Some properties of copper¿graphite interfaces and their effects on the commutation of double-lap-wound d.c. machines

The part played by the oxidation of copper in determining the properties and behaviour of a copper-graphite brush interface has been studied. In general, it has been found that a negative brush wears faster than a positive brush, the degree of asymmetry in the rates of wear increasing with current. Increasing the brush load decreases this asymmetry. Unstable running conditions sometimes occur, and laboratory experiments on two causes of instabilities at the interface, `copper picking¿ and `blistering¿, are described. The basic causes of blisters, their mode of formation and properties have been investigated, using both conventional techniques and electron microscopy. In addition, current interruption across segmented discs has been studied. Finally, arising from the laboratory findings, some experiments are described which seek to explain the nature and causes of alternate-bar marking in double-lap-wound machines.