Armature Reaction

armature reaction is the effect of magnetic field produced by armature current on the flux distribution of main poles of a generator.

The armature magnetic field has two effects: 

1. weakens the main flux and
2. distorts it the flux distribution of the main poles.

When current flows through field coils, flux is generated from the poles, also current flows through the armature and a flux is generated called the armature flux. Without the armature flux present the pole flux would remain evenly distributed. But due to current flowing through the coil, flux is generated in the clockwise direction near the north pole due to current flowing inward in the armature, it produces a force opposite to the direction of the pole flux and distorts it increasing the flux density at the top of the north pole. conversely, an anticlockwise flux is generated in the armature near the south pole, due to outward going current, and pole flux increases lower of the south pole.

How to reduce armature reaction

1. Lamination by reducing the area, resistance in the armature is increased for eddy current. so eddy current decreases. moreover, hysteresis and eddy current increases with the increased density of flux, by lamination small air gaps are created within the iron sheets, thus flux density is decreases as air produces a high resistance path for the flux.
2. Compensating winding In a large dc machine, variation in the load varies flux in the machine either forward or backward. This causes statically induced emf in the armature coils. The effect of armature reaction can be neutralized by using 'Compensating Winding'. These windings are connected in series with the armature windings are placed in embedded slots on the main poles and faces towards the armature conductors. The winding is made in such a way that the flux produced by the compensating windings is in opposite direction to the armature flux with an equal magnitude of mmf as that armature flux mmf. Therefore it neutralizes the armature conductors flux reducing the armature reaction.