Nidec 22765-01, Manual

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Page 33
Insulation Resistance Tests at Low Voltage
Insulation tests are conducted for two reasons: to discern existing
weakness or faults, or to give some indication of expected service
reliability. Insulation resistance tests are based on determining the
current through the insulation and across the surface when a DC
voltage is applied. The leakage current is dependent upon the voltage
and time of application, the area and thickness of the insulation, and
the temperature and humidity conditions during the test.
Refer to the following electrical measurement procedures for testing
detail. Contact Kato Engineering or refer to IEEE Standard 432-1992
when more extensive insulation tests are required.
When checking insulation resistance with a Megger, first verify the
ground path. Connect one test load to a ground point. Then connect
the second test lead to another ground location to prove the ground
connection. Once the ground path has been proven, the second test lead
can be connected to the leads of the component to be tested.
Exciter Field (Stator) and PMG Armature (Stator)
1. Disconnect the exciter leads from the terminals in the ter minal
box or the voltage regulator.
2. Connect exciter leads to one clamp of 500-volt Megger, and
connect the other clamp to the generator frame.
3. Apply 500 V from the Megger, and measure the resistance
reading after one minute. The reading must be a minimum
of 50 megohm. If it is not, refer to the cleaning or dry out
procedures.
4. Ground the exciter field leads to the generator frame for several
minutes after the Megger has been disconnected. This will allow
the voltage build up to be properly discharged.
5. Repeat steps 1–4 for the PMG armature (stator).
Exciter Armature
1. Disconnect the exciter armature leads from the rotating rec tifiers.
2. Connect the leads of the exciter armature to one clamp of a
500-volt Megger, and connect the other clamp to a suitable
connection on the shaft.
3. Apply 500 V from the Megger, and measure the resistance
reading after one minute. The reading must be a minimum
of 50 megohm. If it is not, refer to the cleaning or dry out
procedures.
4. Ground the exciter leads to the shaft after disconnecting the
Megger. This will allow the voltage build up to be properly
discharged.
IMPORTANT: New generators should
measure about 100 megohms of
insulation resistance when meggered.
Generators that read 50 megohms or
less should be dried out according to
the dry out procedures here. Generators
with insulation resistance readings of
10 megohms or less must be cleaned and
then dried out.
Table 2 Temperature Conversion
Factor for Resistance Readings
NOTE: Never apply the Megger to the
rotating rectifier, the voltage regulator, or
generator accessories (e.g., temperature
detectors, space heaters). These devices
can be damaged by the applied voltage.
IMPORTANT: The insulation resistance
tests are usually made on all or parts of
an armature or field circuit to ground.
They primarily indicate the degree of
contamination of the insulating surfaces
or solid insulation by moisture and other
conducting influences and will not usually
reveal complete or uncontaminated
ruptures.
IMPORTANT: The insulation resistance
value increases with de creasing winding
temperatures. All readings should
be corrected to a reference winding
temperature of 40° C. See Table 2 for
converting Megger readings to other
temperatures (e.g., 100 megohms at
50° C is converted to 170 megohms: 1.7 x
100) at 40° C).
Winding Temp
( ° C)
Conversion
Factor
10 0.23
20 0.37
30 0.6
40 1
50 1.7
60 2.7
70 4.5
80 7.5
90 14
100 23
110 38
120 61