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range settings. Consult the instruction
manual for your meter.
3. Polarities supplied by the meters internal
battery may or may not correspond to the
(+) (—) markings on the meter.
INSULATION RESISTANCE -
GENERAL
Insulation resistance is a measurement of the
integrity of the insulating materials that
separate the electrical windings from the
generators steel core. This resistance can
degrade over time or due to contaminants
(dust, dirt, oil, grease, and especially
moisture). Most winding failures are due to
a breakdown in the insulation system. In
many cases, low insulation resistance is
caused by moisture collected when the
generator is shut down. The problem can be
corrected simply by drying out the windings.
Section 5.
Normally the resistance of the insulation
system is on the order of millions of ohms. It
is measured with 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 except for measuring
medium voltage (2400/4160) stators only.
CAUTION: First disconnect any electronic
components, regulators, diodes, surge
protectors, protective relays, etc., will be
destroyed if subjected to the high megger
voltages.
To measure insulation resistance, connect
the red or positive megger lead to the leads
for the winding to be tested, connect the
back or negative megger lead to the
generator frame. Be sure the leads of the
part being tested are not touching any metal
parts of the generator. (If the neutral is
grounded, it must be disconnected.) Take
megger reading (refer to the manual for the
megger).
INSULATION RESISTANCE - MAIN
STATOR
CAUTION: Be sure the regulator, and any
other electronic components, metering,
protective relays, etc., are disconnected
before meggering. High megger voltages
will destroy these parts. All stator leads must
be isolated from ground and connected
together (on most systems with grounded
neutrals - the neutral can be isolated from
ground and used as a test point). Connect the
positive megger lead to the main stator
leads. Connect the negative megger lead to
the generator grounding stud. Take the
megohm reading (refer to instructions for
the megger).
GENERATOR TESTING
The minimum acceptable value can be
calculated using the following formula.
Minimum Insulation Resistance = Gen.
Voltage + 1
(Megohms) / 1000
Example:
For a 480 volt generator
480 + 1 = 1.48 Megohms
If the reading is below the recommended
value, the winding must be dried out or
repaired.
INSULATION RESISTANCE - MAIN
ROTOR
Disconnect the main rotor leads from the
diode bridge on the exciter rotor. Connect
the leads together with the positive megger
lead. Connect the negative megger lead to a
good ground on the rotor assembly such as
the exciter mounting bolt. Take the megohm
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