206 



POLYPHASE CURRENTS. 



[Exp. 



Since line voltage is ED and line current is /s, we may drop 

 the subscripts and write 



Total power = \/3 El cos 0= \/3 El X power factor, 



where E is line voltage and / is line current. This is the custo- 

 mary formula for power in any balanced 3-phase system, no 



Line Voltage 



FIG. 13. 



Delta- or mesh-connection of 

 load resistances. 



matter how connected. In the 

 next paragraph it will be de- 

 rived for a delta-connection. 



21 Delta-connection. Con- 

 nect the same three equal* re- 

 sistances in delta to a 3-phase 

 supply, as in Fig. 13. Measure 

 the current and voltage for each 

 resistance, namely the delta 

 current ID and the delta (line) 

 voltage D. Also measure the 

 line current / and the star voltage s, if the neutral O of the 

 supply system is accessible. It is seen, as above, Es = En-S- V3- 

 Compare the measured values of / and ID with the expression 

 (which should be proved) 



/=V3/D. 



Compute the power for each resistance D/D, and compare 

 with the power found for the same resistances in star-connection. 



For an inductive load, we should multiply by cos to obtain 

 the true power in each resistance. If ED, ID and 6 are the same 

 for each phase, we find total power by multiplying by 3 ; hence 



But 

 hence 



Total power = 3^0/0 cos 6. 

 / = *-*- V3J 



* Some measurements should also be made with unequal resistances. 



