Theory of resistance commutation with particular reference to carbon-fibre brushes

Commutation in small machines depends on changing brush-segment resistances. The manner in which these resistances change, and the consequent brush current densities, can be influenced by the shape of the commutator segments. Simple equations are derived from the current density during commutation, and these are used to show that specially shaped segments can lead to lower maximum values. This conclusion has been verified using carbon-fibre brushes and face-plate commutators.