538 



ELECTRICAL MEASUREMENTS 



Reckoning from lead 1 the currents in A and B are 60 apart. 

 The fiducial position of the coils is given when the power factor 

 of the load is unity; if the power factor is other than unity the 

 current in lead 1 is shifted 0, where is the power-factor angle, 

 and the crossed coils turn through an equal angle. 



The polyphase instruments are independent of frequency, for 

 no reliance is placed on the use of reactances to properly shift the 

 phases of the currents in the crossed coils. On high- voltage cir- 

 cuits the meters are operated through instrument transformers. 



In power-factor meters as actually constructed, see Figs. 318 

 and 320, the fixed coils are made to surround closely the movable 

 system. Economy of space and of materials are thus attained. 

 In this case the scale must be determined by calibration. 



To avoid the use of movable coils, the Westinghouse Company 

 in certain of its power-factor meters and synchroscopes uses the 

 construction indicated in Fig. 321. 



FIG. 321. Westinghouse arrangement of coils for power-factor meter. 



Within the stationary crossed coils A and B is a third fixed 

 coil C with its axis perpendicular to the plane of the paper. This 

 coil carries the line current and magnetizes the soft iron element 

 D, which is mounted in jeweled bearings. The iron, D, thus forms 

 the core of an alternating-current electromagnet and acts as if 

 it were a pivoted coil carrying the line current. 



Another arrangement of circuits, as used in the Punga power- 

 factor meter, is shown in Fig. 322. 



The movable system consists of three flat, rectangular coils 

 lashed to the spindle so that they are 120 apart; they have a 

 common electrical terminal at 0, that is, they are Y connected 

 across the line; the currents are controlled by resistances. 



