The Application of the Helical Groove to Slip-Rings and Commutators

An outline is given of experiments showing that a sliding-brush-slipring contact, even with well seated brushes, only makes electrical contact at a comparatively small number of minute spots at any given instant. Differences are shown in the properties of the contact film formed on a grooved and on a plain slip ring. Commerical applications of the helical grooves are mentioned. It is shown that a helical groove cut in the face of a slip-ring or commutator has the following effects: (1) It provides a means for forcing a more uniform distribution of current over the brush face, and between parallel connected brushes. (2) It permits the escape of air trapped under a brush. (3) It prevents local spot heating and glowing under overload conditions. (4) It raises the contact drop of the brush, producing better commutation and cooler operation. (5) In some cases, an increased brush life.