HO ELEMENTS OF ELECTRICAL ENGINEERING. 



when it flows away from the generator as shown by the arrows 

 l n Fig. 100. From this figure it is evident that a positive current 

 in winding A produces a positive current in main i, and that a 

 positive current in winding B produces a negative current in main 

 I. Therefore 7 X is equal to a b (vector difference). Similarly, 

 7 2 is equal to b r, and / 3 is equal to c a. These relations 

 are shown in Fig. 104, and from this figure we see at once that 

 the value of the current in each main is i/j times the current in 

 each ^-connected armature winding. 



2. Referring to the receiving circuits, (a) Electromotive force 

 relations. In the A-scheme of connection the electromotive force 

 acting on each receiving circuit is identical with the electromotive 

 force between each pair of mains. 



(ft) Current relations. Given the currents, a y b and c in the 

 respective receiving circuits (see Fig. 104), then the currents in 

 the respective mains are represented by the lines f v / 2 and / 3 . 

 Therefore, the value of the current in each ^-connected receiving 

 circuit is equal to the current in each main divided by V '3. 



57. Power in polyphase systems. When the receiving circuits 

 are unlike (system unbalanced) the amounts of power delivered 

 to each are in general different, and the system must be treated 

 as independent single-phase systems. The only case in which 

 the power relations in a polyphase system can be treated on a 

 simpler basis than that corresponding to independent single-phase 

 systems, is the case in which the system is balanced. In this 

 case there are two important matters to be considered : (a) The 

 constancy of power, and (^) the value of the power. 



(a) Constancy of power in balanced polyphase system. In the 

 single-phase alternator the instantaneous value, ei, of the power 

 pulsates as explained in Art. 8, whereas a polyphase generator 

 delivers a perfectly steady flow of energy, that is, a steady 

 power, to balanced receiving circuits. In this case the power 

 delivered to each receiving circuit of course pulsates, but the 

 pulsations are so related that the sum does not pulsate. An 



