grounds. This is the reason that motors and generators
3. The need to increase field current in generators
are periodically tested for grounds.
to maintain normal voltage with the machine running at
normal speed
When testing for grounds (except when a
permanently installed ground detector system is used),
Immediately secure a machine with a
make sure the machine is disconnected from its power
short-circuited field coil. You can find the shorted coil
supply. Make sure the machine cannot be accidentally
by passing normal current through the field circuit and
started while you are performing the tests. Remember
measuring the voltage drop across each coil. The shorted
to use the proper electrical insulation test equipment for
coil will have the lowest voltage.
these tests.
Short circuits occurring in brushless exciters are
commonly the result of one or more defective diodes on
Open Circuits
the rotating rectifier assembly. Loss or reduction of field
voltage combined with smoke or odor are symptomatic
of this failure.
Open circuits in ac stator windings are usually due
to damaged connections at the ends of the windings
Vibration Analysis
where the coils and circuits are connected. You can
u s u a l l y find this type of open circuit by visual
Another method used in troubleshooting motors and
inspection. When this does not suffice, taking resistance
generators is monitoring their vibration signatures. On
measurements between the phase terminals will reveal
a scheduled basis, performance monitoring teams come
the presence of open-circuited coils. An open circuit
aboard ships to perform regular vibration checks on
seldom, if ever, occurs within the stator windings of an
ac generator.
assigned equipment. The data obtained from their
checks are recorded and analyzed to determine if the
Open circuits that develop in the field windings of an
equipment is operating properly.
ac generator that is carrying a load are indicated by the
immediate loss of load and voltage. A generator with an
Vibration analysis is a program established as a
o p e n - c i r c u i t e d field winding should be secured
p r e v e n t i v e maintenance tool. It is a method of
immediately and examined to locate the open circuit. Open
d e t e r m i n i n g a piece of machinery's mechanical
circuits infield windings usually occur at the connections
condition by measuring its vibration characteristics
between poles and can be located by visual inspection.
u n d e r normal operating conditions. It can also
accurately show whether a machine has mechanical
Open circuits can occur in brushless exciters as a
defects and whether a specific part is defective.
r e s u l t of centrifugal forces on the rotating diode
assembly. Diode failure can result in either a reduction
The principle of vibration analysis is based on the
of exciter output (main generator field) voltage or a
fact that all machines vibrate. These vibrations are
complete loss of output voltage. Diode failure by open
caused by the allowable tolerance errors that are
circuiting can occur, but is uncommon.
inherent in the machine's design; they form the baseline
vibration signature of the individual machine. Similar
machines have similar vibration signatures that differ
Short Circuits
from each other by only their manufacturing and
installation tolerances.
A short circuit in the stator of an ac machine will be
A comprehensive vibration analysis program on board
indicated by smoke, flame, or odor associated with
charred insulation. Secure the machine and feel the ends
your ship is a proven preventive maintenance tool. This is
of the coils before they have time to cool. The shorted
made possible through periodic inspections to follow
coil will be hotter than those next to it.
machinery condition trends and pinpoint possible failures.
It also can be used to troubleshoot equipment for suspected
Short circuits in field coils of a machine may be
problems, to verify proper repairs have been made to
indicated by these symptoms:
overhauled equipment, and to check newly installed
1 . Vibration of the rotor due to unbalanced
equipment for proper installation. Most machinery
magnetic pull
problems can be detected in their early stages. In this way,
you can prevent complete equipment failure and possible
2. Smoke or the odor of burning insulation if the
severe damage.
short circuit is severe
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