54 DIRECT-CURRENT DYNAMOS AND MOTORS. 
where ¢ = specific cooling surface of armature; 
S, = cooling surface of armature, formula (7) or 
(9), respectively; 
w, = power lost in armature winding, formula (28); 
$e ‘. * hysteresis, = (29); 
We=  *  “* eddy currents; = (32), 
If the value of s found by (838) is below .4 for a drum, or 
below .75 for a ring armature, the armature would heat 
up excessively, and should therefore be redesigned, as 
indicated in Par. 42. 
4%. Temperature Increase of Armature.—Approxi- 
mate limits of the temperature increase of practical arma- 
tures, as actually found for various values of the specific 
cooling surface s, are givenin Table 21. By means of 
TABLE 21. 
TEMPERATURE INCREASE OF ARMATURE FOR DIFFERENT 
SPECIFIC COOLING SURFACES. 
Temperature Increase. 
Specific 
Cooling 
sur: uce. Drum Armature. Ring Armature. 
Sq. inches ett 
p. watt. ; 
Centigrade. | Fahrenheit. | Centigrade. | Fahrenheit. 
4 50° to 75° | 90° to 135° | 60° to 100° |108° to 180° 
oO 45 ‘** 70 Si ** 126 55 ** 90 99° ** 162 
.6 40 ** 6U 72 ** 108 50 ** 80 90. * tae 
oe 35 ** 60 63. **,.90 AD eS BE *F Tey 
8 BO. tf as §4..**. 8] 40 °F 60 72. **.108 
9 20°" AO AG Fe SES 30. ** 60 G3. fF" Se 
1.0 20° ** 85 36'S) 163 30 ** 46 Seni «| 
1.25 BOM 26 C7 th AS 25 tired 45. §* og 
1.5 Jor: £' 20 18. ‘4:36 96." ** 30 36 ‘© - 64 
2.0 Boots 9) Say 15 ** 625 9°. oe 
