Manual for Induction Motors and Generators
80 - Maintenance
3BFP 000 050 R0101 REV E
For new machines with dry windings, the insulation resistance is very high. The resistance can,
however, be extremely low if the machine has been subjected to incorrect transportation and
storage conditions and humidity, or if the machine is operated incorrectly.
NOTE: Windings should be earthed briefly immediately after measurement in order to avoid
risk of electric shock.
7.6.4.1 Conversion of measured insulation resistance values
In order to be able to compare measured insulation resistance values, the values are stated at
40°C. The actual measured value is therefore converted to a corresponding 40°C value with the
help of the following diagram. The use of this diagram should be limited to temperatures fairly
near to the standard value of 40°C, since large deviations from it could result in errors.
Figure 7-6 Correlation between the insulation resistance and the temperature
R = Insulation resistance value at a specific temperature
R40 = Equivalent insulation resistance at 40°C
R40 = k x R
Example:
R = 30 MΩ measured at 20°C
k = 0.25
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