INDUCTION INSTRUMENTS 



453 



The power is VI cos 6. If a = 0, that is, if the useful potential- 

 coil flux is adjusted so that it is exactly 90 out of time phase with 

 the applied voltage, the torque will be proportional to the power 

 in the circuit. This, of course, is the proper adjustment and 

 must be attained by the addition of special phase shifting or 

 lagging devices by which the useful potential-coil flux in the air 

 gap is brought into quadrature with the line voltage. 



If the useful flux lags behind the applied voltage by less than 

 90, that is, if a is + , the wattmeter is said to be under-lagged ; 

 if a = 0, it is exactly lagged and if a is , that is, if the flux has 

 been caused to lag more than 90, the instrument is -over-lagged. 



V Applied to Load 



| and Potential Coil 

 Lag Angle of Load 



Practically in Phase 

 with Load Current / 



Unlagged, Practically In Phase 

 with I Current in Potential Coil 



FIG. 262. Time-phase diagram for induction wattmeter. 



The construction of the magnetic circuit of the potential coil 

 should be such that the useful potential-coil flux is naturally 

 nearly 90 out of phase with the applied voltage; the required 

 amount of additional lagging is thus reduced The less the re- 

 liance placed on the lagging device the better will the instrument 

 behave when it is subjected to wide variation of frequency and 

 to distorted wave form. 



In the instrument shown in Fig. 261 the desired quadrature 

 relation of the applied voltage and the useful potential-coil flux 

 is obtained by use of a second winding, S, the circuit of which 

 may be closed through the resistance Rz. 



It will be seen that this lagging arrangement is a sort of 

 transformer with a large primary leakage flux, much of which is 

 in the series reactor. There is considerable resistance in the 



