Discussion
The commutator segments of dc machine are made of __________.
Answer: A
Commutator segments are made of hard drawn copper.
The commutator must conduct the full armature current with little loss and also stand up to the continuous rubbing of the brushes and the centrifugal force at speed. Hard drawn (or drop-forged) copper has the conductivity of copper with much greater hardness and wear resistance, and the segments are insulated from each other by mica.
Ordinary annealed copper is too soft and wears and deforms quickly, which is why the plain 'copper' option is not the expected answer. Brass and aluminium have higher resistance, and aluminium is too soft for a sliding contact.
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