130 



ALTERNATING CURRENTS 



higher armature reactance than one with wide shallow slots, 

 other conditions being the same. 



A semi-closed slot, like that shown in Fig. 135 (c), will have 

 considerably more magnetic flux per ampere-conductor than 

 either of the slots of (a) or (6), because of the partially closed 

 magnetic circuit formed by the overhanging tooth-tips. Thus, 

 the reactance of a machine may be controlled in part by the 

 design of the slot. In a smooth-core armature, like that shown 

 in Fig. 134, the armature reactance will be small as compared 

 with that of the slotted type of armature. 



A certain amount of reactance is due to the magnetic flux link- 

 ing the coil ends. Although this is small compared to the reac- 

 tance due to the slot linkage, it cannot be neglected as a rule. 



DIM 



(4) 



FIG. 135. Slot leakage flux which produces armature reactance. 



It is pointed out in Vol. I, Chap. VIII, that the inductance 

 varies as the square of the number of turns. This same rule 

 applies to the conductors in alternator slots. If the number of 

 series conductors in a slot is doubled, the reactance per slot is 

 increased four times, other conditions remaining unchanged. 



As the reactance is proportional to the frequency (X =2irfL), 

 the reactance of a 25-cycle alternator will be considerably less 

 than that of a 60-cycle alternator, other conditions being the 

 same. 



62. Armature Resistance. The armature iron forms a con- 

 siderable portion of the path of the flux which links the armature 

 conductors, Figs. 134 and 135. As this flux is alternating, it is 

 accompanied by hysteresis and eddy-current losses, which occur 

 in the iron immediately surrounding the slots. As this flux is 



