Contact problems in machines using thyristor-assisted commutation

D.C. machines using thyristor-assisted commutation have reached a relatively successful stage of development. One of the most difficult, and not entirely unexpected, problems has been the appearance of black marks on the surface of the segmented rings or `commutator¿. These marks have been associated with the sudden passage of current through a brush when the thyristor connected to it is fired. The paper describes the various current and voltage waveforms expected and observed on such machines and shows the segment marking to be initiated by rapidly changing capacitive currents. So far, the only brush found to be capable of working satisfactorily in the presence of such transients has been one formed from a bundle of carbon fibres. Such brushes, however, have rather high voltage drops when running on common materials, such as copper, in normal atmospheric conditions. But by using a carbon-fibre brush in parallel with a solid carbon brush, with the fibre brush coping with the transients and the solid brush carrying the majority of the current, it has proved possible to eliminate the surface marking from a machine using thyristor-assisted commutation.