226 POLYPHASE CURRENTS. [Exp. 



factor of phase A ; 6 is the phase difference between /A and E&. 

 The method assumes that EA and EB differ 90 in phase and that 

 electromotive forces and currents follow a sine law. 



The advantage of the tangent method is its simplicity and inde- 

 pendence of the calibration of instruments. The method can be 

 used for determining the power factor of a single-phase load, 

 drawn from a 2-phase supply, and a somewhat similar method 

 can be used for determining the power factor of a 3-phase load, 

 28, 38, 41. 



13. The value of and power factor can be found by the 

 sine method directly from (2); thus, sin = W 2 -=- B/A. For 

 a single-phase or 2-phase load there is little advantage in this 

 method, which is useful, however, on 3-phase circuits, 43. 



14. The " cosine " method gives correct power factor by 

 definition and is general, being independent of wave form. The 

 " tangent " and " sine " methods are based on the assumption 

 that voltages and currents follow a sine* law. The " cosine " 

 and " sine " methods require carefully calibrated instruments. 



15. The three methods are seen to be based on the relation, 



- power volt-amperes 



total volt-amperes 



. wattless volt-amperes 



total volt-amperes 



wattless volt-amperes 

 tan 6 = . 



power volt-amperes 



1 6. General Method for Measuring Power; n i Watt- 

 meters. This method consists in selecting any one conductor of 

 a system and considering it as a common return for all the 

 others. One wattmeter is then used for each conductor, except 



*(i4a). With non-sine waves, the value of power factor by the 



tangent method would, theoretically, be a little larger than the true value 



by the cosine method; the value by the sine method would be a little 

 larger than the value by the tangent method. 



