mechanical parts to stick and, if allowed to go
SHORT-CIRCUIT PROTECTION
unchecked, can lead to a short circuit between contacts.
Overload relays and contractors are usually not
The controller should be cleaned periodically, per
designed to protect motors from currents greater than
PMS requirements, to remove dust and dirt from the
about six times the normal rated current of ac motors or
enclosure. Contact surfaces should be kept free of dirt,
four times normal rated current of dc motors. Since
grease, and grime. Seating surfaces of magnetic cores
short-circuited currents are much higher, protection
and armatures should be kept free of grease and scale to
against short circuits in motor controllers is obtained
ensure quiet operation and a good seal of magnetic parts.
through other devices. To protect against short circuits,
Use of compressed air in cleaning is not
circuit breakers are installed in the power supply system,
recommended since it could blow metallic dust particles
thereby protecting the controller, motor, and cables.
with such force as to pierce insulation or cause short
Short-circuit protection is provided in controllers where
circuits.
it is not provided by the power distribution system.
Also, short-circuit protection isn't provided where two
INSULATION
or more motors are supplied power, but the circuit
breaker rating is too high to protect each motor
Insulation of the contractors, wiring, switches, and
separate] y. Short-circuit protection for control circuits
so forth, should be inspected periodical y to ensure there
is provided by fuses in the controller enclosure, which
is no danger of fire or electric shock. A convenient way
provides protection for remote push buttons and
to schedule the maintenance is to accomplish the checks
pressure switches.
at the same time as the maintenance checks for the motor
it serves. Doing a check in this way prevents the need
FULL-FIELD PROTECTION
to de-energize the motor and controller more than once,
which allows the system to stay on line with as few
Full-field protection is required when a shunt field
interruptions as possible.
rheostat or a resistor is used to alter a dc motor field and
obtain different motor speeds. Full-field protection is
LUBRICATION
provided automatically by a relay that shunts out the shunt
field rheostat for the initial acceleration of the motor,
and then cuts it into the motor field circuit. In this way,
The only lubrication that might be necessary is the
the motor first accelerates to 100 percent or full-field
application of light oil to hinge pivots of contractors that
don't operate freely and mechanical interlock
speed, and then further accelerates to the weakened-
field speed determined by the rheostat settings.
mechanisms.
JAMMING (STEP BACK) PROTECTION
CONTROLLER
TROUBLESHOOTING
The controller for an anchor windlass motor
provides stepback protection by automatically cutting
Although the Navy maintains a policy of preventive
maintenance, sometimes trouble is unavoidable. In
back motor speed when needed to relieve the motor of
excessive load.
general, when a controller fails to operate or signs of
trouble (such as heat, smoke, smell of burning
insulation) occur, the cause of the trouble can be found
CONTROLLER MAINTENANCE
by conducting an examination that consists of nothing
Controllers only operate correctly when serviced by
more than using the sense of feel, sight, and sound. On
a planned program of periodic maintenance and
other occasions, however, locating the cause of the
inspection. Since controllers frequently operate several
problem will involve more detailed actions.
times a day, they should be inspected and serviced
Troubles tend to gather around mechanical moving
regularly so that normal repairs or replacement of parts
parts and where electrical systems are interrupted by the
can be accomplished before a failure occurs.
making and breaking of contacts. Center your attention
in these areas. See table 6-10 for a list of common
CLEANING
troubles, their causes, and corrective actions.
When a motor-controller system has failed and
Dust should not be allowed to accumulate inside the
pressing the start button will not start the system, press
controller. An excessive amount of dust can cause
6-20
