88 



ALTERNATING CURRENTS 



If the potential circuits of the three wattmeters have equal 

 resistances, these three potential circuits constitute a balanced 

 Y-load, having a neutral 0' . As coils 1, 2, and 3 and these three 

 wattmeter potential-circuits are both symmetrical systems, O'must 

 be at the same potential as 0. Therefore, no current flows between 

 O and 0' and the line can be cut at X without changing existing 

 conditions. Figure 86 (6) shows the three-wattmeter connection 

 for a three-phase system. It can be shown that the total power 

 is the sum of the wattmeter readings even though the wattmeter 

 potential-circuits have different resistances. Under these condi- 

 tions, however, the wattmeters may not all have the same reading, 

 even with balanced loads. 



Source 



Load 



1 



FIG. 87. Use of the Y-box for measuring 3-phase power. 



The three-wattmeter method is well adapted to measuring 

 power in a system where the power-factor is continually changing, 

 .as in obtaining the phase characteristics of a synchronous motor. 

 If the three instruments have equal potential-circuit resistances, 

 they read alike regardless of power-factor, if the loads are bal- 

 anced. The three-wattmeter method is necessary in a three- 

 phase four- wire system, as a system of n wires ordinarily requires 

 n 1 wattmeters in order to measure the power correctly. 



The Y-box. The use of the Y-box is based on the principle 

 that each of the three wattmeters of Fig. 86 reads the same, if the 

 loads are balanced. Under these conditions the total power P = 

 3TTi. If two resistances, each equal to the resistance of the po- 

 tential-coil of Wij be used in conjunction with this potential-coil, 



