6-B] 



MEASUREMENT OF POWER. 



235 



36. With Y '-Multiplier. With a balanced load and with one 

 wattmeter, the current coil of the wattmeter is connected in one 

 line. One end of the potential circuit is connected to the same 

 line, the other end being con- 

 nected to the junction of two 

 resistances R^ and R 2 , which 

 are connected to the other two 

 lines, as shown in Fig. 5. The 

 resistances R and R 2 are non- 

 inductive and are each equal* 

 to Rw, the resistance of the 



FIG. 5. Measuring power with one 

 wattmeter and a Y-multiplier in a bal- 

 anced 3-phase system. 



w 



potential circuit of the watt- 

 meter. 



True power is three times, the reading of the wattmeter, cali- 

 brated as a single-phase instrument. The resistances are some- 

 times put up in a special volt-box or Y-multiplier for 3-phase cir- 

 cuits; the instrument may then be 

 calibrated so as to read total power, 

 33a. 



37. By Means of T-connection. 

 In a 3-phase system, with three lines 

 X, Y, Z, connect the current coil of 

 the wattmeter in any one line, as Z, 

 Fig. 6. Connect the potential coil 

 from Z to a point O, the middle 

 point of a transformer coil across XY. 

 See Fig. 15, Exp. 6-A. In any balanced 3-phase system, how- 

 ever the load is connected, the wattmeter will now give one half 

 the total power. (This may be seen as follows : If the wattmeter 



*(36a). Provided Ri and Rz are approximately equal to each other, 

 this same method may be used without having R t and -R 2 equal to 7?\y. 

 The instrument is calibrated as a single-phase wattmeter with Ri and R 2 in 

 parallel with each other and in series with Rw', a single reading then 

 gives one half the total power. Compare pa, 333. 



FIG. 6. Measuring power 

 with one wattmeter, T-con- 

 nected, in a balanced 3- 

 phase circuit. 



