MOTORS. 15 
velocity. in accordance with Par. 14, we have from for- 
mula (6) : 
 -D, = 38x SY = 22.8 inches. 
According to Table 3, the percentage of power loss in 
the armature of a 50 K. W. dynamo ranges between .04 
and .08; a good value for a high-class machine therefore, 
is k = .06. The specific cooling surface, according to 
formula (11), can be taken between .5 and | square inch 
per watt; using the average value, we have s = .75. 
For adrum armature of the above found diameter. a good 
shape-ratio is = 1.2 (see Table 4, page 14); hence, the 
approximate diameter of the armature, from formula (12): 
75 X .06 X 50.000 2250 _ ; 
Ds ay / x (1.24+9%) =y/ eH ral ies ties 
This being practically the same as the value obtained 
above from formula (6), the nearest even value between 
the two results, or D, = 224’, may be decided upon. 
18. Example of Ring Armature.—Compute the diam- 
eter of a ring armature for a 200 K. W. direct- 
driven generator which ts to run at 1975 revolutions 
per minute. 
Solution.—The peripheral velocity should in this case be 
taken v = 2.250 feet per minute (see Table 2, page 10); 
hence, we have by (6) : 
2250 ; 
Dp, = 3.8 X to 48.8 inches. 
For the case in hand, the specific cooling surface should 
be taken between .75 and 1; let us assumes = .8. The 
percentage of power loss in the armature of a direct- 
driven 200 K. W. dynamo is about 4 per cent.; hence, 
k= .04. From Table 4, the average ratio of length to 
diameter in this case is m = .375, and the average ratio 
of radial breadth to diameter is about m' =.15. Insert- 
ing these values into (13), we have: 
8 X .04 X 200,000 _,/6400 
D. =\/ oan x (1 — 16) X (8% + 16)" og ESS ins. 
