452 



ELECTRICAL MEASUREMENTS 



Resistor and Reactor 



sistance to total impedance is made very high, so that the current 



in the primary coil is practically independent of the frequency. 

 The Induction Wattmeter. The induction principle is used 



in switchboard wattmeters. 



Fig. 261 shows, in diagram, the magnetic and electric circuits of 



a switchboard instrument made by the Westinghouse Company. 

 The core is built up of thin stampings. The voltage coil, in 



series with a reactor, is connected across the circuit; it magnet- 

 izes the core as indicated by the 

 arrows. This potential coil flux 

 crosses the air gap in the horizontal 

 direction. The current coils give rise 

 to the flux in the vertical direction. 

 These two fluxes are, therefore, in 

 the appropriate space relation for 

 producing a torque on the thin 

 aluminum cylinder which is pivoted 

 in the air gap. 



As the air gaps are large, the fluxes 

 will be proportional to the currents. 

 Consequently at a fixed frequency, 



K,VI cos 7 



Voltage 



Current 



T = 



sin (/3 2 - 



FIG. 261. Electric and mag- 

 netic circuits of Westinghouse 

 induction wattmeter. 



As before, ft ft is the time-phase 

 difference of the fluxes. 

 The time-phase diagram is given in 

 Fig. 262. 



The potential circuit of an induc- 

 tion wattmeter is purposely made 

 highly inductive so that $1 naturally lags 75 or 80 behind the 

 applied voltage; it will not lag 90 on account of the energy losses 

 in the iron cores and in the windings. 



The angle a is the amount by which $1 falls short of being in 

 quadrature with V. 



a+ (ft - ft) + = 90, 



so the torque is given by 



T= 



-a-6) = 



