The Diseases Of Electrical Machinery

k6 THE DISEASES OF Nor does it follow that a machine which runs well with copper brushes will run well with carbon brushes, or vice versa. It depends upon the build of the machine, upon the dimensions of the pole shoes, and upon the number of segments, whether high or low resistance brushes are needed. It must also be pointed out that although a brush may have very good me- chanical properties, yet it may not work satisfactorily. From the question of resistance we now pass to the mechanical properties. Many carbon brushes are too hard to be used ; they break off at the cor- ners, and thereby lead to sparking. Other brushes, again, are too soft ; they dirty the commutator, and wear away very rapidly. The various kinds of carbon brushes which are on the market very rarely fulfil all the requirements which must be expected. Here, again, one firm—the company “ Le Carbone ”—has made itself prominent by the production of really good carbon brushes. Brushes of the finest quality are in- deed fully six times as dear as those of other makes. They are, however, extremely satisfactory in working, and will stand a high current density, so that from a brush of i square inch of working surface one may take, in continued working, a current of 30-40 amperes; and, at times of momentary overload, easily twice as much. The commutator material for use with carbon brushes must be without exception either hard-drawn or drop-forged copper. The use of bronze or cast-