566 



ELECTRICAL MEASUREMENTS 



curate work, the currents in the fixed and movable coils of a 

 dynamometer wattmeter must have the same phase relation as 

 the current and voltage of the load. 



It is one of the imperfections of instrument transformers that 

 they introduce false phase relations. With the potential trans- 

 former the voltage at the secondary terminals is not in exact 

 opposition to the voltage applied to the primary, the departure 

 from exact opposition being small, to be sure, and of the order 

 of magnitude of 10' of arc under normal operating conditions. 



Load 



FIG. 348. Showing connections for measuring power, voltage, and current, 

 using instrument transformers. 



This phase angle may be either a lag or a lead, and depends on 

 the frequency as well as on the number and character of the in- 

 struments in the secondary circuit. 



The current transformer is subject to a much greater phase- 

 angle error than the potential transformer. The displacement of 

 the secondary current from exact opposition to that in the pri- 

 mary may, at low loads, sometimes amount to as much as 3. 

 This displacement depends on the magnitude of the primary cur- 

 rent, on the frequency and on the number and the character of 

 the instruments in the secondary circuit. 



It will be seen that the errors introduced into power measure- 

 ment by the use of transformers are those due to the variation 

 of ratio of both the current and potential transformers, as well 

 as those due to the phase displacement in both instruments. 



It is possible to determine the ratio and phase angle and to 

 make the corresponding correction so that accurate results may 

 be obtained even at low power factors, where the phase-angle 

 errors are most pronounced. These matters are of great prac- 

 tical importance, for instrument transformers are used in con- 



