THE POLYPHASE SYSTEM. 



59. Use of two wattmeters for measuring accurately the power 

 delivered by any three-wire system. The total power delivered 

 by any three-wire system may be measured by two wattmeters 

 with their current coils placed in two of the mains, and their 

 voltage coils connected from each of these two mains to the third 

 main. Thus Fig. 105 shows two wattmeters connected for 



main I 



Fig. 105. 



measuring the power delivered by a three-wire three-phase sys- 

 tem. The receiving circuits may be either A-connected or 

 Y-connected. 



In some cases the two wattmeter readings are to be added, and 

 in other cases the lesser reading must be subtracted from the 

 greater to give the total power delivered. To determine whether 

 the readings are to be added or subtracted, disconnect one watt- 

 meter and put the other in its place, leaving the same terminal 

 of the voltage coil connected to the middle main and connecting 

 terminals i and 2 of the one current coil in place of terminals I 

 and 2 respectively of the other current coil. If the deflection 

 of the transferred wattmeter is reversed, the readings are to be 

 subtracted. 



To show that the sum of the two wattmeter readings gives the total power. Let 

 th^ positive directions in the mains I and 2 and in the three receiving circuits be 

 chosen as shown by the arrows in Fig. 105. These directions are chosen symmetric - 



9 



