The Diseases Of Electrical Machinery 1904
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26 THE DIS EAS ES OF
P
Short-Circuit in the Winding.—While the
above mentioned single armature short-circuit only
causes breakdowns when a second contact has
occurred or is already present, a short-circuit in a
winding, that is to say a contact between two points of
an armature coil, is at once destructive and causes a
serious breakdown. In a dynamo the usual result,
unless the fault has short-circuited only a very small
part of the winding, is that the machine refuses to
excite itself. If the field-magnet is now separately
excited, and the machine is run, it still does not reach
its full voltage ; and one can also clearly observe that
the machine uses up a great deal of power, even when
the external circuit is open. In quite a short time,
| to a minute, a smell of burning will be noticed.
If the machine is at once stopped, and one then
touches the coils to ascertain their heating, one can
find, by the unequal heating of the coils, that one
coil in which the short-circuit exists—it will be much
hotter than the others. This heating results from the
heavy current which is produced in the short-circuited
turn of the armature. To this also is due the great
absorption of power, in spite of the circumstances that
the external circuit is opened and that no current is
being generated for the purpose of excitation. If this
kind of fault occurs in a continuous-current motor the
effect is that the motor runs very slowly backwards.
In certain circumstances the motor may not run at all.
One can then, however, if the field-magnet is excited,