Which finding on commutator bars can cause brush chatter?

• A. Color, Flat Spots, Burned Surfaces, and High or Low Bars
• B. Color Only
• C. Flat Spots Only
• D. Burned Surfaces Only

Answer: (A)

Brush chatter happens when the contact between the brush and the commutator becomes uneven, causing the brush to lose steady contact and vibrate as the machine runs. Any irregularity on the commutator bars that changes the surface or height can trigger that problem. Color changes on a bar indicate overheating and surface oxidation, which roughens the contact area. Flat spots are worn or grooved areas that create abrupt changes in the surface profile. Burned surfaces show overheating damage that roughens or pits the copper. And if bars are higher or lower than their neighbors, the brush seating becomes nonuniform, leading to uneven contact as the rotor turns. Since all of these conditions can cause the brush to chatter, the best answer includes all of them. In maintenance, you’d inspect for color changes, flat spots, burned areas, and height variations across the bars to prevent chatter.