Discussion
Brushes in a dc machine are usually made of
Answer: A
Correct answer: carbon or a carbon composite
Carbon is used for its contact resistance, which forces the current to transfer as the brush passes from segment to segment and so assists commutation; for its self-lubricating property, which reduces wear on the commutator; and because it is softer than the copper segments, so the brush rather than the commutator is the part that wears and is replaced. Copper brushes are used only in low-voltage, very high-current machines such as platers, where the drop matters more than the commutation.
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