Carbon Brushes are used collect the current due to the induced emf from the armature coils in a DC Generator, as all of us know. When a brush is at any particular commutator segment, it shorts out that particular coil and draws current from the rest of the coils.
If the brush is not exactly at the particular segment, say one half is in one segment and other half is in a different one, then it draws current in that proportion.
But it should be mind that, at any particular moment of time, the brushes are always shorting one coil. Now as it is a short circuit, if emf is induced in that coil, huge current will flow. To prevent this, brushes are positioned in such a way that the coil being short circuited has no induced emf in it. As there is no induced emf, there will be no current, and hence no heating loss in brushes and no sparking at commutator segments.
So all the good things start happening once we position brush at the point where emf induced in a coil is zero. So where is that point?
It turns out that, these points lie along the Magnetic Neutral Axis (MNA), as the name suggests has magnetic neutrality along that axis.
As net magnetic field along MNA is zero, hence no emf will be induced. So that’s where one must position the brushes.