A high percentage of a DC machine’s problems occur in the commutator and brushes. Following are some of the test procedures, problems, their causes, and solutions.
Burned commutator bars indicate a serious problem, and the machine will need further testing. If two spots appear at 180 electrical degrees from each other (two adjacent brush holders apart on the commutator of most armatures), the armature may have shorted coils. An armature with shorted coils should be rewound.
In some cases, the shorted coil can be cut open and the commutator segments attached to it can be shorted together. (This should be done only in an emergency.)