o4 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
The modern alternator with its improved’ ventilation and 
cooling arrangements, permits of a high current density 
of the copper, and the usual current densities allow an 
average copper loss of about 10 to 14 watts per lb. The 
economical use of the copper is a most important matter 
owing to its high cost. : 
95. Current Density.—In the design of stationary armature 
windings a current density (amperes per square inch of 
j 
> 
8 
> as 
“me 
RS 
Ss 
4 
% 
LOSSES LW WATTS PER POUND 
O 500 7000 1500 2000 2500 3000 3600 4000 
CURRENT LENSITY IN AMP. PER SOUARE INCH 
Fie. 31.—CoprerR LossES PER PoUND WEIGHT FOR DIFFERENT 
CURRENT DENSITIES. 
conductor section) of 2,000 amperes will create a tempera- 
ture rise of about 40 degrees Centigrade. For large gene- 
rators the armature current densities are somewhat lower 
than the above figure, and for small outputs higher. 
In connection with this subject it will be useful to refer to 
Fig. 31, which gives the energy losses per unit weight in 
terms of the current density. The choice of the latter item 
is dependent on the permissible copper loss and tempera- 
ture rise; consequently, if a higher efficiency is specified 
at ui 
