536 



ELECTRICAL MEASUREMENTS 



correctly at the normal frequency, any change of frequency will 

 render the readings inaccurate, because both the phase and 

 the magnitude of the current in coil B are controlled by an in- 

 ductance and, therefore, depend on the frequency. At low fre- 

 quencies coil B carries too much current, at high frequencies too 

 little current and condition (a) (page 534) is not fulfilled. The 

 result of frequency changes on a single-phase power-factor meter 

 is illustrated by Fig. 317. Single-phase power-factor meters are 

 made but are not in common use. 



Polyphase Power-factor Meters. The indications of poly- 

 phase power-factor meters are correct only on balanced circuits. 

 If the circuit is unbalanced the reading is without significance. 



FIG. 318. Indicating portion of Weston power-factor meter. 



The application of the Tuma phase meter to a balanced two-phase 

 circuit is obvious. The two crossed coils are placed 90 apart in 

 space; their currents, 90 apart in time phase, are obtained by 

 using a resistance in series with each coil and energizing a coil 

 from each of the two phases. To adapt the instrument to bal- 

 anced three-phase circuits it is to be remembered that the angle 

 between the planes of the movable coils should be made equal to 

 the electrical angle between the currents in these coils. The fixed 

 coil is placed in one of the line wires, while the movable coils are 

 connected from this wire through resistances to the other two 

 wires of the circuit, see Fig. 319. 



