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.1 /. TURN A TING CVRRKX 7'N 



169. Transmission Line Reactance; Three-phase. In trans- 

 mission line problems it is convenient to consider the reactance 

 of the individual conductor, rather than the reactance of the 

 looped line or of the entire circuit. The convenience becomes 

 more apparent when three-phase lines are considered. In Fig. 

 345 are shown the three conductors of a three-phase line, symmet- 

 rically spaced. That is, each conductor is at an apex of the same 

 equilateral triangle. The current at the instant shown is flowing 

 outward in conductor A and inward in conductors B and C. 

 The field produced by each conductor is indicated. These fields 

 are continually changing, due to the cyclic variation of the current 

 in the three phases, and this causes a rotating field in the region 



FIG. 345. Three 

 spaced conductors 

 line. 



symmetrically- FIG. 346. Magnetic field pro- 



of a 3-phase duced by conductor C does not link 

 loop AB. 



between the conductors. This rotating field is similar to the 

 rotating field of the polyphase induction motor, and as it cuts 

 all three conductors, it induces electromotive forces in them. 



In treating this problem, however, it is simpler to consider the 

 reactance of each conductor separately. If the spacing is sym- 

 metrical, the flux produced by each conductor does not induce 

 any electromotive force in the circuit composed of the other two 

 conductors. For example, Fig. 346 shows the circular field 

 produced by conductor C acting alone. As none of its lines links 

 the circuit AB, conductor C does not induce any electromotive 

 force in loop AB. Likewise, conductor A induces no electro- 

 motive force in loop BC, and conductor B induces no electronic- 



