16 DIRECT-CURRENT DYNAMOS AND MOTORS. _ 
. The diameter required to give an adequate cooling surface 
being somewhat smaller than that for which the per- 
ipheral velocity is just right, a value between the two 
results obtained may be taken. Their average is about 
D, = 48% inches. 
19. Tables of Dimensions of Modern Dynamos.— 
In order to ‘guide the student in the selection of values 
for the various variable quantities which he has to 
assume in the ‘course of a dynamo calculation, tables of 
dimensions of modern machines are here given, covering 
all the usual cases of dynamo design. The tables include 
drum as well as ring armature machines, and give the 
principal dimensions for high, medium, and low speeds, 
for all ordinary sizes. In Tables 5 to 10 the dimensions 
of some of the machines of the Crocker-Wheeler, Gen- 
eral Electric, and Westinghouse Companies are com- 
piled, and in Tables 11 and 12 the armature dimensions 
of all the machines contained in Tables 5 to 10 are 
tabulated according to size and speed, so as to be readily 
employed for reference. 
In the case of an example calling for the design of a 
machine which is to run at a speed not sufficiently near 
any of the speeds given for that output in Tables 11 or 
12, astandard of comparison may be obtained by inter- 
polating between the next higher and lower speeds found 
for the same output and typeof armature. For instance, 
if a 200 K. W. ring-type dynamo is to be calculated for a 
speed of 300 revs. per min., the result of equations (5) 
and (9), or (10), respectively, should be compared with 
the diameter obtained by interpolating between the di- 
mensions given in Table 11 for the 200 K W. high- speed 
ring armature and the 200 K. W. medium- speed ring 
armature, thus: 
Diameter, 200 K. W. ring armature, 450 revs per min. ........ 8214 ins.. 
a mn * Zn i TUB: oh Ete Sen 01.2. aa Me eee 
oy 2007 «3° 3 '" 800 ‘* ‘*  ** by interpolation: 
450 — ah : 
8216 +4 [= x (50 — 82 4) = 82} + 94 <= 42 inches. 
