14 
DIRECT-CURRENT DYNAMOS AND MOTORS. 
quire less cooling surface than the latter in order to dis-. 
sipate equal amounts of heat. The student should use 
= .6to.75 in (12) for drum armatures, and s = .75 to 
Zin (18) for ring armatures. 
In practice the shape-ratios m or m' in the above formulas 
are usually known from previous designs of the same 
type; but if their approximate values are not given in 
this manner, a number of assumptions of m and m' may | 
be made within the limits compiled in the following 
Table 4: 
TABLE 4. 
SHAPE-RATIOS FOR DIFFERENT ARMATOURES. 
. Ratio m’ of Radial 
Kind and Size of Armature. — bei Berti. xt Thickness to 
; Diameter. 
Ring, above 100” Diameter... . 15 to .25 .05 to .10 
Ring. from 100” to 20” Diam. 
ChE. Pes tion atinss hclsaented Se bar 2 .08 ** ,25 
Ring, below 40” Diameter .. 208 t, 10 * .80 
Drum, above 30” a oD ae 
Drum, from 10” to 30” Diam 
ORAE coos 1c as hahaa eax ewes 70." 2 e — 
Drum, below 10” Diameter... . o. ote — 
A comparison of the value calculated from (12), or (13), 
respectively, with that previously obtained from (6), will 
assist the designer in deciding upon the final diameter of 
the armature. The student, who naturally lacks the ex- 
perience of a professional designer, should also consult 
the dimensions of existing machines of similar output and 
speed, tables of which are given in Par. 19. 
1%. Example of Drum Armature.—Find the approxt- 
mate diameter of adrum armature fora 50 K. W. 
belt-driven generator to run at 500 revolutions per 
minute, 
Solution.—Taking the value of 3,000 for the peripheral 
