THE POLYPHASE CIRCUIT. 295 



quarter period, so that one current passes through its 

 maximum value as the other is at zero, and vice versa. 



The relation between the currents or voltages of the 

 two circuits may be represented by two equal rotating 

 lines at right angles to each other, as in Fig. 141. The 

 lines are not shown joined together, since the quantities 

 represented by them are in two separate circuits. The 

 power supplied by the generator will be twice the power 

 given to either circuit. 



Three-phase Current. If three windings are applied to 

 the armature with equal spacing between the conductors 

 of each winding, the phases of the electromotive forces 

 induced in the windings will differ by one-third period, so 

 that there will be a point of maximum electromotive force 

 in each direction induced in each winding in rotation. 



FIG. 141. RELATION BETWEEN FIG. 142. RELATION BETWEEN VOLTAGES IN 

 VOLTAGES IN A 2-pHASE CIRCUIT. AS-PHASE CIRCUIT. 



If the three windings form completely separate circuits, 

 the armature will have six terminals, and the current and 

 voltage relations may be represented by three rotating, 

 lines, each making an angle of 120 with the other two 

 (See Fig. 142.) The power developed by the machine will 

 be three times the power of each winding. A system of 

 this kind is termed a 3 -phase system, and consists of three 

 alternating circuits, having equal voltage and periodicity, 

 and a fixed phase relation of such a kind that there is one- 

 third period phase difference between the voltages of any 

 pair of circuits. 



From what has just been said, it appears that a 2-phase 

 current requires for its transmission four wires, and a 

 3-phase current similarly requires six wires, and an n-phase 



