1'OLYl'HASE SYSTEMS M 



instrument, if properly connected., reads the total power at all 

 times. 



The two-wattmeter method cannot be used to measure power 

 in a three-phase, four-wire system unless the current in the 

 neutral wire is zero. When the current in the neutral wire of 

 Fig. 91 is zero, the power is correctly indicated by Wi }\'. 

 Now apply load B'O between line B and the neutral. The 

 current to this load will complete its circuit from wire B through 

 the neutral without going through the current-coil of either watt- 



. 91. Two-wattmeter method generally not applicable to a 4- wire system. 



meter. As neither wattmeter can indicate this additional load, 

 the two wattmeters are not sufficient to measure the power in 

 such a four-wire system under all conditions of load. 



TWO -PHASE SYSTEMS 



49. Quarter-phase or Two-phase Systems (Sometimes Called 

 Four-phase). Although three-phase systems are superseding 

 other systems, there arc still many quarter-phase or two-phase 

 system- in existence. 'The two-phase system is rarely used for 

 tran>mi>-ion. but is used for distribution and in some instances 

 it i- specially advan 1 bo u>e two-phase machines. 



Quarter-phase current is generated in the elementary gen- 

 eratn J (-,, by two OOils .1 and H. 90 apart. I 



tin- emf. wa rated by these cnil>. The voltage 



that of H by 90. When one voltage i> a maximum 

 I i ume 92 (c) shows these two-phase voltages 

 represented veetorially. 



