74 SYNCHRONOUS ALTERNATORS. [Exp. 



armature conductors. There are, however, different paths which 

 the flux may follow, causing different inductive effects. 



4. (a) True Armature Reaction. By one path, flux set up 

 by the armature conductors passes into the pole pieces and 

 through the magnetic circuit of the field magnets (Fig. 10), 

 linking with the windings of the field coils. This flux has a 

 demagnetizing effect, weakening* the field by a certain mag- 

 netomotive force produced by the ampere-turns of the armature. 



This flux through the field magnets is maintained by successive 

 armature conductors ; in a single-phase alternator it is pulsating, 

 but in a polyphase alternator, due to the combined effect of the 

 armature currents in the different phases, it is constant both in 

 position and in magnitude. 



5. (b) Local Armature Reactance. By a different path, 

 flux set up by the armature current encircles the armature con- 

 ductors without entering the pole-pieces; this flux (the fine lines 

 in Fig. 9) is entirely in the armature, or parjly in the armature 

 and partly in the air gap. The flux surrounding any particular 

 conductor varies periodically and produces a reactance electro- 

 motive force or reactance drop, XI, in quadrature with the arma- 

 ture current and proportional to it, as in any alternating cur- 

 rent circuit. 



6. By another and somewhat similar path, flux encircles 

 the armature conductors by entering into and returning from the 

 poles without linking with the windings of the field circuit; this 

 flux is shown by heavy lines, Fig. 9. This is cross-magnetizing 

 flux and distorts the field; it does not weaken the field except 

 incidentally to a small extent by saturating the pole pieces. This 

 cross-magnetization is alternating with respect to the armature 

 conductor, as in (b) ; with respect to the pole pieces, it is con- 

 stant in a polyphase and pulsating in a single-phase alternator, 

 as in (a). It may be treated separately; or with (a) or (b). 



*The field is weakened by a lagging current, but strengthened by a 

 leading current, 46-8. 



