578 KLKf'TffJcM. .V/.M 



The true watts must be multiplied by the appropriate trans- 

 former ratios to give the power in the circuit. Obviously, the 

 effect of the phase angles increases as the power factor of the 

 load decreases. 



To illustrate the foregoing, consider the following data: 



The losses in the instrument- arc neglected. 



Inductive load, 25 cycl< 



Reading of ammeter, corrected for calibration, 1.5 amp. 



Reading of voltmeter, corrected for calibration, 110.5 volts. 



Reading of wattmeter, corrected for calibration, 50.0 watts. 



O r is negligible. 



Nominal ratio of current transformer, 8:1. 



Ratio of current transformer from its curve, 1.0125 X 8. 



Phase angle 0, from current transformer curve, 1.79. 



Nominal ratio of potential transformer, 10 : 1. 



Ratio of potential transformer from its curve, 0.995 X 10. 



Phase angle y from potential transformer curve, 0.12. 



F lags behind VY 



50 



Apparent power factor = cos 0' = , ., ' , , = 0.30 Hi. 



i lu.o x i '* q^. 



Apparent power-factor angle, 72.45.^- 



True power-factor angle = = 72.45 + 1.79 + 0.12 = 74.36. 



True power factor = cos = 0.2696. 



O 9AQA 



True value of the load = X 50 X 8.1 X 9.95 = 3,002 watts, 



which is the corrected reading of the wattmeter multiplied by 

 the proper transformer ratios. 



Effect of Phase Angles in Three-phase Power Measurements. 

 -The case which will be considered is that of a balanced three- 

 phase load where the power is measured by the two- wattmeter 

 method using transformers of the same characteristics in both 

 phases. When the power factor of the load is low the watt- 

 meter, in the coils of which the currents are the more nearly in 

 phase, indicates the larger part of the load. The reading of the 

 other wattmeter, which works under much more adverse condi- 

 tions as to the phase displacement of the currents in its coils, is 

 small so that even if the percentage error in its readings be large, 

 the percentage error introduced by it into the measurement of 

 the power will be small. 



