Armature reaction is the effect of the magnetic field set up by the armature current on the distribution of flux under the main poles of a generator. When a DC machine is running, both the fluxes (flux due to the armature conductors and flux due to the field winding) will be present at a time. The armature flux superimposes with the main field flux and, hence, disturbs the main field flux. The armature flux distorts and weakens the magnetic flux produced by the main poles, which is known as armature reaction. The armature reaction has two effects on the main field flux:
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Distorting effect: The armature flux distorts the main field flux, hence the position of the magnetic neutral axis (MNA) gets shifted. The MNA is perpendicular to the flux lines of the main field flux. Brushes should be placed on the MNA, otherwise, it will lead to sparking at the surface of brushes.
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Weakening effect: The armature flux weakens the main field flux. In case of a DC generator, weakening of the main flux reduces the generated voltage.
The effects of armature reaction can be reduced by:
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Compensating winding: An extra number of turns may be put on each pole to neutralize the demagnetizing effect of armature reaction. The compensating winding is connected in series with the armature so that it produces a flux that opposes the armature flux.
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Interpoles: Interpoles are small poles placed between the main poles of the DC generator. The polarity of interpole must be that of the main pole just next to it in the direction of rotation. The interpole windings are connected in series with the armature so that they produce a flux that opposes the armature flux. The function of interpoles is to neutralize the cross-magnetizing effect of armature reaction.
