ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 17 
chined, because with proper care taken in stamping and 
piling the core sheets, a true smooth surface can be 
obtained. 
Regarding iron losses, it is an advantage not to have the 
inner periphery machined. 
Stampings of a diameter less than two feet can be made 
in one dise, but for larger sizes segments are used which 
are piled overlapping. As a rule the greater the number 
of segments used to form a complete ring, the better, 
as assembling is more difficult with large segments. 
34. Ventilation and Air Ducts of Armature Core.—To 
obtain a uniform temperature throughout the armature, 
as well as good ventilation, it has proved economical to 
use an air duct about every three to four inches, parallel 
to the laminations. - These air ducts should not be less 
than, say, 4 inch wide. The distance pieces should be 
so arranged as to provide free access for the air passing 
through the ducts. 
Thick iron end plates are applied in the ducts, $0 support 
the thin stampings, in a manner similar to that of the 
two outside cheeks, see Par. 33. 
In cooling an armature core with ventilating ducts, special 
care is required in order to keep the stampings in proper 
shape and not to distort the slots. 
In all cases it is advisable to note, that the winding space 
of the assembled armature core is less than that of a 
single slot of a stamping, but an additional 0.03 inch will 
make up for this difference. This point requires par- 
ticular attention, because a tight slot may cause con- 
siderable trouble and increase of labor, so that all advan- 
~ tages of the economy in winding space are soon made 
illusory. | 
35. Clamping and Piling.—It is a matter of importance, 
that the armature core should be piled up with exclusive 
