Too ______ pressure causes poor brush contact and unnecessary sparking, and too _____ pressure will cause excessive brush wear.

• A. little / much
• B. much / little
• C. no / high
• D. soft / hard

Answer: (A)

Brush contact pressure in electric machines must be balanced: enough to keep the brush firmly pressed against the commutator for a continuous, low-resistance path, but not so much that it wears the brush or the commutator.

If the pressure is too low, the brush may not stay in firm contact as the machine runs, leading to intermittent contact, arcing, and sparking. This reduces conduction quality and causes unnecessary sparking because the current path is being broken and reformed repeatedly.

If the pressure is too high, the brush experiences excessive friction against the commutator, which speeds wear, generates extra heat, and can degrade the brush and the commutator surface. This is excessive brush wear.

So the statement that little pressure causes poor contact and sparking, while much pressure causes excessive wear, aligns with how contact pressure behaves in practice.