INSTRUMENT TRANSFORMERS 585 



In the first case, 



Vl = -jvi 



and 



V '/iff 



K 2 Tv-i/s/ls. 



a 5 



So 



Fi = D r s DpRp 



With a slight modification, Poggendorf's method of comparing 

 an e.m.f. and a P.D. may be applied to the determination of the 

 voltage ratios and phase angles of potential transformers. 



For comparing two direct potentials the connections shown in 

 Fig. 362 may be used. 



FIG. 362. Connections for determining the ratio of a direct current P. D. 

 to a direct current e.m.f. 



If the detector stands at zero, 



In testing potential transformers, Vi and V 2 are replaced by 

 the primary and secondary terminal voltages, and either RI or 

 Rz must contain an adjustable reactor by which the P.D. across 

 Rz may be brought into time phase with F 2 . This is necessary 

 on account of the phase angle of the transformer. If the adjust- 

 able reactor were not used, the detector could be brought to a 

 minimum but not to zero. 



Except as mentioned, the resistances should be non-reactive, 

 that is, free from both inductance and capacity effects. With 

 very high voltages it may be necessary to use shielded resist- 

 .'ni'-cs 21 to avoid the last. Either RI or R 2 must be adjustable. 

 The detector may well be a vibration galvanometer. 



The connections for the potential transformer test are shown 

 in Fig. 363. 



