What role do graphite brushes play in a direct-current commutator motor?

In a direct-current (DC) commutator motor, stationary graphite brushes serve as the essential electrical interface between the motor's external terminals and the rotating armature.1

These brushes maintain contact with the commutator switch, which is mounted on the rotor shaft, allowing current to flow into the armature coils. As the rotor turns, the commutator reverses the direction of the current in the armature coils to ensure continuous torque production. Because the brushes are part of the electrical circuit, their resistance—along with that of the armature winding—contributes to limiting the initial current when the motor is connected to a power source.1

Graphite is specifically utilized for these brushes due to its excellent electrical conductivity and its physical properties as a lubricant, which help minimize wear during the sliding contact with the rotating commutator.2