Discussion
Blackening of one commutator segment in service usually indicates
Answer: B
Correct answer: an open-circuited or high-resistance armature coil
A single blackened or burnt segment is the classic sign of a fault in the coil connected to it. If that coil is open-circuited or has a bad joint, the current cannot pass through it and arcs across the gap as the brush leaves the segment, burning it locally. Uniform blackening across all segments points instead to brush grade, spring pressure or atmospheric conditions, and a regular pattern of marked segments to a fault repeating once per pole pitch.
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