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Conversion to the reference temperature
When measuring at winding temperatures other than 40 °C, convert the measured value to
the reference temperature of 40 °C according to the following equations from IEC 60034-27-
4 and IEEE 43-2000.
In this case, doubling or halving the insulation resistance at a temperature change of 10 K is
used as the basis.
• The insulation resistance halves every time the temperature rises by 10 K.
• The resistance doubles every time the temperature falls by 10 K.
For a winding temperature of approx. 25 °C, the minimum insulation resistances are 20 MΩ
(U ≤ 1000 V) or 300 MΩ (U > 1000 V). The values apply for the complete winding to ground.
Twice the minimum values apply to the measurement of individual assemblies.
• Dry, new windings have an insulation resistance of between 100 ... 2000 MΩ, or possibly
• Over its operating lifetime, the motor winding insulation resistance can drop due to
Measuring the polarization index
1. To determine the polarization index, measure the insulation resistances after one minute
2. Express the measured values as a ratio:
Measuring the polarization index is not helpful for insulation resistances > 5000 MΩ, and is
therefore not included in the assessment.
1LE8 轴高 80 ... 355
操作说明, 11/2022, A5E52373573A
R
C
R
= K
· R
K
C
T
T
T
R
T
40
K
= (0.5)
(40-T)/10
10
T
T
even higher values. An insulation resistance value close to the minimum value could be
due to moisture and/or dirt accumulation. The size of the winding, the rated voltage and
other characteristics affect the insulation resistance and may need to be taken into
account when determining measures.
ambient and operational influences. Calculate the critical insulation resistance value
depending on the rated voltage by multiplying the rated voltage (kV) by the specific
critical resistance value. Convert the value for the current winding temperature at the time
of measurement, see above table.
and ten minutes.
Polarization index = R
insul 10 min
Some measuring devices display these values automatically following the measurement.
R
/ R
(10 min)
(1 min)
≥ 2
< 2
Insulation resistance converted to 40 °C reference temperature
Temperature coefficient, see below
Measured insulation resistance for measuring/winding temperature T
in °C
Reference temperature in °C
Halving/doubling of the insulation resistance with 10 K
Measuring/winding temperature in °C
/ R
insul 1 min
Insulation in good condition
Dependent on the complete diagnosis of the insulation
5.2 Preparing for installation
Assessment
Assembly
175
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