lo-CJ SYNCHRONOUS CONVERTER. 325 



9. Reactance Control. A lagging current through a reactance 

 always causes a drop in voltage; but a leading current, when 

 sufficiently in advance of the electromotive force, will cause a rise 

 in voltage. With reactance in a circuit, therefore, the voltage 

 delivered will depend upon how much the current is lagging or 

 leading, and this in a synchronous motor or converter is con- 

 trolled by the excitation. 



10. Fig. 2 shows a reactance X located in one line; in practice, 

 an equal reactance is located in each line (20) either as separate 

 coils or as leakage reactance* in transformers. Let Eo be the 

 line voltage and ET be the terminal voltage at the collector rings 

 of the converter. 



When the current / is in phase with ET, the reactance causes a 

 voltage drop in quadrature with / so thatEx is somewhat less than 

 Eo, as shown in Fig. 3, Exp. 3~B. This corresponds to a particu- 

 lar excitation of the converter. 



When the current 7 is lagging, the drop through the reactance 

 is more effective and ET is much less than Eo, as in Fig. 4, 

 Exp. 3-B. 



When the current I is leading corresponding to an increased 

 converter excitation ET is increased, as in Fig. 5, Exp. 3-8. 



It is seen, therefore, that the A.C. voltage supplied to the con- 

 verter (and so the D.C. voltage delivered) can, when series react- 

 ance is used, be increased or decreased by increasing or decreas- 

 ing the motor excitation. 



ii. In the laboratory, and sometimes in practice, this change 

 in excitation can be made by a hand adjustment of the field rheo- 

 stat. In practice, however, it is usually made automatic by com- 

 pounding the converter, by means of series turns carrying the 

 D.C. load current. As the load increases or decreases, this in- 

 creases or decreases the excitation and the voltage. By properly 

 proportioning the reactance and the series winding (or a shunt 



* ( loa). The unavoidable reactance of the generator, line, transformers 

 and converter may be sufficient. 



