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NOTE: The insulation resistance indicated in the test report is normally
considerably higher than the values measured on site.
Minimum values for insulation resistance
The following criteria apply to windings in a normal condition.
Generally, the insulation resistance values for dry windings should exceed the
minimum values significantly. Definite values are impossible to give, because
resistance varies depending on the machine type and local conditions. In addition,
the insulation resistance is affected by the age and usage of the machine.
Therefore, the following values can only be considered as guidelines.
The insulation resistance limits, which are given below, are valid at 40°C, and
when the test voltage has been applied for 1 minute or longer.
Rotor:
•
For rotors:
R
(1-10 min at 40 °C)>1.5 M .
•
Carbon dust on naked copper surfaces lower the insulation resistance values
of the rotor.
Stator:
•
For new stators:
R
(1-10 min at 40 °C)>1000 M .
•
For used stators:
R
(1-10 min at 40°C)>100 M .
If the values indicated here are not reached, the reason for the low insulation
resistance should be determined. A low insulation resistance value is often caused
by excess humidity or dirt, the actual insulation being intact.
Stator winding insulation resistance measurement
The insulation resistance is measured using an insulation resistance meter. The
test voltage is 1000 VDC. The test time is 1 minute, after which the insulation
resistance value is recorded. Before the insulation resistance test is conducted,
check that:
•
The secondary connections of the current transformers (CT's), including spare
cores are not open, see
Figure 7-3. Connection of the stator windings for
insulation resistance measurements, part a.
•
All power supply cables are disconnected.
•
The frame of the machine and the stator windings not being tested are earthed.
•
All resistance temperature detectors are earthed.
•
Possible earthing of voltage transformers (not common) must be removed.
The insulation resistance measuring should be carried out in the terminal box. The
test is usually performed to the whole winding as a group, in which case the meter
is connected between the frame of the machine and the winding, see
part a
and
part b
of
Figure 7-3. Connection of the stator windings for insulation resistance
measurements
. The frame is earthed and the three phases of the stator winding
remain connected at the neutral point, see part A of
Figure 7-3. Connection of the
stator windings for insulation resistance measurements
. In the figure M
represents the insulation resistance tester.
If the measured insulation resistance of the whole winding is lower than specified,
and the phase windings can easily be disconnected from each other, each phase
can also be measured separately. This is not possible in all the machines. In this
measurement, the tester is connected between the frame of the machine and one of