POWER MEASUREMENT 333 



and 3. If it is necessary to reverse the current coils, the smaller 

 of the readings is considered negative. 



The Polyphase Wattmeter. To avoid the necessity of using 

 two separate instruments the polyphase wattmeter has been 

 devised. 



The instrument consists of two complete wattmeters mounted 

 in the same case. The two movable coils are attached to the 

 same rigid stem and consequently act against the same spring. 

 The deflection is thus made to depend on the sum of the torques 

 of the two elements, so that the total power is read directly from 

 the scale. The electrical connections are the same as for two 

 single-phase wattmeters, see Fig. 194. 



Ample insulation must be provided between the two elements 

 and it is imperative that the stray field from one element have 

 no influence on the torque generated by the other element. 

 Protection from both external and internal stray fields may 

 be obtained by the use of laminated shields. 



In careful tests at low power factors the use of the polyphase 

 wattmeter in connection with instrument transformers is to be 

 avoided, for the necessary corrections for the ratio and phase 

 angle of the current transformers cannot be made. 



Fig. 197 shows two forms of polyphase wattmeter. The 

 laboratory instrument is read by means of a torsion head and 

 the effect of the stray field from the upper element on the torque 

 of the lower element and vice versa is minimized by placing the 

 elements with their axes perpendicular. 



In switchboard instruments the interference between the 

 elements, which would naturally be large, may be compensated 

 by an ingenous device due to Edward Weston. Ordinarily the 

 potential circuits of both elements of a polyphase wattmeter 

 are connected directly to lead number 2 (see Fig. 195). Then, 

 if there is no interference and the resistance of each potential 

 circuit is ri + r ohms, the turning moment acting on the mov- 

 able system is, when the potential coil currents are in and isz, 



K C T K C T 



= ip I iiiitdt + ^ I ijiw 



