THE POLYPHASE SYSTEM. 



IOI 



basis of this choice of signs, the clock diagram Fig. 92 represents the facts. Then the 

 vector sum A -\- B represents the electromotive force from main one across to main 

 two, and the vector difference, a b, of the currents in x and y represents the current 

 in the middle main. If, however, the positive directions in winding B and receiving 

 circuit^/ be chosen opposite to the directions shown in Fig. 91, then, if the physical 

 facts remain unchanged, the lines B and b in the clock diagram must be reversed, 

 the electromotive force from main one to main two will be represented by the vector 

 difference A B, and the current in the middle main will be represented by the 

 vector sum a -\- b. If in the first case A is 90 ahead of B, then in the second case 

 A must be considered as 90 behind B because of the change in the choice of signs. * 



51. The three-phase alternator. A three-phase alternator is an 

 alternator upon whose armature three distinct windings A, C and 

 B are placed, each having, in the most general case, a separate 

 pair of collector rings, and the windings are arranged so that the 

 electromotive forces A, C and B are 60 apart in phase, f 



The four-ring arrangement of the three-phase alternator. A 

 number of batteries can supply currents to a number of distinct 

 bell circuits, and all of the bell circuits can have a common re- 



Fig. 93. 



turn wire as shown in Fig. 93. In the same way the three 

 armature windings, A, B and C, of a three-phase alternator can 

 deliver currents to three receiving circuits x, y and z, a common 

 return wire being used as indicated in Fig. 94. In this case one 



* See A Discussion of some Points in Alternating-current Theory [Section (b) On 

 the necessity of choosing signs in complicated networks], by W. S. Franklin, Tran- 

 actions of the American Institute of Electrical Engineers, Vol. 21, page 589, 1903. 



| It will be found convenient later to consider the three electromotive forces A, B 

 and C to be 120 apart in phase, the change being merely a matter of the choice 

 of signs. See Art. 53. 



