+3 
—: 
DIRECT-CURRENT DYNAMOS AND MOTORS, 35 
layers nearly filling the space within the slot lining. If 
this condition is not obtained by the first selection of the 
slot number, a second, and eventually a third, must be 
chosen within the given limits, and, if necessary, even 
the size of conductor must be changed by suitable sub- 
division of the cross‘section. 
It usually happens, in case of smooth as well as of toothed 
armatures, that a particular size of wire will not suit a 
certain case, and it is often very difficult to find any regu- 
lar size that will fit exactly right. In fact, it is a large 
part of the work of designing a machine to make these 
relations come out satisfactorily. If necessary, a special 
size of wire can be made to order, but this should be done 
only as a last resort. 
29. Example of Smooth-Core Armature.—How many 
No. 6 B. & S. wires can be wound upon a smooth-core 
drum armature 10 inches in diameter? 
Solution.—Here D, = 10’; d, = .162’, d,? = .026244 
(compare with column 3 of Table 13); and from Table 
14 we find that the average winding depth for this size. 
of armature is about .5 inch; hence, by formula (16): 
495 x 10x35 . 
= 026244 = $332 wires. 
In order to verify this result, we find in Table 13 that the 
_ diameter of No. 6 B. & §. wire, including its double cot- 
ton covering, which is almost always used for armature. 
winding, is ./77 inch. The circumference of the arma- 
ture is 10 X wz = 31,416 inches, 90 per cent. of which, or 
48.27 inches, is occupied by the winding; hence, the num- 
_ber of wires per layer is 28.27 + .177 = 160, 
The core insulation and binding in an armature of the size: 
under consideration will amount to a height of about 4 
inch; hence, the number of layers that can be placed on 
5—+4 
177 
ductors, therefore, 160 x 2 = 820, which agrees very 
well with the result obtained by formula (16). 
this armature is 
= 2, and the total number of in- 
