108 
DIRECT-CURRENT DYNAMOS AND MOTORS. 
design as well. By increasing the length of the arma- 
ture, the same length of conductor will occupy less height, 
thereby reducing the gap space and correspondingly 
diminishing the number of ampere-turns required. 
479. Determination of Shunt Winding for Four-Pole 
Toothed-Armature Generator.—Inserting into 
(49) the value of the ampere-turns found in Par. 69 for 
the machine shown in Fig. 26, which in this case we will 
assume to be a 125-volt generator, we obtain 
dy? = Steir “35) = 4,030 circular mils. 
Herein 28.6 is the approximate mean length of one turn 
on a 68-inch core for a multipolar machine, found from 
Table 2%, as shown on page 98. The factor 4 in the 
denominator indicates that in the present case the four 
magnets are to be connected in series (see Par. 73). 
The deduction of 25 volts represents an allowance of 20 
per cent. for regulating resistance. 
The above value of d,," is almost exactly the area of No. 
14 B. &S. wire having a diameter of .071 ee when 
insulated (see Table 28). 
Using the value 2}” given in Table 2% for the average 
winding depth of a 6-inch multipolar magnet, and taking 
3+ inches as the available length of each winding space, 
the cooling surface is found: 
Sin = (68 + 2 x 2h) X w xX BE X 24 28.6 x WAX 
= 855 square inches. 
In this case. only one end surface of each coil is consid- 
ered as cooling surface, because in a multipolar frame the 
polepieces cover up theinner ends. For a rise of 30° C.,. 
the shunt current is therefore obtained by (51), thus: 
30 X 355 
Yn = ee = BB. 
Csn 75 CBO 4 amperes, 
giving a specific cooling surface of 
eee 2.5 square inches per watt. 
50 X 2.84 
