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• Dry, new windings have an insulation resistance of between 100 and 2000
M Ω , 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:
PI = R
/ R
insul 10 min
insul 1 min
Many measuring devices display these values automatically following the
measurement.
For insulation resistances > 5000 M Ω , the measurement of the PI is no
longer meaningful and consequently not included in the assessment.
R
/ R
(10 min)
(1 min)
≥ 2
< 2
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 insulation resistance at shorter intervals.
Limit values of the anti-condensation heating insulation resistance
The insulation resistance of the anti-condensation heating with respect to
the machine housing should not be lower than 1 M Ω when measured at
500 V DC.
Insulation in good condition
Dependent on the complete diagnosis of the insulation
Assessment
85
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