ELECTRICITY METERS 



505 



tion watt-hour meter is used on a three-phase load of power 

 factor unity, the error due to incorrect lagging may be appre- 

 ciable, for in this case, although the power factor of the load is 

 unity, one of the elements of the meter operates at a power fac- 

 tor 0.866 leading while the other element operates at a power 

 factor 0.866 lagging (see page 332). 



For other three-phase power factors the conditions under which 

 the elements operate are shown by Fig. 294. 



-.3- 



-.1 



.3- 



X 



.2 .3 



3<f> P 



P.F. 



543 



Lag. 



1 



.3- 



:I 



0.0 



-.1- 



FIG. 294. Showing the power factors at which the two elements of a 

 polyphase watt-hour meter operate when the balanced three-phase load has 

 different power factors. 



Suppose the upper element is underlagged while the lower ele- 

 ment is overlagged. Then, when the upper element operates at 

 a lagging power factor and the lower element at a leading power 

 factor, both elements tend to make the meter register too little. 

 If the elements are interchanged, both tend to make the meter 

 register too high. 



Polyphase induction watt-hour meters, operated through in- 

 strument transformers, are often used in determinations of the 



