THREE-PHASE POWER MEASUREMENT 



25. Correction for Loss in Wattmeter 



I a't timing to the diagram of Fig. 31, which shows the connections 

 of the wattmeter circuits, we notice that when the fine-wire or shunt 

 circuit is arranged as shown by the full line, the power measured 

 includes that wasted in the shunt circuit of the wattmeter, whereas 

 with the dotted line connection the power measured includes that 

 wasted in the current coil of the wattmeter. A correction is easily 

 made in either case by calculating the power wasted from the known 

 resistances of the circuits and the currents passing through them 

 The error may also be compensated for by providing an auxiliary 

 fine- wire winding on the main coil, having a number of turns equal 

 to that on the main coil, and arranged so as to oppose the effect of 

 the main winding; the auxiliary winding being included in the 

 fine-wire circuit. This arrangement suffers, however, from the dis- 

 advantage of increasing the self-inductance of the shunt circuit. 



26. Power Measurement in Three-phase 

 Circuits 



The measurement of power in a single-phase circuit is readily 

 effected, as explained in 21, by the use of a wattmeter. A similar 

 method, involving the use of two wattmeters, is applicable to a two- 

 phase circuit, one wattmeter being included in each phase. If the 

 phases are coupled, the wattmeter connections remain unaltered, and 

 are as shown in Fig. 35, where Ci and 

 2 denote the current coils of the watt- 

 meters, pi and p% their pressure coils, and 

 r\ and r% the high non-inductive re- 

 sistances connected in series with the 

 pressure coils. The sum of the two 

 wattmeter readings gives the total power 

 in the two-phase system. If only a 

 single wattmeter is available, it may be 

 used to obtain the two readings in suc- 

 cession; this, however, is not so satis- 

 factory as a simultaneous reading of two 

 wattmeters. 



The measurement of power in a three- FIO. 35. Power Measurement in 

 phase circuit is a much more compli- Two- phase Circuit, 



cated matter, and it is only in certain 



special cases that it approaches the simplicity of single-phase power 

 measurement. We shall first consider the most general method of 



