6-B] 



MEASUREMENT OF POWER. 



2 33 



I. .80 .60 40 .20 O +.20 +40 +.60 +.80 +1. 



cos o .064 .143 .240 .359 -5 -655 .803 .918 .982 i. 



Intermediate values can be found by interpolation. It is seen 

 that the curve is not symmetrical. 



30. Errors of calibration are avoided if one wattmeter is 

 used, successively, in the two positions to determine W and W 2 . 

 Since the assumption is made that the current in the wattmeter 

 is the same for the two readings (7 x = /y = /), greater accuracy 

 is obtained if the current in the two cases is actually the same 

 current. This is accomplished by using the one wattmeter method 

 of Fig. 7, which is more accurate for determining power factor 

 than is the two-wattmeter method. In either case, corrections 

 may be made (42) for slight variations in voltage. 



31. Negative Reading of Wattmeter. Referring to Fig. 2, 

 it is seen that for all values of 6 from o to 90, the projections of 

 /Y upon Ezx have the same sign; the wattmeter reading W l is, 

 therefore, in all cases positive. 



The projection of /Y upon EYZ decreases as 6 increases, be- 

 comes zero when = 6o, and then changes sign. The watt- 

 meter reading W 2 , accordingly, changes sign, being positive when 

 6 is less than 60 (power factor more than 0.5) and negative 

 when is more than 60 (power factor less than 0.5). In all 

 cases W^ is the larger, and W 2 is the smaller, reading. 



On non-inductive load, = o and IV 1 = W 2 ; each wattmeter 

 reads half the total power. When # = 90, W= W 2 and the 

 total power is zero. 



32. Three-Phase Power with One Wattmeter. With only 

 one wattmeter, 3-phase power can be measured by the two-watt- 

 meter method (23) by using suitable switches for throwing the 

 wattmeter from one position to the other. This procedure gives 

 the true power for unbalanced as well as balanced loads and is 

 generally the best one to follow. 



The transfer of the current coil of the wattmeter from one 

 line to another is not always convenient or possible and, when 



