DIRECT-CURRENT DYNAMOS AND MOTORS. 13 
TABLE 83. 
‘POWER LOSS IN ARMATURES OF VARIOUS SIZES. 
Percentage of Power Loss, &. 
Size of Machine. 
Usual Limits. Average. 
Above 500 K. W .01 to .03 .02 
Between 500) and 100 ‘* ‘¢ 02: ** 06 -035 
ty Pr eS BO SF <8 OP OF .045 
| Eta 18 9 cali te -04:**-.08 .06 
‘hg a Ss Sie teatl OBS ** 10 .075 
66é 5 ce 1 66 666 .06 €é <t2 .09 
Below -1¢¢2.% 210: ** 30 15 
Substituting in formulas (8) and (10) the value of S, given 
in formula (11), we obtain: . 
for drum armatures: D,?x( m++)=sxkx W, or 
Doa=za/SXkXK W 
7H SR (12) 
for ring armatures: 27 D,?(1—m') (m+m')=sxkx W, or 
Do= phe Jal Aa. 13 
V oe et (ue-+m’) di 
Since in formula (12) one of the end surfaces is neglected, 
woereas in (13) the entire core surface is considered as 
cooling surface, the actual cooling surface of a drum 
armature is larger than that given by (8), while the 
effective cooling surface of the ring armature is smaller 
than the value found by (10). Consequently, in order to 
reduce both formulas to the respective actual cooling sur- 
face, it is necessary to take a somewhat larger value of s 
for a ring armature than for a drum, in spite of the fact 
that the former, owing to its better ventilation, will re- 
