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In addition, a critical insulation resistance is specified for the stator winding, which is the
minimum requirement for further operation, e.g. following longer machine downtimes.
Table 7-1
Stator winding insulation resistance at 40° C
U
rated
[V]
U ≤ 1000
1000 ≤ U ≤ 2500
2500 < U ≤ 5000
5000 < U ≤ 12000
U > 12000
U
= rated voltage, see the rating plate
rated
U
= DC measuring voltage
meas
R
= critical or minimum insulation resistance at reference temperature of 40° C
C
R
= minimum insulation resistance after cleaning/repair at 40° C
C
R
= critical insulation resistance during operation at 40° C
C, operation
R
= insulation resistance converted to current measuring/winding temperature
T
PI = polarization index R
T = current measuring/winding temperature
Note the following:
● When making measurements at winding temperatures other than 40 °C, the measured
● Dry, new windings have insulation resistance of between 100 and 2000 MΩ, or possibly
● Over its operating lifetime, the motor winding insulation resistance can drop due to ambient
Example calculation
Critical resistance for a rated voltage (V
LOHER CHEMSTAR 1PS5
Operating Instructions 03/2014
U
meas
[V]
500
500 (max. 1000)
1000 (max. 2500)
2500 (max. 5000)
5000 (max. 10000)
/ R
(T ≤ 40° C)
insul 10 min
insul 1 min.
value must be converted to the reference temperature of 40 °C. The conversion is made
using the formula from IEEE 43‑2000 specified in the table. In so doing, doubling or halving
the insulation resistance for a temperature change of 10 K is used as basis.
– The insulation resistance halves every time the temperature rises by 10 K.
– The resistance doubles every time the temperature falls by 10 K.
even higher values. Insulation resistance close to the minimum value could be due to
humidity and/or dirt accumulation. However, 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.
and operational influences. Depending on the rated voltage, the critical insulation resistance
value is to be calculated by multiplying the rated voltage (kV) by the specific critical
resistance value and then converted to the current winding temperature at the time of the
measurement, see previous table.
3.3 kV x 1.8 MΩ / kV = 6 MΩ at 40 °C
7.3 Testing the insulation resistance and polarization index
R
R
C
C, operation
[MΩ]
[MΩ]
≥ 5
0.2 MΩ/kV
(≈0.5 MΩ/kV at 25° C)
100
1.8 MΩ/kV
(≈5 MΩ/kV at 25° C)
) of 3.3 kV:
N
Commissioning
R
PI
T
[MΩ]
> 2,0
71
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