234 



POLYPHASE CURRENTS. 



[Exp. 



the load is balanced, the power in a 3-phase system can be 

 measured with only one wattmeter without such transfer by one 

 of the following methods. 



33- With Neutral Available. When the neutral is available, 

 the current coil of the wattmeter can be connected in any one 

 line circuit and the potential coil connected from that line to the 

 neutral. For a balanced load, the total power will be three times 

 the reading* of the wattmeter. The power factor is equal to 

 W-t-EI, where / is the line current and E is the star voltage. 

 When the load is not balanced the total power will be the sum of 

 three readings, one on each phase. 



34. With Artificial Neutral. When the neutral is not avail- 

 able, an artificial neutral can be created, as by means of three 

 equal star-connected non-inductive resistances, R lf R 2 , R s in 

 Fig. 4. The method of 33 can then be applied. 



It is necessary that these resistances be relatively low, as com- 

 pared with the resistance of the potential circuit Rw of the 



wattmeter. The current in 

 them will then be relatively 

 large, so that the potential of 

 the neutral will not be dis- 

 turbed by the connection of 

 the potential circuit of the 

 wattmeter. The power taken 

 in the resistances may be in- 

 cluded or not in the measured 

 power as desired; correction for this power can be made when 

 necessary. 



35. Strictly speaking 7^ and R 2 should each be equal to the 

 joint resistance of R s and Rw in parallel. In this case there is 

 no need of making the resistances low ; this leads to the method 

 of 36 in which R z is omitted entirely, that is, R 3 = ao. 



*(33a). Calibration for Total Power. In this method, or in any 

 method depending upon a single reading, the wattmeter can be calibrated 

 to read total power. 



FIG. 4. Measuring power with one 

 wattmeter connected to the neutral in 

 a balanced 3-phase circuit. 



