Conversion to the reference temperature
When measuring with winding temperatures other than 40° C, convert the measuring value to
the reference temperature of 40° C according to the following equations from IEEE 43-2000.
(1)
R
C
= K
T
· R
T
R
C
Insulation resistance converted to 40° C reference temperature
k
T
Temperature coefficient according to equation (2)
R
T
Measured insulation resistance for measuring/winding temperature T
in °C
(2)
K
T
= (0.5)
(40-T)/10
40 Reference temperature in °C
10 Halving/doubling of the insulation resistance with 10 K
T Measuring/winding temperature in °C
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 and 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:
PI = R
insul 10 min
/ R
insul 1 min
Many measuring devices display these values automatically following the measurement.
For insulation resistances > 5000 MΩ, the measurement of the PI is no longer meaningful and
consequently not included in the assessment.
R
(10 min)
/ R
(1 min)
Assessment
≥ 2 Insulation in good condition
< 2 Dependent on the complete diagnosis of the insulation
Assembly
5.1 Preparations for installation
SIMOTICS TN Series N-compact 1LH8
46 Operating Instructions 06/2017
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