The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 45 Loss of Magnetism.—The first phenomenon is due to the faet that cast-iron or cast-steel of a dynamo may gradually lose its remanent magnetism until the remaining field is no longer powerful enough to gene- rate a high enough voltage to produce a sufficient current in the field coils. The second fault is more important, since it occurs oftener. Here various occur- rences may lead to the demagnetisation of the poles. It may be caused by an extraordinary armature re- action, or by the occurrence of the currents in the magnet coils in the wrong direction. In the first case, with shunt machines, a heavy short-circuit may result in an armature current heavy enough to overcome the field induced by the magnet winding. Such cases have often occurred in practice. The possibility has also been proved that a shunt machine can be de- polarised during accumulator charging. Currents in the wrong direction in the magnet winding occur principally with compound machines working with accumulator plants. Reversed Poles.—The by no means uncommon sudden reversal of the polarity of a continuous-current dynamo follows immediately after the above. As has been there remarked this can be easily caused, in the case of a shunt machine, by a heavy short-circuit, when the current in the armature overpowers the magnetic action of the polar windings, and not only destroys it but even reverses the field. The author has met several cases, which were formerly considered impos-