Insufficient brush spring pressure causes what at the carbon-to-copper junction?

• A. Electrical Noise
• B. Electrical Resistance
• C. Thermal Transfer
• D. Mechanical Wear

Answer: (B)

When the brush spring pressure is too low, the brush doesn’t press firmly against the copper commutator, so the real contact area at the carbon-to-copper junction is reduced and uneven. That smaller, inconsistent contact increases the electrical resistance at that interface. The higher resistance shows up as a greater voltage drop across the junction and localized heating, which can also lead to sparking and noise, but the fundamental issue at the junction is the increased resistance. The other effects—noise, heat transfer, and wear—are related consequences or different aspects, but the direct property changing at the junction with insufficient pressure is electrical resistance.