6-B] MEASUREMENT OF POWER. 225 



That the method is not generally correct will be seen by assum- 

 ing the current coils of the two wattmeters to be connected in 

 two of the lines, as A and B ; neither wattmeter would then 

 record a single-phase load drawing current from the other two 

 lines, A'B'. 



On a 4-wire system, with unbalanced load, at least three watt- 

 meters must be used, 16. 



ii. Power Factor in a Two-phase Circuit. If E, I and W 

 are measured on one phase of a 2-phase circuit, W -\- El is the 

 power factor for that phase, 4. This may be called the cosine 

 method for determining power factor, since W -=- El = cos 

 when currents and electromotive forces are represented by sine 

 waves. 



12. The following tangent method for determining power 

 factor from two readings of the wattmeter will be found simple 

 and often convenient. 



The current coil of the wattmeter is connected in one line of 

 phase A ; the potential coil is connected across phase A, whose 

 voltage is E\. The wattmeter now reads the power volt-amperes 

 or true watts 



(1) W l = E A I A cosO. 



Transfer the potential coil to phase B, whose voltage is EE- 

 The wattmeter now reads the wattless or quadrature volt- 

 amperes (sometimes called wattless, or quadrature, watts), 



(2) W 2 = B /A sin 0. 

 Dividing the second reading by the first, 



Tan 6, and hence power factor (cos0), is determined by the 

 ratio of the two readings. Usually B = A, so that tan = 

 W 2 -4- W^. The power factor thus determined is the power 

 16 



