The Diseases Of Electrical Machinery

22 THE DIS EAS ES OF S the poles of the magnets one above the other, it is customary to decentre the armature by making the air-gap between the upper pole and the armature smaller than that between the armature and the under pole, in order, by the attraction of the upper pole, to relieve the bearings of a part of the weight of the armature. This must not be done with multipolar machines having more than two paths through the armature winding. It is done in some railway motors. The heating of the commutators of low-voltage machines is often caused by there being too heavy masses of copper, too massive segments, in which, owing to the heavy currents, eddy-currents ensue. The heating roughens the commutator surface, and so leads to sparking. Having now dealt sufficiently with those faults which show themselves by heavy sparking, we pass on to other phenomena. The Different Kinds of Armature Short- Circuits.—One often hears the cause of a break- down referred to as a short-circuit or, as workmen call it, a “short” in the armature. Very few of those who use these words picture to themselves the various and differing faults which are included in this name. First, the insulation, between.the armature winding and the iron core of the armature, may have broken down at some point. Secondly, the insulation of one coil may have given way in two places, so that