POWER MEASUREMENT 



331 



The two wattmeters evaluate the integrals. 



Two -wattmeter Method. Three-phase system, load con- 

 nected in Y. 



By Blondel' s theorem two wattmeters are required. Referring 

 to Fig. 194, the instantaneous power is 



p = Vwii + vwiz + vaois 



Viz = #10 ~h #02 



#32 = #30 4- #02 



t'l 4- 2*2 4- 2*3 = 



P = *l(#12 + #20) 4- 2*2#20 4- ^(#32 4- #2fl) 

 4- *3#32 4- (il + it + ^) #20 



(28) 



The two integrals are given by the wattmeters. 



FIG. 194. Power measurement 

 with 7 load. 



FIG. 195. Power measurement 

 with A load. 



A A-connected load is similarly dealt with. 



To draw the vector diagram corresponding to Fig. 195, assume 

 that the load is balanced. Let V 12 , F 23 , V 3 i be the line voltages 

 and & the phase angle between the potential applied to, and the 

 current in, any branch of the circuit. 



From the figure it is evident that instrument No. 1 gives the 

 mean product of the instantaneous value of Viz and I\ while No. 

 2 gives the same product for VM and 7 3 . Using the effective 

 values of line voltage and line current (refer to Fig. 196) 



Reading of No. 1 = VI cos (30 + 0) = VI (- sin sin 30 

 -f cos cos 30) 



