ELECTRICITY METERS 



503 



the same purpose is the Drysdale phase-shifting transformer, the 

 principle of which is explained on page 290. 



This transformer as designed for meter tests, together with the 

 connections necessary in testing three-phase meters, is shown 

 in Fig. 289. 



The phase-shifting transformer should be used on circuits 

 which have sinusoidal voltage waves, otherwise the wave form in 

 the secondary will change with the adjustment of the phase 

 displacement. 



In order to lag an induction meter it is necessary to operate 

 it at two power factors and usually 1 and 0.5 are chosen. The 

 double-motor generator set or the phase-shifting transformer 

 previously described may be used, but these two particular 

 power factors may be obtained from a three-phase circuit. Fig. 

 290 shows the connections. 



FIG. 290. Arrangement for obtaining power factor 0.5 from a three-phase 



circuit. 



As shown, the current is in phase with E u , the voltage at the 

 meters in phase with EI Z and the power factor is 0.5 leading. 

 To obtain 0.5 power factor with lagging current the voltage coils 

 would be connected between leads 3 and 2. 



To determine whether one is dealing with a lagging or leading 

 current, a small inductance, L, of low resistance, may be included 

 in the potential circuit of the standard dynamometer wattmeter; 

 normally this inductance is short-circuited. If the current is lag- 

 King, the insertion of the inductance will slightly increase the 

 apparent power factor and will decrease it when the current is 

 leading. 



