INSTRUMENT TRANSFORMERS 



569 



transformation." The phase angle of the potential transformer is 

 designated on the diagram by 7. This angle is the departure 

 from exact opposition of Vi and V 2 . It is usually very small and 

 in reality may be either an angle of lead or of lag, according to 

 circumstances. The phase angle of the current transformer will 

 be denoted by /3. 



Fig. 350 shows the results of experimental determinations of 

 the constants of certain commercial current and potential trans- 

 formers. It gives an idea of the order of magnitude of the 

 changes to which the ratios and phase angles are subject. 

 Similar curves, giving representative values of ratios and 

 phase angles, which have been determined by testing a few 

 transformers all made in accordance with the same specifications, 

 may be obtained from the makers of such instruments and are 

 sufficiently accurate for much commercial work. 



In tests where the greatest accuracy is desired, the ratios and 

 phase angles for the transformers should be determined at the 

 frequency and voltage and with the same connected burden of 

 instruments and leads as are to be used in the subsequent work. 



FIG. 351. Pertaining to polarity tests of instrument transformers. 



When using instrument transformers in connection with watt- 

 meters and power-factor meters it is necessary to know the rela- 

 tive polarities of the secondaries of the transformers with respect 

 to their primaries, otherwise the meters may be so connected that 

 they will not read properly. 



Some manufacturers arrange the internal connections so that 

 the corresponding terminals of the primary and of the secondary 



