Answer Posted / keepup

as per my limited knowledge i think the reaso y brush is
made of carbon is coz carbon has a negative temperature
coeficient of resistance, that is due to friction between
the brush and rotor, temperature is developed. as a result
the resistance of carbon brushes will decrease. for
instance if copper or aluminum brush is use, then when temp
increases at the contact pt between the brush and rotor,
resistance of the copper or aluminum brush will increase
resulting in loss of energy usually in the form of heat and
may lead to burning of the brushes.

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