To get the concept of armature reaction we should know about the basic construction of a dc machine.
In any direct current machine,there are two winding present namely field winding(stationary) & armature winding(rotating). When the dc source is applied to the machine, the field winding get excited and produce a magnetic flux in between the two field windings.
As the name suggests,the field winding produces the field current & the armature winding carries the armature current .But as the armature winding is nothing but a closed coil so there should be a magnetic flux as armature current is passing through the armature conductor.
Thus,there are two magnetic field acting inside the machine.One is due to the field winding,called as main flux & another produced by the armature ,namely armature flux.Obviously,there is a interaction in between those fluxes.
The main flux,which is produced by field winding is very much useful because this flux creates the magnetic field.But the interaction of the armature flux with main flux distorts and weakens the main flux & creates problems for the proper operation of the DC machine.
Thus,the effect of armature flux on the main field flux is called armature reaction
The adverse effects of armature reaction are:
The main flux is distorted due to the interaction.
An unequal flux density occurs at pole tips.
So,we can say that armature reaction can reduce the efficiency of a dc machine.
The effect of armature reaction can be reduced by proper commutation & by providing a special type of winding, namely compensating winding.
Another answer of " What is armature reaction? "
In electromechanical energy converters the rotor conductors decrease the effective field flux, this can be said armature reaction at a basic level. To understand it in a deeper level we need to use some diagram first.
Here are three diagrams are drawn. In figure (a) there is no current flow through the rotor conductors and the flux flow from the N pole to S pole is undisturbed and straight.
When current runs through the rotor conductors, there is a flux generation which is inevitable. Now this rotor flux superimpose with the main flux and thereby disturbing the natural flow of the main field flux, which is shown in figure (b)
Now, when these two flux interact there is a resultant flux generated which tilts the geometric neutral axis (GNA) and there is a new axis for the flow of resultant flux, which is shown in figure(c).