Some Important Aspects in the Phenomenon of Commutator Sparking

Carbon brush contact has been included as part of the commutation problem study since the beginning. The current conduction into commutator copper via the carbon brushes is not simple. Whereas brush contact conduction on a slip ring is uniform and depends on few factors, the conduction phenomenon on the commutator has a variety of associated problems, attributable not only to the nature and properties of the brush itself (including brush pressure), but also upon the parameters of the machine coils into which the current is directed, and upon machine speed. The complication is known to be caused by short-circuiting of one or more of the coils by the brush face, and current change in the coil cannot take place without its stored inductive energy being destroyed. Usually, the coil currents cannot change and build up to their new value within the time of short circuit, and the remaining uncommutated current sends an arc from the trailing edge to leaving segment. However, the are formation is sometimes facilitated by the prearcing period voltage rise. In what follows, a complete picture of the formation and rupture of the arc and the effects of associated parameters, of both the brush and machine, on the commutated current, is attempted and substantiated by detailed oscillograms of the process.