Commutator and carbon brush functions

In a DC generator the emf induced in the armature conductors is alternating. The commutator and brushes together convert it into unidirectional emf at the external terminals and also carry the current from the rotating armature to the stationary load.

Sketch

        N                         S
     +-----+                   +-----+
     |     |    armature coil  |     |
     |     |   +-----------+   |     |
     +-----+   |           |   +-----+
               |           |
           seg1|           |seg2    commutator
            +--+--+     +--+--+     (split ring)
            |/////|     |/////|
           ===B1===     ===B2===    carbon brushes
              |            |
              +---[LOAD]---+

Commutator

It is a cylinder of wedge-shaped hard-drawn copper segments, insulated from one another and from the shaft by mica, mounted on the shaft.

1. Rectifies the emf: it reverses the connection of each coil to the external circuit at the instant the coil emf reverses, so the current in the load is always in one direction.
2. Collects current: it connects the armature winding to the brushes.
3. Keeps the armature emf and current in the proper phase, so the torque and the output polarity stay constant.

Without the commutator the machine would give AC at the brushes (as an alternator with slip rings does).

Carbon brushes

They are blocks of carbon or graphite held in brush holders by spring pressure, and rest on the commutator surface.

1. Carry current from the rotating commutator to the stationary external circuit.
2. Self-lubricating: carbon has a low friction coefficient, so wear of the commutator is small and the brush is replaced cheaply.
3. High contact resistance helps commutation (reduces the sparking current in the short-circuited coil).
4. Soft: the brush wears instead of the costly copper commutator.
5. Spring pressure keeps good contact despite vibration.

The brushes are placed at the magnetic neutral axis (MNA), where the coil sides are not cutting flux, so that the coil is short-circuited when its emf is zero and sparking is minimum.