The answer to your question is that the carbon brushes do not rotate with the slip rings, instead they stay fixed and slide along the rings. The rings themselves are mounted on the motor or generator rotor, and therefore rotates. The brushes are connected to the exterior electrical circuit and push on the slip rings. This is the standard arrangement for any electrical device that has moving parts which rotate and is designed so that an electrical voltage can be transmitted outside the generating area (which is in rotation) into a stationary frame. Because of friction that happens when sliding on the rings from time to time the carbon brushes need to be replaced.