2 3 2 



POLYPHASE CURRENTS. 



[Exp. 



which is seen to be the expression for the total power in a 3-phase 

 circuit (20, 21, Exp. 6-A). The two-wattmeter method for 

 a balanced 3-phase load is thus established. 



28. Pozver Factor. Subtracting W 2 from lV lt we have 



W 2 = 2EI sin 30 sin = EI sin 0. 



Hence, by dividing, we have 



tan0 



The value of 6 and of power factor (cos 6) for a balanced 

 3-phase circuit is, accordingly, determined by the tangent formula 



The larger reading is W lf and is always positive; the smaller 



reading, W z , may be positive or negative. 



29. To save labor in com- 

 putation, it is convenient to 

 plot a curve, Fig. 3, with 

 power factor (cos 6) as or- 

 dinates and the ratio of watt- 

 meter readings, W 2 -=- W^ as 

 abscissae. Points on this curve 

 are determined by the relation 



jativ 



11 



cos 



30) 



w, 



cos 



-1.00-.50 ,60 -.40 ,20 + 20 +.40 ^60^80 +1 JO 

 Ratio of Wattmeter Readings, Wf* "i 



30) 



CUFVC, tllC 



g means Q f 



* 



FIG. 3. Power factor of balanced P Ower fact r f r a balanccd 



3-phase circuit for different ratios of load is readily determined 

 wattmeter readings in two-wattmeter frQm the ratio of the two 

 method. 



wattmeter readings. For plot- 

 ting the curve in Fig. 3, the following points were -determined: 



