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Armature ReactionAuthor: E.E. Kimberly When a generator is carrying a load, the current in its armature produces a magnetic pole at the armature surface at every interpolar space. See Fig. 10-21 (a). When the flux ϕa of the field produced by the loaded armature exists jointly with the flux ϕf of the main field, shown in (b), the resultant flux has the direction ϕt in (c). As a result, the commutating axis, that is, the axis on which commutation should occur, would shift from xy in (b) at no load through the angle a to X1y1 in (c) at some particular load. The influence of armature flux in distorting the main-field pattern is called armature reaction. In order that destructive sparking between the brushes and the commutator may be avoided, it is necessary that the brushes be moved forward in the direction of rotation to the axis X1y1 corresponding to the particular load. Since the angle a varies as the load varies, the brushes must be moved to a new position corresponding to every value of load current. Furthermore, if the direction of rotation of the generator armature be reversed, the commutating axis will shift to the opposite side of xy and it will be necessary to provide for a possible brush shift of angle 2a from the position X1y1 shown in (c).
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