ARMATURE REACTION:

We know that an emf is induced in the coils of armature when there is a change in magnetic flux associated with it.So when there is no load connected to the armature, the current flowing through the armature winding is zero.Therefore the net magnetic flux of the system is created by the field system.

When there is a load connected to the armature winding the current flowing through the armature winding is non-zero value.So since there is a current flow through the winding,it causes generation of magnetic field and hence magnetic flux.Now the net magnetic flux get reduced since the magnetic flux created by armature winding either opposes or crosses perpendicularly to that produced by field system.

Any dc machine has two types of planes known as Magnetic Neutral Plane(MNP) and Geometric Neutral Plane(GNP).

The position of MNP and GNP are important on case of dc machine.GNP is always peerpendicular to the line connecting the centres of magnetic poles.Whereas MNP is always perpendicular to the resultant magnetic flux or mmf.

So when the load connected to armature winding is zero, resultant mmf is from north pole to south pole.So the MNP and GNP coincides.

But when a load is connected to armature winding,the net mmf rotates in a direction along the rotation of armature and hence there is a shift in MNP.Now MNP and GNP never coincides.