Interpoles are additional poles which are placed between the main
poles in a dc machines. These poles are designed to counteract the
effects of armature reaction in the dc machines.

Interpoles are placed at the geometric mean axis. They are wound byconductors which are connected in series to the main armature winding.

The polarity of the interpoles is the same of the next approaching pole.

The chief function of the interpoles is to compensate the reactanceemf and to minimize sparking across the brushes.

Armature Reaction refers to a phenomenon in DC machines where the magnetic field of the poles is distorted by the magnetic field produced by the armature current. 

In a generator operating on no load, the armature current is zero and the only field inside the machine is that produced by the poles.  When this generator is connected to a load, current starts to flow in the armature windings.  This current causes a magnetic field around the armature conductors. 

This magnetic field interacts with the magnetic field caused by the generator poles.  The effect of this magnetic field is both demagnetising and cross-magnetising as reduces the net effect of the main field on the armature conductors while distorting the magnetic field at the same time. 

Effects of the armature reaction. 

The principal effect of armature reaction is related to commutation.  The brushes on a dc machine are placed in the neutral plane.  The neutral plane refers to the plane where the armature brushes move in parallel to the magnetic field of the poles.  At this point the emf of the armature conductor is zero.  This facilitates easy commutation across the commutator segments. 

Due to armature reaction, the natural direction of the magnetic field of the poles is distorted.  Thus the neutral plane is also altered.  Therefore, the armature conductors are not at zero potential when they come in contact with the brushes.  This leads to sparking across the brushes and loss of power.

Overcoming Armature reaction.

There are two methods of addressing the effects caused by armature reaction.  One method is through the use of compensating winding in the field poles. Another method is the use of interpoles between the main poles to prevent distortion of the main field.