The Diseases Of Electrical Machinery

ELECTRIC AL MACHINERY. 35 circuiting of one coil the total resistance has been reduced to half of the previous value, a short current of double strength will now, at the same voltage, pass through the one remaining coil. This will naturally result in great heating of the still good coil, but there will be no sparking. Damp often causes a gradual complete carboni- sation of a coil. Contact between. the Magnet-Winding and the Iron.—In the case of shunt machines, the existence of a short-circuit between the winding and the iron, that is to say, of a contact between the copper winding (or more usually one of the coil ends) and the iron of the frame, though in itself of no account, may become dangerous. For when two such faults are present, either both in the winding of the magnets, or one in the magnet and the other in the commutator or in the armature-winding, there will be a breakdown. Further, with machines in a three-wire system, with earthed middle wire, when the frame is earthed, or in the case of railway motors working with an earthed return, any single fault of the kind under consideration will result in complete breakdown, just as has already been shown + FlG. 13. when treating of the armature breakdowns. Fig. 13 shows the following example : A 'contact is sup* D 2