A commutator is an electrical switch that periodically reverses the current in an electric motor or electrical generator. It enables a motor to run on, and a generator to produce, a direct current instead of an alternating current.
It typically consists of a set of copper contacts fixed around the circumference of the rotating part of the machine (the rotor), and a set of carbon brushes fixed to the stationary part of the machine (the stator) that complete the electrical circuit from the rotor's windings to the outside of the machine. Friction between the copper contacts and the brushes eventually causes wear to both surfaces. The carbon brushes, being made of a softer material, wear faster and are designed to be replaced easily without dismantling the machine. The copper contacts are usually inaccessible and are not designed to be repaired.
In the diagram, right, conventional direct current flows from the battery at bottom as shown by the dashed arrows. The commutator itself is the red and blue curved segments. The brushes are illustrated as dark gray contacting the commutator contacts, and the rotor winding is shown in violet. As the motor rotates, the commutator contacts will turn through 180° and the current flowing in the winding will reverse. However the fixed magnetic field that the rotor is in has also switched polarity relative to the rotor winding, and so rotation continues in the same direction.
See also: slip ring