Please read Armature reaction in DC generators before going through Compensating Windings and Interpoles in DC Generator.
Armature reaction causes many undesirable effects on a DC machine-like sparking on brushes. So, necessary steps should be taken to minimize the effects of armature reaction.
How to Minimise Effect of Armature Reaction?
One easy and simplest way is to shift the brushes to the new position of magnetic neutral plane. Shifting the brushes to the advanced position (the new neutral plane) does not completely solve the problems of armature reaction.
The effect of armature reaction varies with the load current. Therefore, each time the load current varies, the neutral plane shifts. This means the brush position must be changed each time the load current varies.
Where Compensating windings and Interpoles are Used?
In small generators, the effects of armature reaction are reduced by actually mechanically shifting the position of the brushes.
The practice of shifting the brush position for each current variation is not practiced except in small generators. In larger generators, other means are taken to eliminate armature reaction. Compensating Windings or Interpoles are used for this purpose.
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.
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 cross magnetizing effect of the armature magnetic field.
The neutral plane will remain stationary and in its original position for all values of armature current. Because of this, once the brushes have been set correctly, they do not have to be moved again.
Another way to reduce the effects of armature reaction is to place small auxiliary poles called “interpoles” between the main field poles. The interpoles have a few turns of large wire and are connected in series with the armature.
Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole ahead of it, in the direction of rotation. The field generated by the interpoles produces the same effect as the compensating winding.
This field, in effect, cancels the armature reaction for all values of load current by causing a shift in the neutral plane opposite to the shift caused by armature reaction. The amount of shift caused by the interpoles will equal the shift caused by armature reaction since both shifts are a result of armature current.