DIRECT-CURRENT MACHINERY 



161 



behind them the e.m.f. between terminals is decreased. This 

 may be seen by reference to Fig. 126. With the brushes on 

 the neutral points the e.m.f. 's generated in all the conductors 

 in series between terminals act in the same direction and com- 

 bine to give the maximum e.m.f. When the brushes are moved 

 ahead the e.m.f. between terminals is only that generated in 

 conductors a-b or d-c since the resultant of the e.m.f.'s generated 

 in conductors a-d and b-c is zero. Thus advancing the brushes 

 corresponds to a decrease in the number of armature conductors. 



Maximum Voltage 

 Generator 



Generator 



FIG. 126. Effect of moving the brushes. 



105. Saturation Curve. The flux <i> crossing the air gap can 

 be varied by varying the current in the field winding. This is 

 accomplished by connecting a field rheostat in series with the field 

 winding. When the resistance is increased the current decreases 

 and the flux decreases as shown in Fig. 127, which is the saturation 

 curve of a machine plotted with flux per pole on a base of field 

 current. 



At first the flux increases almost directly as the current, while 

 the iron parts of the circuit are unsaturated, but as the flux den- 

 sity increases the magnetic circuit becomes saturated and a greater 

 increase of current is required to produce a given increase of flux 

 than on the lower part of the curve. 



Since the e.m.f. generated varies directly with the flux, by mul- 

 tiplying the ordinates of the curve (1) in Fig. 127 by the constant 

 Kn, curve (2) is obtained of the same shape as before giving the 

 generated e.m.f. or terminal e.m.f. at no load on a base of field 

 current. 



