ELECTRICITY METERS 477 



Keeping these adjustments the same, the meter is then tested 

 at the lower power factor. If it is correctly lagged, the values 

 of K from the two tests will be the same. Suppose, however, 

 that the constant given by the second test is greater than that 

 obtained at unity power factor. This shows that the meter 

 runs too slow at low power factors. Therefore, if the current 

 lags, see equation (6), 



cos (0 -|- a) < cos 

 + a > 6 



therefore a is a positive angle (see Fig. 262) and the meter is 

 underlagged. This means that the resistance R 2 in Fig. 272 

 must be decreased. After the change in R 2 has been made the 

 test is repeated, and so on until the two constants agree. 



If the current had been leading, 9 negative, the result would 

 have been 



cos ( e + a) < cos ( B) 



- e + a > - e. 



Here a must be a negative angle and the meter is overlagged. 



In this connection, attention may be called to the fact that the 

 statement that a power factor is 0.5, for example, may give little 

 indication of the conditions under which the meter is operating, 

 for both the P.D. and current waves may be irregular. With a 

 distorted P.D. wave, one may obtain various current waves, 

 depending upon the method of regulation which is used, the power 

 factor always being 0.5. For instance, if inductances be used, 

 the upper harmonics in the current wave will be suppressed to a 

 certain extent. If the change from unity to a low power factor 

 (0.5) is made by using a three-phase circuit, as shown on page 

 503, the fundamental will be lagged 60, but the harmonics will 

 not appear in their proper phase relations. 



The most exacting test for the lagging is when = 90, for 

 in that case, the meter will register unless a = 0. It is difficult 

 to adjust 6 to ^xactly 90. A natural method is to take the 

 voltage and current from the two phases of a two-phase circuit, 

 but the two e.m.f 's may not be exactly 90 apart and the regulat- 

 ing devices together with the current coils of the instruments 

 may shift the phase of the current slightly. 



Correct lagging is especially important when induction meters 



