36 
DIRECT-CURRENT DYNAMOS AND MOTORS. 
20. 
Assuming that a commutator of 54 divisions is to be used 
with this armature, the number of conductors must be 
modified as follows: ._Number of wires per layer of arma- 
ture section is 160 + 54 = 2.96; taking 3 wires, we have 
54 X 8 = 162 wires per layer; and since there are 2 
layers, the actual number of wires on this armature 
will be 1462 x 2 = 324, or 2 X 3 = 6 wires per commu- 
tator division. 
Example of Toothed Armature.—find the ap- 
proximate number of No. 9 B. & S. wires that can 
be placed on a toothed armature having a diameter of 
18 inches. 
In this case D, = 18’, h, = 1.15 (by interpolation from 
Table 14), and d,? = .013, nearly, (from Table 13); 
therefore, by formula (17): 
18 X 1.15 
.013 
This result may be checked as follows: The circumference 
of the armature under consideration is 18 X 7 = 56,55 
ins., about one-half of which, or 28.3,ins., is occupied by 
the slots. The slots being 1.15 inch deep, the total slot 
area is 28.3 x 1.15 = 32.5 square inches. Of this area, 
only about four-fifths is actually occupied by the con- 
ductor, including its own cotton covering; hence, the 
effective slot area is 32.5 x 4 = 26 square inches. 
According to Table 13, the number of No. 9 wires, D. C. 
C., which can be wound in one square inch is 60.23; con- 
sequently we have 26 x 60.28 = 1566 as the total num- 
ber of conductors, which agrees with the above result 
obtained by formula (1'7). 
According to Table 15, the average number of slots for an 
armature of the size considered is about 80; hence, the 
number of wires per slot is 1590 + 80 = 20. The width 
of the slot is the available circumference divided by the 
slot number, or 28 3 + 80:= .85 inch, while its depth, 
ha, is 1.15, _Deducting +5 inch for lining of the sides and 
N= = 1590 wires. 
