Discussion
In a 230 V dc machine, the brushes are made up of :
Answer: B
In a 230 V dc machine the brushes are made of electrographite.
Electrographite brushes are carbon baked at a very high temperature so that it turns to graphite. They are self-lubricating, wear slowly, and have a fairly high contact resistance, which helps commutation by limiting the current in the coil being short-circuited by the brush. That makes them the usual choice for ordinary industrial dc machines at a few hundred volts.
Copper and copper-graphite (or copper-carbon) brushes have a low contact drop and suit low-voltage, high-current machines such as electroplating and welding generators; on a 230 V machine they would cause sparking and rapid wear of the commutator.
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