Armature reaction is the reaction or intereaction between the magnetic field produced by the flowing of current in the coil wound on the armature and the main magnetic field of the D.C. genarator or the motor.
The conversion of mechanical energy to the electrical energy in the electric dc motor or generator is due to this interaction.
Armature reaction causes the neutral plane, a plane parallel to armature windings with the magnetic flux, to shift in the direction of rotation. Thus armature reaction distort the magnetic flux (cross magnetisation) and lead the applied field not radial and reduction in magnetic field (de magnetisation ) which changes the efficiency of the output of the dc generator or dc motor.
In order to minimise the armature reaction, two simple methods can be taken up :
1. Shifting of the position of the brushes such that its plane are in the neutral plane (but this is not feasible as single machine is used as both generator and motor).
The brushes of a generator must be set in the neutral plane; that is, they must contact segments of the commutator that are connected to armature coils having no induced emf. If the brushes were contacting commutator segments outside the neutral plane, they would short-circuit "live" coils and cause arcing and loss of power.
2. Installation of interpoles in the generator or motor to nullify the effect of armature reaction although it reduces available flux.
Compensating windings or intrrpoles are used for this purpose.Their function is to neutralize the cross magnetizing effect of armature reaction. The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature in such a way that the current in them flows in opposite direction to that flowing in armature conductors directly below the pole shoes.
The series-connected compensating windings produce a magnetic field, which varies directly with armature current. Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the effects of the armature magnetic field
Armature reaction is response generated by armature of motor or generator to the change in the flux linked with it.This is the main cause for back emf in motors.
The armature reaction generates eddy currents in the armature which may result in losses in the machine.