In leveling, which condition can warp and raise commutator bars?

• A. Vibration
• B. Stalling
• C. Overloading
• D. Misalignment

Answer: (B)

When a motor or generator stalls, it draws a very high current because there’s little back-EMF to oppose the applied voltage. That excessive current heats the winding and the commutator area intensely. The heat causes the copper bars of the commutator to expand or soften relative to the surrounding insulation, leading to warping and lifting (raising) of the bars. Once the bars warp, the brush contact becomes uneven, increasing resistance and arcing, which can further damage the commutator. So this specific overheating condition during stalling is what makes warping and raising the commutator bars likely, more than simple vibration, misalignment, or overloading alone.