What is the function of carbon brushes in dc machine?

The carbon (graphite) brushes in a generator are used to connect external circuits in the generator to the rotor through the commutator. But you want to know what that means, right? Darn it, we always see these "trick" questions here. Okay, this is the scoop. The rotor in a generator is (usually) a group of windings. These are coils of wire. But they need to be connected to the "outside" or non-moving parts. Each coil has two ends, and these ends are connected to copper segments (sometimes called "lands") on the commutator. The segments are all in a ring around the end of the rotor, and the brush or brush sets ride on them. The other end of the brush(s) is(are) connected to wires to electrically connect them to appropriate places in the non-moving parts of the generator. The brush is conductive (naturally), and it is soft enough to provide a good "wear point" against the copper segments. The brushes do most of the "wearing away" in this application, and it is a fairly easy and straight forward evolution to replace them. In the motors of power tools, some of the units have a way to access the brush holder without taking the housing apart. Particularly the older ones. Anyway, it is said that a picture is worth a thousand Words. Use the link below and take a look at the pictures. It's all there.