MarelliMotori MXB-E 180 Serie Manual De Instalación, Funcionamiento Y Mantenimiento página 36

R > 5 MΩ
 Stator
Insulation strength ( Rc ) @ 40°C
< 10 MΩ 10 MΩ < Rc < 100 MΩ 100 MΩ < Rc < 1 GΩ > 1 GΩ
Poor Check with IP Good Very Good
NOTE: The insulation strength indicated in the test report usually is significantly higher than the values
measured at the yard.
5.2 POLARIZATION INDEX (IP)
Measuring the polarization index according to the standard IEEE 43, it is possible to test the insulating system
of the electric machine.
Measure and record the insulation resistance at environment temperature at different times: T1', T2' , .....,T10'.
The measurements interval is conventional (e.g. 1 minute).
Fig. 4
Figure 4 Quality trend of the insulation resistance in function of time.
High temperatures can cause unpredictable changes in the polarization index, so the test cannot be used at
temperatures over 50ºC.
Dirt and humidity accumulated in the winding reduce the insulation resistance and the polarization index, as well
as their dependence on the temperature. Windings with open leakage distances are very sensible to the effects
of dirt and humidity.
There are different rules to establish the lowest acceptable value with which it is possible to start the machine in
safety. For the polarization index (IP), the values usually vary from 1 to 4, where 1 indicates that the windings
are wet and dirty.
Poor Acceptable Good
IP IP < 1.5 1.5 < IP < 2 IP > 2
5.3 Reconditioning of the stator windings
The active parts shall be dried by a flow of hot air. The hot air flow must be directed as much as possible toward
the winding heads.
If the machine is provided with anti-condensation resistances, it is not allowed to use them as device to dry the
winding. The heaters must be powered during the usual stops of the machine only in order to avoid
condensation.
The stators can be directly heated having direct current circulate in them (using for instance an industrial
welder). In that case we recommend that the current circulating in the windings is about 25% of the rated current
of the machine, controlled in order to achieve the desired temperature.
When possible, the windings of the electric machine must be suitably reconnected so as to adapt their
resistance to the value of the available generator in direct current.
963857252_= 35