🔍 Key Concepts

• Purpose of a commutator in a D.C. motor (providing current to the armature and allowing reversal of current direction) • Proper surface condition of the commutator for good brush contact (smooth, clean, correct film) • Typical maintenance practices: machining/undercutting vs. cleaning vs. coating or lubricating electrical contact surfaces

💭 Think About

• Think about what kind of surface the brushes need to run on for minimum wear and sparking: should it be oily, varnished, bare shiny copper, or have a specific type of film? • Which of these actions is normally done only during major overhaul or machining, and which is done as part of routine preventive maintenance? • Is the thin oxide ‘patina’ on copper usually helpful or harmful for electrical sliding contact surfaces like a commutator?

✅ Before You Answer

• Verify which choice describes routine cleaning or conditioning rather than a one‑time machining/overhaul procedure. • Check if any option would insulate the commutator surface from the brushes (oil film or varnish) – that would be undesirable. • Consider whether removing a normal, smooth brown contact film is recommended, or whether that film is actually beneficial for commutation.