DIRECT-CURRENT DYNAMOS AND MOTORS. 101 
75. 
76. 
In the calculation of a differential winding, the proceeding 
is practically the same as in a compound winding. The 
only difference between the two is that in a differential 
winding, since the series coils are wound 7n opposition to 
the shunt coils, ATJ, is greater than AT, hence AT,, 
becomes negative. 
The calculation of magnet windings by means of fortnulas 
(37) to (55), respectively, is best understood by apply- 
ing them to specific cases. 
Data for Magnet Winding.—Table 28, page 102, gives 
the areas, the usual diameter, the number of turns per inch, 
and the weight and resistance of insulated magnet wire. 
Magnet wires, except the large sizes down to No. 3, are 
usually insulated by a single cotton covering (S. C.C.), 
the large sizes being double cotton covered. This table is 
intended to be employed in calculating magnet windings, 
and its use will be seen from the following paragraphs. 
Determination of Series Field Winding for 
Bipolar Smooth-Armature Machine.—To show 
the application of the method given in Par. 72, let us as- 
sume that the machine represented in Fig. 25 is a 35 H. P. 
series motor to besupplied with 500 volts and 65 amperes. 
The ampere-turns required for compensating armature 
reaction, though in this case not exactly the same as de- 
termined in Pars. 66 and 6%, are approximately correct, 
because the number of turns for 500 volts must be in- 
creased in about the same proportion as the current 
strength in them is reduced. In taking the total number 
of ampere-turns as obtained above, no serious error is 
therefore committed. 
The number of turns is obtained by dividing the total num- 
ber of ampere-turns by the current, which is 65 amperes. 
In case of the smooth-armature design, A7'’ was found 
17,630 ampere-turns; therefore, by (46), 
17630 
T= —— = 272 turns. 
65 
