ALTERNATING CURRENTS 



assumed) forming a two -phase, and Fig. 20 the graphs of the currents 

 forming a three-phase system. 



The vector diagram for a two-phase system consists, as shown in 



\ 



FIG. 20. Graphs of Three-phase Currents. 



Fig. 21 (a), of two vectors at right angles to each other; while that 

 for a three-phase system, Fig. 21 (&), consists of three vectors making 



angles of 120 with each other. 



(6) 



(a) iU 



FIG. 21. Vector Diagrams of Two- and Three-phase Systems. 



Algebraically, the expressions for the currents forming a two- 

 phase system may be written 

 ii = Ii sin pt 



i z = I 2 sin (pt -f | J = 1 2 cos pt 



