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.
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Carbon Brushes .
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It was interesting when you explained that carbon brushes have to be placed along the Magnetic Neutral Axis to avoid being affected by the magnetic field. When I think about it, the precision of the location must be really important in industrial settings where high-powered generators are used. I’m glad I read your article and learned more about the function of carbon brushes in generators!
I would say that it is misleading to say that the Carbon Brushes have to be placed along the Magnetic Neutral Axis. It is not correct to take the reference of the Carbon Brushes as being the Magnetic Neutral Axis as the termination of the leads of the coils could be skewed at any angle from the iron slots to the copper segments or risers, on the commutator. So I would say that it is better to say that the Carbon Brushes should be placed on those segments which lead to the coils which are placed such that they are on the Magnetic Neutral Axis where the coils would not carry any voltage which is short-circuited by the brushes. It is to be mentioned that the carbon brushes themselves can come in a variety of resistances to try and control the short circuit effect by the brush. Hence when replacing brushes one needs to look up the equivalent resistor of the carbon in the brush. Normally, brushes for starter motors are different from those on DC motors or generators.