372 



ELECTRICAL ENGINEERING 



on the same shaft. For over-compounding, a series field winding 

 is added. Its action is explained in Art. 242. 



235. Ratios of E.M.Fs. and Currents. With the brushes 

 fixed on the no-load neutral line, the direct and alternating e.m.f. 's 

 generated in the converter bear a definite relation to each other 

 and one cannot be varied without varying the other. At unity 

 power factor the alternating and direct currents in the armature 

 also bear a definite relation to each other if the current required 

 to supply the converter losses is neglected. 



Since the alternating and direct currents flow in the same arma- 

 ture conductors and in opposite directions, the e.m.f. consumed in 

 the armature is small and the power loss is small. In the following 

 analysis these quantities will be neglected and the alternating 

 current will be assumed to be in phase with the impressed e.m.f. 

 This condition can be obtained in practice by properly adjusting 

 the exciting current. 



Take first the case of the single-phase converter, Fig. 353. 



FIG. 353. Single-phase converter. 



Let E = direct voltage of the converter. 



I = direct-current output. 



EI = effective value of the alternating supply voltage, 

 whether single phase or polyphase. 



I 1 = alternating current in the supply lines. 



/' = alternating current in the armature winding. 



