🔍 Key Concepts

• Commutator construction in DC machines – how copper segments and mica insulation work together • Relative hardness and wear – why the brush/commutator surfaces must wear in a controlled way • Purpose of mica – electrical insulation vs. mechanical wear characteristics

💭 Think About

• Think about which part of the commutator you want to wear first in service – the copper segments, the mica, or the brushes, and why. • Consider what would happen to the commutator surface if the insulating material were much harder or much softer than the copper segments. • Ask yourself: for good brush contact, do we want the mica to stand proud of the copper, or to stay slightly recessed as the commutator wears?

✅ Before You Answer

• Verify which material should carry the current (copper or mica) and therefore which should be exposed to the brushes. • Check which arrangement will keep the commutator surface smooth and uniform over time, with minimal sparking. • Confirm how the wear rate of mica vs. copper affects the need for undercutting and maintenance of the commutator.