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TM 5-685/NAVFAC MO-912
Table 4-2. Generator trouble shooting-Continued
test that can indicate the contamination of insula-
tion by moisture, dirt or carbonization. There are
other tests available to determine the quality of
insulation, but they are not recommended because
they are generally too complex or destructive. An
External field resistance in out
Adjust resistance
insulation resistance test should be conducted im-
mediately following generator shutdown when the
Voltage regulator contacts dirty
Clean and reseat contacts
windings are still hot and dry. A megohmmeter is
the recommended test equipment.
Open phase winding
Cut open coil out of circuit and
c. Before testing the insulation, adhere to the fol-
replace at first opportunity. Cut and
replace the same coil from other
(1) Take th e equipment to be tested out of ser-
vice. This involves deenergizing the equipment and
Rotor not centered
Realign and replace bearings, if
disconnecting it from other equipment and circuits.
(2) If disco nnecting the equipment from the cir-
Unbalanced circuits
Balance circuits
cuit cannot be accomplished, then inspect the in-
Loose connections or wrong
stallation to determine what equipment is con-
Tighten connections or correct
polarity coil connections
wrong connections
nected and will be included in the test. Pay
particular attention to conductors that lead away
Shorted coil
Cut coil out of circuit and replace
from the installation. This is very important be-
at first opportunity
cause the more equipment that is included in a test,
the lower the reading will be, and the true insula-
Shorted field coil
Replace or repair
tion resistance of the apparatus in question may be
masked by that of the associated equipment. It is
Improper ventilation
Remove obstruction, clean air
always possible, of course, that the insulation resis-
tance of the complete installation will be satisfac-
tory, especially for a spot check. Or, it may be higher
Reversed field coil
Check polarity. Change coil leads
than the range of the megohmmeter, in which case
Static charge
High-speed belts build up a static
nothing would be gained by separating the compo-
nents because the insulation resistance of each part
Connect alternator frame to a
would be still higher.
ground strip
(3) Test for foreign or induced voltages with a
volt-ohm-milliammeter. Pay particular attention
4-13. Insulation testing.
once again to conductors that lead away from the
a. The failure of an insulation system is the most
circuit being tested and make sure they have been
common cause of problems in electrical equipment.
properly disconnected from any source of voltage.
Insulation is subject to many effects which can
(4) Large electrical equipment and cables usu-
cause it to fail; such as mechanical damage, vibra-
ally have sufficient capacitance to store a dangerous
tion, excessive heat or cold, dirt, oil, corrosive va-
amount of energy from the test current. Therefore,
pors, moisture from processes, or just the humidity
discharge capacitance both before and after any
on a muggy day. As pin holes or cracks develop,
testing by short circuiting and grounding the equip-
moisture and foreign matter penetrate the surfaces
ment and cables under test. Consult manufacturer's
of the insulation, providing a low resistance path for
bulletins and pertinent references to determine,
leakage current. Sometimes the drop in insulation
prior to such shorting or grounding, if a specified
resistance is sudden, as when equipment is flooded.
"discharge" or "bleed" or "grounding" resistor should
Usually, however, it drops gradually, giving plenty
be used in the shorting/grounding circuit to limit
of warning, if checked periodically. Such checks per-
the magnitude of the discharge current.
mit planned reconditioning before service failure. If
(5) Generally, there is no fire hazard in the
there are no checks, a motor with poor insulation,
normal use of a megohmmeter. There is, however, a
for example, may not only be dangerous to touch
hazard when testing equipment located in inflam-
when voltage is applied, but also be subject to burn-
mable or explosive atmospheres. Slight sparking
may be encountered when attaching test leads to
b. The electrical test most often conducted to de-
equipment in which the capacitance has not been
termine the quality of armature and alternator field
completely discharged or when discharging capaci-
winding insulation is the insulation resistance test.
tance following a test. It is therefore suggested that
It is a simple, quick, convenient and nondestructive
use of a megohmmeter in an explosive atmosphere

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