Interpoles 

are designed in DC motors to overcome the effects of the

armature reactance and the self-induction of the machine. Most shunt
and compound DC motors over one-half horsepower have interpoles
located 90 electrical degrees from the main poles. Some motor
designs use only one interpole with satisfactory results.
Interpoles are wired in series with the armature. Any changes in the armature
current due to armature reactance, loading, or self-inductance pass through the
interpole windings. This creates a changing magnetic field equal to and opposite
that of the armature, thereby cancelling the effect.
Polarity of an interpole depends on the direction of rotation. The interpole's
polarity will always be the same as that of the main pole which precedes it.
In Figure 7-45, the direction of rotation is clockwise as viewed from
the commutator end of the motor. When direction of rotation of the motor is
changed, the direction of current flow through the interpoles must be reversed at
the same time.

One easy and simplest way is to shift the brushes to the new position of the
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.
Compensating Windings
The cross-magnetizing effect of armature reaction may cause trouble in
d.c. machines subjected to large fluctuations in load. In order to neutralize the
cross magnetizing effect of armature reaction, a compensating winding is used.
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.
To neutralize completely the effects of armature reaction, a second set of auxiliary
field windings, known as the compensating windings, is used in high-power d.c.
machines.
Compensation windings consist of a few turns of low-resistance copper bar laid
in slots in the faces of the main shunt field pole pieces and so connected that the
windings carry current in the reverse direction to that of the immediately adjacent
armature conductors.
The compensating windings are connected in series with each other and with the
armature winding in a manner similar to the interpole windings so that they also
oppose the field set up by armature reaction. The current in them is then equal to
that in the armature.
The field resulting from the compensating windings is wide in comparison with the
commutating fields but weaker since the flux is less concentrated.
The effect of the two windings acting in conjunction is to neutralize completely the
effects of armature reaction in respect to the shifting of the neutral plane and to
eliminate almost completely the distorting effects.
Thus it is ensured that the neutral plane will remain in a fixed position throughout
the entire range of load and speed of the machine, and, in the ease of a motor, in
both directions of rotation.
Good commutation is thus affected with the brushes located in a fixed position.
Interpole (Commutator Field Winding)
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. 
The commutating field, or interpoles, as they are sometimes called because of
their position relative to the main poles, consist of a series of small poles similar to
the main field poles in construction and method of fastening, but having a winding
that consists of a few turns of heavy copper bus bar of high current capacity and
low resistance.
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.
The commutating pole (Interpole) windings are all connected in series with each
other and with the armature circuit. A resistor connected in parallel with the
commutating pole (Interpole) windings is adjusted and permanently set at the
factory to give the commutating pole strength that results in the best commutation.
Most of the armature current goes through the commutating pole windings; only a
small amount goes through the shunting resistor.
Since the armature reaction increases when the armature load current increases,
and the effect of the commutating poles (Interpoles) also increases, the result is
that the neutral or commutating plane is maintained in a fixed position throughout
the load range.
With this method of correction, some distortion of the field still remains because
the commutating fields, being small, are not completely effective in correcting the
distortion in the vicinity of the main pole tips. This latter condition is especially
true of the high-power, compact machines used for submarine propulsion.
Use of interpoles or compoles
With the brushes connected to coils lying in the G.N.P., sparking
occurs when the generator is loaded because the M.N.P. moves away
from the G.N.P. If the position of the M.N.P. can be made to always lie
along the G.N.P. at all loads, good commutation may result. The
cause of the M.N.P. moving is the armature m.m.f., whose maximum
value lies along the G.N.P. which is between the poles. If this m.m.f.
can be neutralised along this axis, then there will be no armature
flux to cause the M.N.P. to move. To do this, additional narrow poles
are required between the main poles and these must set up an m.m.f.
equal and in opposition to the maximum armature m.m.f. for all load
conditions. This means that the winding on these poles must be
connected in series with the armature.
Whilst theoretically a triangular-peaked m.m.f. is required, in practice a
rectangular m.m.f. is satisfactory.