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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
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.
• Over its operating lifetime, the motor winding insulation resistance can drop due to 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.
Measuring the polarization index
1. To determine the polarization index, measure the insulation resistances after one minute and
ten minutes.
2. Express the measured values as a ratio:
Polarization index = R
Many measuring devices display these values automatically following the measurement.
Measuring the polarization index is not helpful for insulation resistances > 5000 MΩ, and is
therefore not included in the assessment.
NOTICE
Damage to insulation
If the critical insulation resistance is reached or undershot, this can damage the insulation and
cause voltage flashovers.
• Contact the service center.
• If the measured value is close to the critical value, you must subsequently check the
1FU9
Operating Instructions, 07/2020, A5E46712792A
insul 10 min
R
/ R
(10 min)
(1 min)
≥ 2
< 2
insulation resistance at shorter intervals.
/ R
insul 1 min
Assessment
Insulation in good condition
Dependent on the complete diagnosis of the insulation
Assembly
5.3 Preparing for installation
45
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