The Diseases Of Electrical Machinery

46 THE DIS EAS ES OF sible of belief, which can be explained only on these grounds. That the reversal of compound machines may easily be caused by the back flow of current from accumulators is well known to everyone. Since the reversal of polarity only occurs in accumu- lator stations, or in plants where several generators work in parallel, all that is necessary to do is to lift the brushes from the armature and to send a current through the field from the ’bus-bars. Heating of Magnet Coils by a too Heavy Shunt Current.—Every dynamo is built for a certain voltage and for a certain speed. It follows, hence, that it is not allowable to run a dynamo either at normal voltage and higher or lower speed, or at normal speed with too high or too low pressure. We have already considered, in the part dealing with sparking, the faults caused by a machine running at normal pressure and too high speed, or at normal speed and low pressure, and have seen that in these circum- stances the weakening of the field is the cause of the trouble. The other fault, namely, the running of the machine at normal speed and too high voltage, or at normal voltage and low speed, we have not yet con- sidered. This fault is shown by abnormal heating of the armature iron and of the field coils. It occurs oftener than one would think in plants with accumu- lator batteries. Dynamos for charging accumulators are, at their normal highest voltage (165 volt for 60 cells), already very heavily loaded. This in itself is of