MarelliMotori MXB-E 180 Serie Manual De Instalación, Funcionamiento Y Mantenimiento página 35

 If the measured value is considered too low, the winding must be cleaned and/or dried. If that is not
enough, it is necessary to refer to experts
 The machines about which you suspect an humidity issue must be dried with the utmost care,
regardless of the insulation resistance value measured
NOTE: The insulation strength indicated in the test report usually is significantly higher than the values
measured at the yard.
5.1.3 Conversion of the measured insulation resistance values
In order to compare the measured insulation strength values, these are established at 40°C; with the aid of the
following graph. The actual measured data is then converted to a value corresponding to 40°C. The application
of this graph should be limited to temperatures near the standard value of 40°C because more significant
variations may cause errors.
Figure 3 Correlation between insulation resistance and temperature.
R C = k x R
5.1.4 Minimum values for the insulation resistance
Criteria related to windings in standard conditions
In general, the insulation resistance values for dry windings must significantly exceed the minimum values; it is
impossible to provide definitive values, because the resistance changes depending on the type of machine and
the local conditions. The insulation resistance as well is subject to the effects of ageing and of machine use.
Thus, we recommend to follow the values indicated here solely as guidelines.
The minimum value of the insulation resistance is one of the main requirements for the stator electric safety. It is
not recommended to start the machine if the values are lower than the minimum value.
The limits of the insulation resistance, indicated below, are valid at 40°C and when the test voltage has been
applied for over one minute (yet not over 10 minutes).
 Rotor
963857252_=
Winding temperature °C
RT Value of the insulation resistance at a specific temperature
RC Equivalent insulation resistance at 40°C
K Correction coefficient for insulation resistance
Example:
Ω
RT = 400 M measured at 20°C
k = 0.25
Ω
RC = 0.25 x 400 M = 100 M
Fig. 3
Ω
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