The Diseases Of Electrical Machinery

i o THE DIS EAS ES OF mistake (which the author has met with several times) of inserting carbon brushes in the holder without any grinding of the surface whatever. One cannot suffi- ciently strongly point out that a badly ground brush has been the destruction of many a commutator. This work, therefore, must not be carelessly done; but a fair amount of time should be devoted to it, and the machine only set again in full working when one is convinced that there is a good and perfect contact at the sliding surface. It is for this reason advisable to run a machine with new brushes for several hours without load, so that the brushes may become thoroughly bedded to the outline of the commutator. Brushes are ground to shape by bending a piece of glass-paper (rough side uppermost) round the com- mutator, placing the brushes in the brush-holder, and screwing them up tight against the glass-paper. The brush rocker is then moved backwards and forwards until the brushes, by rubbing upon the glass-paper, have taken the same curvature as the commutator surface, and bed properly against it. Unsuitable Commutator Material.—We now come to the actual faults which may lead to spark- ing. We must in the first place mention the purely mechanical cause of unsuitable material. For moclern commutators with carbon brushes, the segments are made of hard-drawn or drop-forged copper, with layers of mica insulation between the segments. With these commutators unsuitable material and improper con-