60 PCW'KK AND I'< AVER-FACTOR. 



The readings of an ammeter or voltmeter are independent 

 of the phase-difference between current and voltage. Thus, 

 the product of ammeter and voltmeter readings will only 

 give the true value of the power in the circuit when current 

 and voltage are in phase with one another. Under these con- 

 conditions the apparent power, i.e., the product of amperes 

 and volts, is also the real power. 



Under any other circumstances the product of ammeter 

 and voltmeter readings will give a value of the " apparent 

 watts," which is greater than the true power. 



The greater the difference of phase between current and 

 voltage, the greater is the negative power, and the less the 

 resultant output. If the phase difference is exactly 90, or 

 a quarter of a period, the positive and negative power are 

 equal, and the average power developed is zero. This 

 should be verified by the student, who should repeat the 

 curves shown in Figs. 25 and 26 with 90 difference of phase, 

 and calculate the average power under these conditions. 



Thus it follows that only in a non-inductive circuit is 

 the power equal to the product (volts x amperes). 



The Power-factor is the ratio of the true watts to the 

 apparent watts or volt-amperes. 



f actual power in watts 



Power-factor = - - ' 



volts x amperes. 



Power of circuit = volts x amperes x power-factor. 



As explained in the next chapter, an ammeter in a circuit 

 carrying a current with a maximum value of 25 amps., would 

 read 17-68 amps., and similarly the voltmeter would give a 

 reading of 28'3 volts with a maximum voltage of 40. 



The product of these readings, 17-68 x 28-3 == 500, is, 

 therefore, the apparent power in both cases shown in Figs. 

 25 and 26. 



Thus the power-factor of the circuit for which Figs. 25 

 and 26 were drawn is the ratio of the height of the line A B 



430 



to the height of CD, i.e., _ fk , r = '86, in the case of Fig. 25, 



ouu 



and 4- = 1-0 in the case of Fig. 26. 



The comparison of the power in a circuit as measured on a 

 wattmeter with the " apparent watts " or volt-amperes 

 forms one of the most direct methods of determining the 

 power-factor of a circuit. 



