MarelliMotori MXB-E 225 Serie Manuel D'installation page 35

should be limited to a temperature close to the standard 40°C value since major variations may generate errors.
Figure 3 Correlation between the insulation resistance and the temperature.
R C = k x R
5.1.4 Minimum insulation resistance values
Criteria for windings under normal conditions
Generally, the insulation resistance values for dry windings must considerably exceed the minimum values;
definite values cannot be provided since the resistance varies according to the type of machine and local
conditions. The insulation resistance is also affected by the age and use of the machine; the following values
should thus be followed only as a guideline.
The minimum insulation resistance value is one of the basic requirements for the electric safety of the stator. It
is absolutely not recommended to start the machine if the values are lower than the minimum value.
The insulation resistance limits indicated below are valid at 40°C and when the voltage test has been applied for
more than one minute (and anyhow no more than 10 minutes).
 Rotor
R > 5 MΩ
 Stator
< 10 MΩ
Poor
963857253_=
Winding temperature ° C
RT Value of the insulation resistance at a specific temperature
RC Insulation resistance equal to 40C°
k Corrective coefficient for insulation resistance
Example:
RT = 400 MΩ measured at 20°C
k = 0,25
RC = 0,25 x 400 MΩ = 100 MΩ
Insulation resistance ( Rc ) @ 40° C
10 MΩ < Rc < 200 MΩ
Check with PI
Fig. 3
200 MΩ < Rc < 1 GΩ
Good
> 1 GΩ
Very Good
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