The Diseases Of Electrical Machinery

6 THE DISEASES OF If the brushes are kept for a long time in the wrong position, in addition to the bad effects of the sparking on the surface of the commutator and the brushes, a not inconsiderable heating of the commu- tator may result. The wrong placing of the brushes may also be the cause of unusual armature heating. Wrong Speed.—Sparking may also be caused by the machine being run at too high a speed, although the machine itself is all right. It is by no means unimportant whether the prescribed speed be adhered to or not. If we considerably increase the number of revolutions per minute, we must, if the voltage remains constant, diminish the flux in the magnets by weaken- ing the exciting current. If this be carried too far, the action of the currents in the armature finally be- comes too great, and the machine commences to spark, as otherwise at normal speed it would only do under a heavy over-load. This may be caused, in the case of dynamos, by a wrong ratio in the gearing. With motors, also, we have the same fault sometimes in cases where the speed is to be regulated by a resistance in the shunt circuit. It must therefore be insisted that such increases of speed must only be allowed when agreed to by the makers of the machine. The regu- lator above mentioned should, if possible, be without a zero-contact, so that it is impossible to switch off unwittingly the shunt current while the armature is still in circuit. The result of this would be destruction of the motor, either by heating or running away.