Generac 86640 - Manual (Page 14)

Section 1.3- INSULATION RESISTANCE TESTS
Effects of Moisture and Dirt
The insulation in RV and IM generators is moisture
resistant. However, prolonged exposure to moisture will grad­
ually decrease the resistance of Stator and Rotor insulation.
If moisture is allowed to remain in contact with generator
windings, some of the moisture will be retained in cracks and
voids of the iunsulation. This will result in a reduced insulation
resistance and, eventually, the unit’s AC output will be af­
fected.
Dirt can make the problem worse since it tends to hold
the moisture Into contact with the windings.
Salt, as from sea air, can also worsen the problem since
salt absorbs moisture from the air. When salt and moisture
combine, they make a good electrical conductor.
Because of the detrimental effects of dirt and moisture,
the generator should be kept as clean and dry as possible.
Stator and Rotor 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 moisture. After drying, a
second test of the Insulation should be performed. If resis­
tance Is still low after drying, replacement of the defective
windings may be necessary.
Insulation Resistance Testers
Figure 22 shows Just one type of hi-pot tester. The tester
shown is one of many brands that are commercially available.
It has a "Breakdown^ lamp which will glow to Indicate insula­
tion breakdown during the test.
Normally, the resistance of insulation is measured on the
order of millions of ohms, it can be measured w'rth a device
called a 'megger', which is a megohm meter (meg is for
million) and a power supply. The power supply voltage varies
but the most common Is 500 volts. A megger voltage over 500
is NOT recommended.
When using a hi-pot tester or megger to measure Insu­
lation resistance, be sure to follow the manufacturer's instruc­
tions carefully.
CAUTION: Before attempting to measure Insula­
tion resistance In any winding, first disconnect
any electronic components, regulators, diodes,
surge protectors, protective relays, etc., from the
circuit. Such components will be destroyed by the
high voltages generated by a megger or hl-pot
tester.
To measure Insulation resistance, connect one tester
lead to the leads for the winding to be tested and the other
tester lead to the generator frame. Make sure no leads of the
part being tested are touching any metal parts of the gener­
ator. If the 'Neutral' is grounded, it must be disconnected.
Follow the tester manufacturer’s instructions and perform the
resistance test.
Insulation Resistance- Main Stator
All stator leads (No. 11, 22, 33, 44, 2 and 6) must be
Isolated from ground and connected together. Connect one
hi-pot or megger test lead to the main stator leads. Connect
the other hi-pot or megger lead to the generator grounding
stud. Set the hi-pot tester or megger voltage to '500'. Follow
the tester manufacturer’s Instructions and take the reading.
Do not apply the voltage for longer than one (1) second.
If a hi-pot tester like the one shown in Figure 22 is being
used, a 'Breakdown' lamp on the tester will glow during the
one second test to indicate an Insulation breakdown.
If a megger is used to perform the test, the minimum
acceptable value can be calculated using the following for­
mula:
Minimum Insulation в
Resistance (Megohms) Generator Voltage +1
-------------TOOO------
Example: For a generator rated 240 volts AC:
2 ^+ 1 b 1.24 Megohms
1000
H the reading Is below the recommended value, the
winding must be dried out
Rotor insuiation Resistance
Before testing Rotor insulation, гтшке sure the Rotor Is
completely Isolated. This can be done by removing both
brush holders with their brushes.
Connect one hi-pot tester or megger lead to the positive
Rotor slip ring, the other test lead to a clean frame ground
(such as the Rotor shaft). Set the tester to '500' volts and
apply voltage - DO NOT APPLY VOLTAGE LONGER THAN
ONE SECOND.
If a hi-pot tester is used. Its 'Breakdown' lamp will come
on to Indicate an insulation breakdown.
If a megger Is used, the minimum acceptable value Is 1.5
megohms. ,
If insulation Is breaking down under the applied voltage,
the Rotor must be dried out. After drying, retest the Rotor. If
insulation still breaks down after drying, replace the Rotor
assembly.
Cleaning the Generator
Greasy dirt or dirt that Is caked may be loosened with a
soft brush or a clean, damp cloth. A vacuum system may be
used to clean up loosened dirt. Loose dust and dirt may also
be blown away with clean, dry, low pressure air (25 psi
maximum).
Drying the Generator
To dry the generator without disassembling it, proceed
as follows:
1. Open the generator’s main circuit breaker. NO ELECTRI­
CAL LOADS MUST BE APPLIED TO THE UNIT WHILE
DRYING.
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