The Diseases Of Electrical Machinery

2 THE DISEASES OF under the head of armature breakdowns, yet do not fali into any of the above categories. These we will consider by themselves. Sparking.—By far the most usual cause of spark- ing is bad or unsuitable handling of the commutator or of the whole machine. In many cases where complaints have been made regarding continuous- current machines, it has turned out that there was no ground for complaint in the machine itself, but that the whole blame lay upon the dynamo attendant, through whose carelessness or ignorance the machine has been damaged. For example, by simply care- lessly switching a dynamo into parallel with another dynamo, or with a battery, or through working the starter of a motor too suddenly, i.e. in general by suddenly putting in or taking out an unusually heavy current, a burn-spot will be caused on the commutator, which in spite of careful treatment with glass-paper, cannot quite be got rid of. This causes the brushes, when passing over the spot, to spark, which further increases the size of the spot, so that a burn which extended originally over one or two seg- ments only, is so much extended that in a short time sparkless commutation is impossible, even if the commutator is rubbed do;vn with glass-paper. Often in such a case as this the fault will be attributed to other causes, as for example, the pro- jection of the mica. insulation between the segments, to the varying degrees of hardness of the single seg-