Sparking and arcing in electrical machines

The extent of sparking or arcing, as commutator bars come under or leave the brushes in an electrical machine, is often accepted as a guide to machine performance; yet the basic causes and effects of these phenomena are not widely known. The paper presents a background theory of the events occurring at the make and break of both stationary and sliding electrical contacts. Special emphasis is placed on graphite and copper contacts, and experimental results show good agreement with the theory. Flashes of light, usually called sparking, at the make or break of low-voltage contacts (above 2¿3V for a graphite-copper or a graphite-graphite contact) are produced mainly by incandescence of the very small hot conducting bridges and possibly by exothermic oxidation of the bridge material. At higher voltages, including induced voltages, the probability of a discharge produced by field emission increases. When the minimum voltage for a drawn arc (10¿20V) appears across separating contacts, an arc is drawn out, and appreciable damage is caused to the contacts. These processes are discussed, together with experimental observations on the erosion produced at make and break under various conditions. Finally, the relevance of this work to commutation problems, such as the sparking limit, leading-edge sparking, arc damage and flashover, is discussed in some detail.