In DC Generator, A magnetic flux setup  by armature current which produce the effect over the distribution main pole magnetic flux this reaction is called armature reaction.

When there is no load to the generator, the current in the armature conductors is zero.Under these conditions there exists only the mmf of the main poles, which produces the main flux (ϕ). The flux is distributed symmetrically with respect to the polar axis, that is, the central line of the north and south poles. The Magnetic Neutral Axis (MNA) or plane is perpendicular to the axis of the flux.

The magnetic neutral axis coincides with the geometrical neutral axis or plane (GNA). Brushes are always placed along the magnetic neutral axis line. Hence, MNA is also called as the Axis of Commutation
The figure below shows the armature conductor carrying current with no current in field coils.the direction of current in armature conductors may be determined by flaming's right hand rule.the current flows in the same in all conductors lying under one pole. the direction of flux produced by armature conductors may be determine by cork screw rule.the conductors on the left hand side of the armature carrying current in the direction into the paper the flux produced by current in these armature conductors shown in figure. 

these conductors combine their mmfs to produce a flux through the armature in downward direction similarly , the conductor on the right hand side of the armature carry current in the direction out of paper. these conductor also combine their mmfs to produce a flux through the armature in the downward direction. thus, the conductors on both sides of armature combine their mmfs in such manner as to send a flux through the armature in the downward direction. This flux ϕA is represented by their arrow as show in the figure given uper . the armature flux produced is analogous to that produced in the equivalent iron cored solenoid with its axis along the brush axis.

Figure shows the condition when field current and armature current are acting simultaneously. this occurs when generator is no load. now there two flux inside the machine, one produced by main field pole of the generator and other by current in the armature conductors. these two fluxes now combine to form a resultant flux ϕR as shown in figure. 

It is seen that the field flux entering the armature is not shifted but also distorted. the distortion produces crowding of the  flux (increase flux pole density ) in the upper pole tip in N-pole and the lower pole tip in the S-pole. Similarly , there is a reduction of flux   (decrease flux pole density ) in the lower tip in N-pole and Upper tip S-pole . the direction of resultant flux has shifted in the direction of rotation of generator. since MNA is always perpendicular  to the axis of resultant flux , the MNA is also shifted.

Because of the nonlinear behavior and saturation of the core , the increase in flux one pole tip consequently , the main pole flux is decreases the terminal voltage of generator increase with load.  

Effect of Armature Reaction 
  1. Magnetic flux density is increase over one half of pole and decrease over the other half pole. but flux produced by each pole is slightly reduced and , therefore , the terminal voltage slightly reduced. the effect of total flux reduction by armature reaction is known as demagnetization effect.
  2. The flux wave is distorted and there is shifted in position of the magnetic neutral axis (MNA) in the direction of rotation for the generator and against the direction of rotation for the motor.
  3. Armature reaction establishes a flux in the neutral zone (or commutating zone) Armature reaction flux in the neutral zone will induce conductor voltage that aggravates the commutation problem.  



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