The Diseases Of Electrical Machinery

14 THE DISEASES OF mutators may be caused by the shrinking of the in- sulation between the commutator segments and the damping rings. If vulcanized fibre has been used for insulation, in the course of time this will always happen. On the other hånd, the damping rings which hold the segments in place may be responsible. In any case, it will hardly be the duty of the engineer who erects the machinery to correct this. Commu- tators which have become loose on their shaft naturally cause sparking, at any rate with carbon brushes, since they become untrue, and cause the brushes to jog up and down. With metal brushes, owing to their greater flexibility, sparking from this or other of the above causes is much less to be feared. Commutator out of Truth.—A commutator may also become untrue in consequence of the varying hardness of the segments. This fault is indeed less to be feared when using hard-drawn or drop-forged copper ; but it often appears when the segments used are of cast metal, such as simple brass, or (better) bronze, such as silicium-bronze or phosphor-bronze. The use of such bronzes for commutators is therefore not quite satisfactory, although it is not to be con- demned absolutely. Brushes and Brush-Holders.—The brushes have an important influence upon sparking. Errors may arise, of a disastrous kind, both in form, number and material. In modern machines, having two or more rows of collecting brushes, it is the invariable practice