There are two practical ways of improving commutation i.e. of making current reversal in the short - circuited coil as sparkless as possible. These methods are known as (i) resistance commutation and (ii) e.m.f. commutation (which is done with the help of either brush lead or interpoles, usually the later).
This method of improving commutation consists of replacing low - resistance Cu brushes by comparatively high - resistance carbon brushes.
It is seen that when current I from coil C reaches the commutator segment b, it has two parallel paths open to it. The first part is straight from bar 'b' to the brush and the parallel path is vis the short - circuited coil B to bar a and then to the brush. If the Cu brushes (which have low contact resistance) are used, then there is no inducement for the current to follow the second longer path, it would preferably follow the first path. But when carbon brushes having high resistance are used, then current I coming from C will prefer to pass through the second path because (i) the resistance r1 of the first path will increase due to the diminishing area of contact of bar 'b' with the brush and because (ii) resistance r2 of second path will decrease due to rapidly increasing contact area of bar a with the brush.
Hence, carbon brushes have, usually, replaced Cu brushes. However, it should be clearly understood that the main cause of sparking commutation is the self - induced e.m.f. ( i.e. reactance voltage), so brushes alone do not give a sparkless commutation; though they do help in obtaining it.
The additional advantage of carbon brushes are that (i) they are to some degree self - lubricating and polish the commutator and (ii) when sparking occur, they would damage the commutator less than when Cu brushes are used.
But some of their minor disadvantage are : (i) Due to their high contact resistance (which is beneficial to sparkless commutation) a loss of approx. 2 volts is caused. Hence, they are not much suitable for small machines where this voltage form an appreciable percentage loss, (ii) Owing to this large loss, the commutator has to be made some what larger than with Cu brushes in order to dissipate heat efficiently without greater rise of temperature, (iii) because of their lower current density (about 7-8 A/cm2 as compared to 25-30 A/cm2 for Cu brushes) they need larger brushes holders.
In this method, arrangement is made to neutralize the reactance voltage by producing a reversing e.m.f. in the short - circuited coil under commutation. This reversing e.m.f. as the same shows, is an e.m.f. in opposite to the reactance voltage and if its value