Insulation Test; General Considerations - MarelliMotori MXB-E 180 Serie Manual De Instalación, Funcionamiento Y Mantenimiento

5.1 Insulation test

5.1.1 Measurement of insulation strengths
At the group manufacturer, if the alternator remained inactive for a long time (over a month), before its
commissioning it is highly recommended to perform an insulation test to ground of the main stator windings.
Detailed instructions are provided in the international standard IEEE Std. 43-2000.
Before performing such test, it is necessary to disconnect the connections which go to control devices (voltage
controller or other devices).
The measurement of the insulation resistance between the windings and the ground is performed with a specific
measurement device (Megger or equivalent) powered by direct current and with output voltage (test voltage)
equal to 500 V for low voltage machines. The value of the insulation resistance is to be recorded after 1 minute
from the application of the test voltage.
For the measurement of the insulation strength, proceed as indicated below:
 Main stator: the measurement of the insulation resistance shall be executed after disconnecting the
connections which go to the control devices (voltage control or other devices) or to any other device of
the group. The measurement shall be executed between a phase and ground with the remaining two
connected as well to ground together with the auxiliaries (operation to be repeated for all three phases).
See Figure 1 Measurement of the insulation strength on the stator winding.
 Exciter stator: disconnect the cables + and – from the controller and measure the insulation strength
between one of these two terminals of the winding and the ground.
 Rotor windings: measure the insulation resistance between a terminal of the winding of the main rotor
on the bridge rectifier and the rotor ground (shaft).
(Se Figure 2 Measurement of the insulation strength on the rotor winding).
The measured values shall be recorded. In case of doubt measure as well the polarization index as described in
Chapter 3.
Polarization index
In order to avoid electrocution dangers, connect the windings to ground for a short time just after the
measurement.
Figure 1 Measurement of the insulation strength on the stator winding.
Figure 2 Measurement of the insulation strength on the rotor winding.

5.1.2 General considerations

Please, take note of the following considerations, before deciding the actions to be undertaken based on the
insulation resistance tests:
963857252_=
Fig. 1
Fig. 2
33