580 



ELECTRICAL MEASUREMENTS 



lag adjustment is made. However, a perfect adjustment is not 

 possible, for both the ratios and the phase angles vary with the 

 load. 



In careful industrial tests, the combination of watt-hour meter 

 and instrument transformers may be calibrated in the laboratory 

 without undue expenditure of power by using fictitious loads 

 (see page 500). The test conditions may then be reproduced, 

 wave form excepled. 



An attempt is sometimes made to correct for the variation of 

 the ratio of the current transformer, which is most troublesome 

 at light loads, by altering the light-load adjustment of the meter. 

 If the ratio increases at the light loads the adjustment is set so 

 that the meter runs a little fast, creeping being avoided. There 

 is no means of making even an approximate adjustment for the 

 variation of the phase angle. 



DETERMINATION OF THE RATIOS AND PHASE ANGLES OF 

 INSTRUMENT TRANSFORMERS 



From what has preceded it is evident that if accurate measure- 

 ments are to be made, it is necessary to know both the ratios and 

 the phase angles of the transformers. Among the various meth- 

 ods which have been devised for their determination, a few of 

 those based on the potentiometer principle have become gener- 

 ally accepted as being the best. They give results of high 



accuracy and are convenient 

 because they do not require 

 currents and voltages to be 

 held at fixed values. 



Ratio and Phase Angle of 

 Current Transformer. The 

 71 determination of the ratio of 



FIG. 357. Connections for determining a current transformer is sim- 

 ply the determination of the 



ratio of two currents. To make such a measurement using 

 direct currents the connections shown in Fig. 357 are used. 

 If the detector stands at zero, 



To Detector 



To Source of 7o 



7, 



/2 



Ri 



