Explain the need for carbon brush and for what purpose do we use it
It is the static part of the DC or Universal Motor which supplies the electric power to the rotating part of the motor (armature) and makes physical contact with the commutator segment. Carbon itself is a semiconducting element having a negative temperature coefficient.When the temperature rises due to friction the resistance of carbon brushes goes down. Carbon brushes are used to transmit current from a rotating part of the equipment to a static part. For example, you can use the same principle to transmit electric current from the rotor winding to the terminal box of a slip-ring motor for adding starting resistance.
Why carbon brush is only used?,. Because, it is soft, adjusts to the curvature of commutator or slip rings provides full contact. and having negative resistance coefficients effect, as the heat increases the resistance decreases. So that voltage drop due to contact resistance will be negligible.