DIRECT-CURRENT MACHINERY 



201 



Fig. 171 shows n as a function of E; the locus is a straight line 

 passing through the origin. 



FIG. 170. 



Impressed Voltage E 



FIG. 171. Variation of speed 

 with impressed voltage. 



This method of varying speed is uneconomical as a large amount 

 of power is lost in the control resistance; it is the product of the 

 current input and the voltage consumed in the resistance and is 

 = / X IR = PR watts. 



Field Current If 

 FlG. 172. 



The resistance R must not be connected in series with the field 

 winding as it would then decrease the field current and therefore 

 the flux and tend to cause an increase in speed. 



