In testing for a grounded brush holder or rigging,
touch one test lamp prod of the armature test set to the
motor housing. With the other test prod, touch each
brush holder individually. If the lamp lights, there is a
ground in the brush holder.
Remove all leads to the brush holders and
brushes before you attempt this test.
The color of the commutator and slip rings will
indicate the type of trouble. An even chocolate-brown
color indicates a normal condition and a black color
indicates brush arcing. You can remove slight burns on
the commutator segments by polishing the commutator
as the armature rotates. Use a canvas pad, as shown in
figure 7-61. To remove the deeper burns, use fine
sandpaper instead of the canvas pad. When a
commutator is deeply scored, it must be reconditioned
in a lathe or with a special tool.
Never use emery cloth to polish commutators
because the emery particles can lodge between
the segments and cause the commutator circuits
Slip rings used on rotors are usually made of bronze
or other nonferrous metals. Under normal conditions,
the wearing surface should be bright and smooth. When
the rings are pitted, they should be polished. When
excessively worn and eccentric, they should be trued
with a special tool.
REASSEMBLY.After you have inspected all
parts and repaired or replaced the faulty ones, you are
ready for reassembly. To assemble motors, follow in
reverse order the procedures of their disassembly. Be
sure to check any available literature you may find. Be
sure to oil or grease the bearings as required. Remove
the relief plug in the bottom of the housing while you
Figure 7-61.Fabricated cleaning pad.
MOTOR CONTROLLER MAINTENANCE
The most important rule to remember when you are
making repairs or inspecting motor controllers is as
Be sure the controller is disconnected from the
power source before touching any of the
Control equipment should be inspected and
serviced on the same maintenance schedule as motors.
Motor starters can normally be repaired on the job site at
the time of inspection.
After the power has been
secured, the first thing you should do to keep controllers
operating at maximum efficiency is to keep them free of
dirt, dust, grease, and oil, both inside and out. Clean the
operating mechanism and contacts with a clean, dry,
tintless cloth, or vacuum cleaner. Small and delicate
mechanical parts may be cleaned with a small, stiff
bristle brush and a Navy-approved solvent.
Check the contacts to ensure proper electrical
connections. When contacts open and close, the rolling
and rubbing action keeps the contacts bright and clean.
Infrequently operated contacts or contacts under heavy
loads can overheat and create oxidation on the contacts.
Copper contacts are used for most heavy-duty
power circuits, and, in many cases, in relay and
interlock circuits. They should be inspected regularly.
If projections extend beyond the contact surfaces or if
the contacts are pitted or coated with copper oxide, they
should be sanded down with fine sandpaper.
Welding of contacts sometimes occurs, in spite of
all precautions. Low voltage is the most common cause.
Welding may also result from overloads, low-contact
pressure resulting from wear or weak springs, loose
connections, or excessive vibrations. If welding occurs,
it is an indication of trouble in the electrical system. The
contacts must be replaced, but it is useless to replace
them unless the cause of the welding is found and
Carbon contacts are used when a contactor is
frequently opened and closed. It is essential that the
contactor be open when it is de-energized. Since carbon