FIELD-EXCITATION OF ROTARY CONVERTERS 209 



f 

 To overcompound the converter the right line PQ should be 

 straightened more, by raising the point O higher on the right line BY; 

 but the principle always remains the same, namely, to draw the right 

 line PQ in such a way that it shall come as close as possible, at every 

 point, to the desired characteristic, which should itself be drawn 

 first. 



Application to the General Case of Self-Excitation. In the 

 general case of self-excited converters, the solution is nearly the same, 

 but it is complicated by the fact that the shunt-excitation varies with the 

 direct-current voltage. Hence, instead of only one right line PQ to 

 indicate the lags, there will be a series of parallel right lines correspond- 

 ing to various values of e. 



O' O J P~---\p> 



i I 



I 



FIG. 24. 



In the case of constant potential regulation, the method of deter- 

 mining the excitation-windings and the reactance of the circuit remains 

 exactly the same as given in the preceding paragraph, since the shunt- 

 winding must, by hypothesis, produce constant excitation. But the 

 small variations of A d have to be considered in the final calcula- 

 tion of the differences of voltage due to the imperfection of the 

 regulation. 



For values of E lower than the normal value, the shunt-excitation 

 decreases of itself and, consequently, the right line PQ comes closer 

 to d, remaining parallel to itself. On the other hand, this line recedes 

 from d when e increases. It is easy to see, in Fig. 24, that this effect 

 tends to increase the voltage-differences as compared with these dif- 

 ferences when the shunt-excitation is constant. 



When the machine is to be overcompounded the zero-load and 

 the normal load no-lag conditions determine, as before, the reactive 



