472 



ELECTRICAL MEASUREMENTS 



P.D.s = potential difference at terminals of current and lag coils. 



8p = phase displacement of potential-coil current with respect to V. 

 0c = phase displacement of current in current coils with respect to 



line current, 

 co = 27r times frequency. 



Fia. 271. Diagrams for lag coil of commutating meter. 



On account of the reactance of the field coils, the current I c in 

 them will lag behind (PD) S as indicated. The vector sum of 

 I s and I c is the total or load current, I. By adjusting the resist- 

 ance R s the current in the field coil I c may be made to lag behind 

 the main current I by an angle equal to the lag of the current in 

 the potential coil behind the applied voltage; in other words, 6 C 

 may be made equal to P . The instrument then becomes 

 practically correct. 



Analytically, for the potential circuit, 



/, = v-^^- 



The lag of this current behind the potential will be 



BP = tan- 1 -- 



For the shunted current coils 



R P 



(PD)s 



R s 

 Rc - i 



I c lags behind PD S by the angle 

 The main current is 



tan- 1 



R 



Rc 2 + 



<Lc 1 

 f"(coLc) 2 J 



