The commutator should have a smooth film and consistent color. The film should be from light to dark brown. A streaky film is often a warning that the commutator might thread. If this is the case, fine copper dust will be found on the brushes and brush rigging. Bar marking, where some bars are lighter or darker than others, is usually considered to be normal and related to the number of conductors per slot. However, if some bars have a burnt or grayish appearance, it may indicate an armature winding fault or a high resistance riser connection.
If some bars are burnt along the trailing edge, the problem could either be the neutral positioning of the brushes or a brush grade with insufficient commutating ability.
The face of the brush will reflect the condition of the commutator surface. For example, if the commutator surface is threaded or rutted, the brush face will likewise be threaded or rutted. Running a finger across a threaded brush face will indicate the depth of the threading on the commutator.
To see these and other commutator conditions at a glance, click the link below.