Nidec 4P10, Instruction Manual, Page: 31 / 46

Page 31
Insulation Resistance Tests at Low Voltage
Insulation tests are conducted for two reasons: to discern existing
weakness or faults, or to approximate 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 depends upon the voltage and the
time of application, the area and thickness of the insulation, and the
temperature and humidity conditions during the test.
The insulation resistance test determines the insulation condition
prior to application of more extensive testing measures. Refer to the
following procedures for testing details.
When testing insulation resistance with a megger, first verify the
ground path. Connect one test lead 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)
1. Disconnect the exciter field leads from the terminals in the
terminal box or the voltage regulator.
2. Connect 500 V megger: one clamp to the exciter field leads, the
second clamp to the exciter field frame.
3. Apply 500 V from the megger. After 1 minute, observe the
resistance reading. The reading must be a minimum of 50
megohms. If it is not, refer to the cleaning or dry out procedures.
4. Disconnect the megger and wait a minimum of 1 minute. Ground
the exciter field leads to the exciter field frame for several
minutes to discharge residual voltage.
PMG Armature (Stator)
1. Disconnect the PMG armature leads from the terminals in the
terminal box or the voltage regulator.
2. Connect 500 V megger: one clamp to the PMG armature leads,
the second clamp to the exciter field frame.
3. Apply 500 V from the megger. After 1 minute, observe the
resistance reading. The reading must be a minimum of 50
megohms. If it is not, refer to the cleaning or dry out procedures.
4. Disconnect the megger and wait a minimum of 1 minute. Ground
the PMG armature leads to the PMG armature frame for several
minutes to discharge residual voltage.
Exciter Armature (Rotor)
1. Disconnect the exciter armature leads from the rotating rec tifiers.
2. Connect 500 V megger: one clamp to exciter armature leads, the
second clamp to a suitable connection on exciter sleeve or shaft.
3. Apply 500 V from the megger. After 1 minute, measure the
resistance reading. The reading must be a minimum of 50
megohm. If it is not, refer to the cleaning or dry out procedures.
4. Disconnect the megger and wait a minimum of 1 minute. Ground
the exciter armature leads to the exciter sleeve or the shaft to
discharge residual voltage.
CAUTION: Insulation resistance
tests are usually made on all or parts of
an armature or field circuit to ground.
The tests 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.
NOTE: The insulation resistance value
decreases as the winding temperature
increases. All resistance values must
be corrected to winding temperatures:
multiply the resistance value by the
conversion factor to obtain the corrected
value. See Table 1. (e.g., 100 megohms at
50° C is converted to 170 megohms: 1.7 x
100 at 40° C).
Table 1 Temperature Conversion
Factor for Resistance Values
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
WARNING: Never apply the
megger to the rotating rectifier,
the voltage regulator, or generator
accessories (e.g., temperature detectors,
space heaters).
NOTE: New generators should measure
about 100 megohms minimum of
insulation resistance when meggered.
Generators with insulation resistance
of 50 megohms or less should be dried
out according to the dry out procedures
here. Generators with an insulation
resistance of 10 megohms or less should
be investigated.