Insufficient brush spring pressure creates an increase in what at the carbon-to-copper junction?

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

Answer: (B)

The real contact pressure at the carbon-to-copper junction governs how well the two surfaces conduct current. When brush spring pressure is too low, the contact is poor—areas touch only lightly or intermittently, and films or oxides on the surfaces aren’t wiped away effectively. That reduces the effective contact area and introduces nonuniform, higher-resistance paths for the current. The result is an increase in electrical resistance at the junction, which also leads to more heat and a larger voltage drop across the contact.

Electrical noise and wear can occur with bad contact, but the primary change described by insufficient pressure is the rise in resistance. Thermal expansion isn’t directly caused by the spring pressure itself, but by the heat generated from the higher resistance.