Current Flow during Commutation.



The sketch below shows what happens to the flow of current into each segment when any two segments are shorted out by the carbon brush. Because the coils are inductive the current carries on flowing around the coil and it then flows across the face of the brush (hence the name transverse current) the current flowing through the coil decays so that when the segment moves from under the brush face the flow of current changes and flows in the opposite direction. 

For example lets assume that the normal current flowing through the brush is 10 amps. This current divides into two paths five amps flows into one segment and five amps into the other segment. When two segments become short circuited by the carbon brush and the current flows across the face of the brush let's say for example three amps, this current flow will reduce the current on one side of the brush by 5-3=2 amps. It will increase the flow of current by 5+3=8 aamps on the other side of the brush. This 8 amps will increase the current density on half of the brush face and this higher current may cause arcing as the edge of the brush clears the segment.



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