504 



ELECTRICAL MEASUREMENTS 



A power factor of zero may be obtained from a balanced three- 

 phase circuit, as shown in Fig. 291. 



The currents I\ z and /i 3 must be equal and the resistances 

 non-reactive. 



A power factor of zero may also be obtained from a two-phase 

 circuit, the voltage being taken from one phase and the current, 

 through a non-reactive resistance, from the other. It must be 

 assured at the beginning that the two phases are really in time 

 quadrature and that the inductances of the current coils do not 

 cause an appreciable phase displacement. Where a two-phase 



FIG. 291. Arrangement for obtaining zero power factor from a 

 three-phase circuit. 



current is obtained from a three-phase circuit by Scott trans- 

 formers, unless the wave forms of the primary supply are 

 sinusoidal, the wave forms in the secondaries may be badly dis- 

 torted, one being flat-topped, the other peaked. 



With any of these phase-shifting devices it is important that 

 the voltage and current waves be sinusoidal; for a 60 displace- 

 ment of the fundamental in the current wave with respect to the 

 fundamental in the voltage wave implies a 180 displacement of 

 the third harmonics, a 300 displacement of the fifth harmonics 

 and so on. A statement that the load has a power factor of 0.5 

 gives little idea of the conditions under which the watt-hour 

 meter is operating. The changed phase relation greatly compli- 

 cates the behavior of induction meters. 



Testing Polyphase Induction Meters. When a single-phase 

 induction watt-hour meter is used on a non-inductive load, the 

 error due to incorrect lagging is negligible. If a polyphase indue- 



