ARMATURE REACTION IN DC MACHINE

•    When dc generators are loaded, more current flows through the armature.

•     This armature current flowing through the armature conductors produces magneto motive force in the armature. This is called as armature mmf.

•      Therefore under load conditions there are two mmf’s acting on dc generators.

                       (i) The field mmf produced by the exciting current flowing through the field winding.
                       (ii) Armature mmf produced by the armature current flowing through the armature winding.

•     The effect of the armature mmf on the field mmf is called as armature reaction.

•     In dc generators this armature reaction depends upon the position of the brushes fixed.

•      The brushes are normally fixed at magnetic neutral axis.

•     To understand the armature reaction consider a two pole dc generator rotating in clockwise direction carrying field and armature currents as shown in the fig

The effects due to armature reaction may be minimized or compensated by the following arrangements.


(i) In case of small machines by increasing the air gap length, the ratio between main mmf and armature mmf may be increased.

(ii) By fixing small magnetic poles called interpoles in between main magnetic poles at the magnetic neutral axis the effect of armature reaction may be reduced.

(iii)    By providing compensating winding on pole faces, the effect of armature reaction may be reduced.