General Considerations - MarelliMotori MXB-E 225 Serie Manual Del Usuario

Before performing this test, disconnect the connections from the regulators (voltage regulators or other devices).
The measurement of the ground mass insulation resistance of the windings is carried out with the appropriate
measuring tools (Megger or equivalent) powered with continuous current and with an output voltage (test voltage)
of 500V for low voltage machines. The insulation resistance value should be recorded 1 minute after being
subjected to the voltage test.
Proceed as follows to measure the insulation resistance:
 Main stator: the insulation resistance is measured by disconnecting the connections from the regulators
(voltage regulators or other devices) or from any other devices on the unit. The measurement will be
performed between a phase and ground mass with the remaining two also ground connected along with
the auxiliaries (the operation must be repeated for all three phases).
See Figure 1 Measuring the insulation resistance on the stator winding.
 Exciter stator: the + and - cables must be disconnected from the regulator and the insulation resistance
must be measured between one of these two winding terminals and the ground mass.
 Rotor windings : measure the insulation resistance between one winding terminal of the main rotor on
the rectifier bridge and the ground mass of the rotor (shaft).
(See Figure 2 Measuring the insulation resistance of the rotor winding.)
The values measured will be recorded. In case of doubt, also measure the polarization index, see Chapter 5.2.
WARNING! In order to avoid the risk of electroshock, shortly connect the winding to the ground immediately
after the measurement.
Figure 1 Measuring the insulation resistance on the stator winding.
Figure 2 Measuring the insulation resistance of the rotor winding.

5.1.2 General considerations

The following considerations must be taken into account, before deciding which actions to take following the
insulation resistance test:
 If the value measured is too low, the winding must be cleaned and/or dried. If the measures indicated
are not sufficient, rely on the help of experts.
 The machines for which humidity issues may be suspected must be carefully dried, regardless of the
value of the insulation resistance measured.
NOTE: The insulation resistance reported on the test report is usually considerably higher than the values
measured on site.
5.1.3 Conversion of the values of the insulation resistance measured
In order to compare the insulation resistance values measured, these are set at 40°C; using the following graph,
the actual data measured is thus converted into a value corresponding to 40°C : the application of this graph
963857253_=
Fig. 1
Fig. 2
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