The Diseases Of Electrical Machinery 1904
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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.