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Section 3
INSULATION RESISTANCE TESTS
EFFECTS OF DIRT AND MOISTURE
Moisture and dirt are detrimental to the continued
good operation of any generator set.
If moisture is allowed to remain in contact with the
Stator and Rotor windings, some of the moisture will
be retained in voids and cracks of the winding insula-
tion. This will result in a reduced Insulation resistance
and, eventually, the unit’s AC output will be affected.
Insulation used in the generator is moisture resistant.
However, prolonged exposure to moisture will gradu-
ally reduce the resistance of the winding insulation.
Dirt can make the problem worse, since it tends to
hold moisture Into contact with the windings. Salt, as
from sea air, contributes to the problem since salt can
absorb moisture from the air. When salt and moisture
combine, they make a good electrical conductor.
Because of the detrimental affects of dirt and mois-
ture, the generator should be kept as clean and as
dry as possible. Rotor and Stator windings should be
tested periodically with an insulation resistance tester
(such as a megohmmeter or hi-pot tester).
If the Insulation resistance is excessively low, drying
may be required to remove accumulated moisture.
After drying, perform a second insulation resistance
test. If resistance is still low after drying, replacement
of the defective Rotor or Stator may be required.
INSULATION RESISTANCE TESTERS
Figure 3-1 shows one kind of hi-pot tester. The tester
shown has a “Breakdown” lamp that will glow during
the test procedure to indicate an insulation breakdown
in the winding being tested.
Figure 3-1. – One Type of Hi-Pot Tester
*
DANGER! INSULATION RESISTANCE
TESTERS SUCH AS HI-POT TESTERS AND
MEGOHMMETERS ARE A SOURCE OF HIGH
AND DANGEROUS ELECTRICAL VOLTAGE.
FOLLOW THE TESTER MANUFACTURER’S
INSTRUCTIONS CAREFULLY. USE COMMON
SENSE TO AVOID DANGEROUS ELECTRICAL
SHOCK
DRYING THE GENERATOR
GENERAL:
If tests indicate the insulation resistance of a winding
is below a safe value, the winding should be dried
before operating the generator. Some recommended
drying procedures include (a) heating units and (b)
forced air.
HEATING UNITS:
If drying is needed, the generator can be enclosed in
a covering. Heating units can then be installed to raise
the temperature about 15°-18° F (8°-10° C) above
ambient temperature.
FORCED AIR:
Portable forced air heaters can be used to dry the
generator. Direct the heated air into the generator’s
air intake openings. Remove the voltage regulator and
run the unit at no-load. Air temperature at the point
of entry into the generator should not exceed 150° F.
(66° C.).
CLEANING THE GENERATOR
GENERAL:
The generator can be cleaned properly only while it is
disassembled. The cleaning method used should be
determined by the type of dirt to be removed. Be sure
to dry the unit after it has been cleaned.
NOTE: A shop that repairs electric motors may
be able to assist you with the proper cleaning of
generator windings. Such shops are often expe-
rienced in special problems such as a sea coast
environment, marine or wetland applications, min-
ing, etc.
USING SOLVENTS FOR CLEANING:
If dirt contains oil or grease a solvent is generally
required. Only petroleum distillates should be used to
clean electrical components. Recommended are safe-
ty type petroleum solvents having a flash point greater
than 100° F. (38° C.).
*
CAUTION!: Some generators may use epoxy
or polyester base winding varnishes. Use sol-
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