RECTIFYING DEVICES 



385 



giving a condenser current /</, the line current is found 

 at /'; the e.m.f. to overcome the inductance drop is pro- 

 portional to this current and 90 ahead of it and is shown at 

 E L f . By subtracting this E L ' from E we find the converter 

 voltage at E R f , which gives a voltage somewhat greater than 

 the line vcltage E. (It is to be noticed that in constructing 

 this diagram E L and E must always so combine that E R is 

 found in phase with I R ; I R and the converter voltage E R 

 must be in the same phase and on the vector diagram this 

 is shown to be possible by the phase shifting of E.) 



For still further increases in the field excitation of the 

 converter the currents are shown by I" and /'" and the 



'i * E; E; 



FIG. 252. Vector Diagram of Compounding Action. 



corresponding voltages are shown by E R " and E R ". It 

 is thus seen that, when there is inductance drop in the 

 line supplying the a-c. power to the machine, it is possible 

 to bring about an increase in the voltage impressed on the 

 converter by sufficiently overexdting the field, even though the 

 line voltage E, remains constant. 



Compounding Accomplished Automatically. The ratio 

 of the alternating voltage to the continuous voltage is 

 practically constant so that this increase brought about 

 in the impressed voltage means a corresponding increase 

 in the continuous voltage of the converter. The over- 

 excitation of the converter field is automatically increased 

 as the load increases by means of a series field winding in 



