6-A] 



GENERAL STUDY. 



207 



Total power =V3 D/ cos 0, 

 = V3 El cos 0, 

 = V3 El X power factor, 



where E and / are line voltage and line current. This is the 

 customary power formula for any balanced 3-phase system, as 

 has already been found for the star-connection. 



22. The currents and voltages for the delta-connection can 

 be laid off by the radial method (see Appendix I.) from a com- 

 mon center, giving a diagram similar to Fig. 12. 



Another method is shown in Fig. 14, in which the voltages are 

 laid off as a triangle (polygon method) and the currents radially 

 from the corners. The cur- 

 rents in Fig. 14 are drawn 

 as lagging. These currents 

 are /XY (from X to F), IYZ 

 (from Y to Z), and /zx (from 

 Z to X). With sign reversed, 

 the latter becomes /xz, meas- 

 ured from X to Z. The sum* 

 of /XY and /xz gives /. If 

 we wish to select signs so that 

 the sum of these three vectors 



is zero, we must reverse the 



sign of I so as to give the line 



current /' ; we now have /', /XY 



and /xz all measured from X, so that Law (3) of Appendix I. 



is satisfied. 



23. Transformer-connections on 3-Phase Circuits. Trans- 

 former secondaries and primaries like any generating or re- 

 ceiving circuits can be connected to a 3-phase circuit by A-, F-, 

 T- or F-connections, shown in Fig. 2. 



*(22a). The current / is the sum of /xz and /XY (both measured 

 from X), or the difference between /zx and /XY (measured one towards 

 and the other away from X). See Laws (3) and (4), Appendix I. 



FIG. 14. Currents and voltages in a 



^-<'f* 3-Phase dr C uit,_ P oiy- 



gon or mesh method of representation. 



