128 



USRL TEST STATIONS 



>»$LS^JM^ 



OUTPUT 



2 



-vW — t — ° 



T 



£. 



Qm 



Figure 66. Circuit schematic of thermocouple wattmeter; 

 circuit arrangements are shown for: (A) power measure- 

 ment, (B) current measurement, (C) voltage measurement. 



dances from 10 to 300 ohms and has power scales of 

 100, 500, and 1,000 watts. The indicated and actual 

 scale factors agree within 1.5 per cent from to 100 

 kc. Operation outside the impedance limits will not 

 only affect the accuracy and overload the thermo- 

 couples but even may destroy them. 



While the thermocouple wattmeter is direct-read- 

 ing and fairly accurate, the time required to reach 

 temperature equilibrium delays the readings so that 

 point by point measurements are required. The pos- 

 sible destruction of the thermocouples from sudden 



changes in load impedance and the limited impe- 

 dance range for any one power scale are distinct dis- 

 advantages. The recording wattmeter was designed 

 to overcome these difficulties. 



Recording Wattmeter. 66 In measurements with the 

 recording wattmeter, two signals designated 2 and A 

 are obtained and recorded. The signal 2 is obtained 

 by adding a signal which is n times the current i to 

 one which is m times the voltage e. The signal A is the 

 difference between ni and me. It can be shown that 



A 2 



■imn 



ei cos 6 = Power, 



(2) 



where 8 is the phase angle between the current and 

 voltage. The 2 and A signals are recorded on the in- 

 termediate-frequency systems in the usual manner. 

 The measurement of total power is correct only for 

 sinusoidal waves. For portable use, separate ampli- 

 fiers and meters may be used in place of the recorders. 



Figures 67 and 68 show the connections used in 

 obtaining the 2 and A terms for unbalanced and bal- 

 anced circuits. 



In the actual circuit for an unbalanced load, a coil 

 and pad replace the i? :) resistors in turn to obtain the 

 2 or A signal. The voltage component is obtained by 

 short-circuiting R., and the current component, by 

 disconnecting R 1 . 



In the balanced condition, the only new factor to 

 be considered is the stray capacity between the two 

 high potential terminals of the driving coil. This may 

 be neglected for the usual load impedances. The im- 

 pedances of the secondary windings will not affect the 

 measurements. 



It can also be shown that, if R 2 is center-tapped to 

 ground, the meter will measure the power delivered 



'iCT-7 



P'E 



1 



R 2 



Figure 67. Recording wattmeter circuit for unbalanced 

 load. 



.W 



cv-^ 



'4 R 3 



!feR, 



K~V 



!fe«, 



£«-A 



n_i- 



Figure 68. Recording wattmeter circuit for balanced load. 



