The Diseases Of Electrical Machinery 1904
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8
THE DISEASES OF
of the necessary suppression of the sp3.rking with the
unusually weak fields. If we regulate these machines
still lower, down to about 10 volts, as the author
many times has had to do, it is impossible to prevent
heavy sparking, due to the influence of the armature
currents upon the weak magnetic field. It may be
here remarked that the care of these machines should
be given only to experienced and conscientious hands.
In many cases it will also be well to render un-
necessary any considerable weakening of the field by
providing the booster with a two-speed pulley, so that
with low voltages one can also work at a slow speed
and with a stronger magnetic field. This course
might often be pursued, and above all in those cases
where the booster is driven by a belt from the main
machine.
Excitation of Booster Fields.—It should here
be remarked that a booster should always be provided
with separate excitation of the field magnet. Separate
excitation makes the magnetism of the machine more
stable, and is in any case much to be preferred for
machines where the voltage must be regulated down
to a very low value. As is well known to practical
men (and explained by the theory) a shunt machine
can only be regulated down to about one-third of
the normal voltage. From this point downwards the
voltage becomes unstable, i.e. the machine is inclined
to lose its voltage altogether. With separate exci-
tation this is completely prevented. For this reason