The continual friction of the brushes against the _________ _____________ wears the brushes down and tends to force them out of alignment, which causes trouble in commutation.

• A. Stationary commutator
• B. Armature
• C. Commutator housing
• D. Rotating commutator

Answer: (D)

The main idea is that the surface the brushes press against is what wears because it is in motion. In a DC machine, the brushes stay still while they contact the commutator, which is mounted on the rotor and rotates with it. That continual rubbing wears the brushes and can push them out of alignment, which disrupts the timing of current reversal in the windings and causes sparking or uneven current—trouble in commutation. The other parts don’t fit because the commutator that the brushes ride on is the rotating surface; a stationary commutator would contradict how the device is built, the armature refers to the winding assembly rather than the contact surface, and the commutator housing is just a stationary shell around the rotating parts. So the rotating commutator is the component in contact with the brushes and subject to wear.