Siemens SIMOTICS XP 1MB1 series Operating Instructions Manual page 56

Assembly
5.1 Preparing for installation
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)
(2)
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 =
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.
54
R
C
K
T
R K R
= ·
R
C T T
T
40
10
K
= (0.5)
T
(40-T)/10
T
R R
/
insul 10 min insul 1 min
R / R
(10 min) (1 min)
≥ 2
< 2
Insulation resistance converted to 40° C reference temperature
Temperature coefficient according to equation (2)
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
Insulation in good condition
Dependent on the complete diagnosis of the insulation
1LA5/6/7/9, 1LG4/6, 1MA6/7, 1MB..1/2/3/4/5 - SH 63 ... 355
Assessment
Operating Instructions, 06/2018, A5E44455710A