TRI-PHASE CURRENTS. 145 



shows the resultant leading before the curve i Q sin pt by one- eighth 

 of a period, and lagging behind the curve i Q sin (pt 4- 9 J by the 



same amount. 



TRI-PHASE CURRENTS. 



96. If, instead of tapping the armature of an alternator in 

 two places 180 apart, as for monophase currents, or in four places 

 90 apart, as for di-phase currents, it is tapped at three equidistant 

 points 120 apart, each of the three portions of the armature will 

 be the seat of an alternating E.M.F., but the E.M.F.s will be 

 mutually out of phase by one-third of a period. Each line con- 

 ductor will therefore carry an alternating current of the same 

 intensity and frequency, but mutually differing in phase by one- 

 third of a period. 



Fig. 49 shows diagrammatically such a tri- phase generator in 



FIG. 49. 



which the internal field-magnet rotates whilst the external 

 armature is stationary. 



If the current in one of the line wires is represented by ?' sin^, 



those in the other two wires will be i sin (pt + * J and i sin 

 (pt + --_ J respectively. 



