Commutation phenomena in electrical machines. Part 2: Further experiments in flashover

In an earlier paper, it was established that a major mechanism of flashover involved the generation of a series of intersegmental arcs, and further relevant work has been carried out on the properties and characteristics of such arcs and on means by which they may be extinguished. The dependence of the flashover voltage (per gap) on gap width, the velocity of sliding and the brush grade have been investigated. This voltage increases markedly with increasing gap width; but, except at low speeds, it is independent of the velocity for a given gap width. The flashover voltage was found to be only marginally dependent on the brush grade when brushes selected from the main types were tried. Sequences of ciné photographs showed a phenomenon known as `jetting¿, in which molten copper, formed at an upper edge of a segment, is thrown outwards by the pressure of vaporising copper. The method considered whereby short arcs could be extinguished was based on a transverse magnetic field. Under the earlier conditions, the arcs accelerated uniformly; but it was suggested that a terminal velocity should be reached, which has been shown to be correct using additional apparatus. A formula describing the dependence of this terminal velocity on basic parameters has been derived. Further work has been carried out on the practical aspects of extinguishing short arcs by means of an air blast.