‘ 
DIRECT-CURRENT DYNAMOS AND MOTORS. 55 
this table the designer will be able to predict, from the 
result obtained by formula (33), the approximate rise of 
temperature in the armature. 
48. Circumferential Current Density of Armature. 
—An excellent check on the heat calculation of the arma- 
ture, and in many cases the only investigation made as to 
its heating properties, is the computation of the circum- 
ferential current density, that is, the total num- 
ber of amperes carried per inch of the armature circum- 
ference. The circumferential current density is found by 
dividing the total number of amperes all around the arma- 
ture by the core periphery: 
xe | 
4 2Ny pa ae at Be: (34) 
o. D.x x. 6.283 XD, XK Np 
c = circumferential current density, in amperes per inch 
of armature circumference; 
N = total number of armature conductors; 
C = total current at armature terminals, in amperes; 
2n, = number of parallel circuits in armature winding; 
D, = diameter of armature core, in inches. 
Having found c from (84), a comparison with Table 
22, compiled from actual machines of various types, 
gives an idea of the temperature increase that may 
be expected in the calculated armature when completed. 
By comparing the average increase derived from Table 
22 with the average obtained from Table 21, a very 
close approximation to the actual value of the tempera- 
ture increase of the armature can usually be gained. 
49. Example of Smooth-Drum Armature.—A bipolar 
smooth-drum armature to produce 40 amperes at a 
constant E. M. fF. of 125 volts when running at 1,800 
revolutions per minute, has been calculated as follows: 
Diameter, 5% inches; length, 94 inches; radial depth 
of core discs, 1g inches; winding, 336 conductors, 
