Usually the width of the brush is equal to the two segments of the commutator. Whenever a brush contacts two or more commutator segments, the connected to those segments are short-circuited. After the period of short-circuiting the current on those coils changes their current direction in it. The change that takes place in the coil after the period of short-circuiting of that coil is called commutation.
When that changes take place slowly, that commutation is known as smooth commutation and when that changes take place suddenly, that commutation is known as rough commutation.
If the commutation is not smooth, the spark may be more and that will damage the commutator surface, commutator segments and so the winding. The remedies for rough commutation are resistance commutation method and emf commutation method.
