POLYPHASE SYSTEMS 



89 



the wattmeters TT 2 and W 3 are not necessary. As a rule these 

 two equal resistances are mounted in the same box and are 

 connected as shown in Fig. 87. Accurate results can be obtained 

 with this method only when the loads are balanced. 



48. Two-wattmeter Method. The power in a three-wire, 

 three-phase system can be measured by two wattmeters con- 

 nected as shown in Fig. 88. The current-coils of the two instru- 

 ments are connected in two of the lines and the potential-coil 

 of each instrument is connected from its respective line to the 



FIG. 88. Two-wattmeter method of measurinR 3- phase power. 



third line. Under these conditions the total power passing 

 through the system 



P = w l W> 



Hess of power-factor, balance, etc. The choice of the plus 

 or the minus siirn will he explained later. 



One method of proving that these instruments give the correct 

 [K>\ver is as follows: Let e\, e z , * and /,, i' a , is be the respective 

 voltaurs and currents of t In- t hree loads at any particular instant. 

 lieinn instantaneous values, the power at the instant 

 under consideration is equal to their products regardless of 

 power-fiic tor. Thai is. the instantaneous power 



Hut 



p = 



i'i + it + t = ( KirchhofT's first law) 

 ii = -O'i -f t'i) 



