Removal Of Moisture From Windings; Maintenance; Inspection And Maintenance Intervals - MarelliMotori MJB 160 Nstrucciones Y Advertencias De Seguridad

The ratio between those insulation resistances is called polarisation index (PI):
POLARISATION INDEX
=
PI
The slope in insulation resistance versus time curve indicates the dryness and cleanliness of a winding.
Winding insulation could be considered GOOD if the diagram obtained is similar to the curve A.
Winding insulation could be considered UNSATISFACTORY if the diagram obtained is similar to the curve B. In that case insulation is
affected by moisture or dirt and should be dried-out and cleaned.

4.9 Removal of moisture from windings

An increase of insulation resistance between phase and earth is normally obtained by removing the moisture.
Several methods can be followed for this scope:
•
Stator winding drying by internal heat source.
Heaters have to be distributed below the generator main winding stator.
•
Stator winding drying-out by self heating method.
The stator can be heated with the circulation of low voltage DC current (i.e. obtained by an industrial welding set) through the windings.
A current of about 25% of the full load current, as marked on the generator rating plate, should be used.
If both phase terminals are available, generator winding can be re-connected to adjust its internal resistance , in order to suit the direct
current supply available. A thermometer should be placed inside the stator windings.
Temperature should not be allowed to exceed 80°C.
Could be useful to cover the machine to conserve the heath.
In case it is possible have to be unclosed all the openings on frame, if available. Those openings if positioned on the top of the
generator (i.e. removing terminal box cover or removing end shields for vertical constructions) can improve moisture escape.
•
Drying of stator with oven heating
You brings the oven to 110 – 150°C maximum, the drying of winding for generators MJB 160 – 200 could continue for 2 – 4 hours
depending on the starting condition insulation resistance.
If the insulation resistance doesn't reach at least the recommended value, it's possible that the cause is a solid contamination.
It will be in this case necessary to clear the winding once more and then repeat the drying process.

5. MAINTENANCE

For safety purposes it is necessary that any testing or maintenance carried out on electrical machine are performed by
qualified and authorised personnel, and all operation must be performed when the machine is stopped, at ambient temperature and
disconnected from any supply source (including the auxiliary circuits such as the anti-condensation heaters). Furthermore all
measures must be taken to avoid restarting of gen-set during maintenance.
The environment in which the generator is put to work must be clean and dry.
In order to block the screws use Loctite® 243 thread-lock, ensuring that they are not dirty with oil/grease (if necessary use Loctite®
7063 or equivalent solvent).
ATTENTION! In the case of electrical connections, the Loctite® must not cover the electrical contact surfaces!

5.1 Inspection and maintenance intervals

Inspection and maintenance should take into account the importance of the plant ambient conditions (dust etc.) and operating
conditions.
As a general rule, the machine should be subjected to a first inspection after approx. 100 operating hours (in any case not more than 1
year) and subsequent inspections when performing maintenance on prime mover.
When performing inspection check that:
- The generator operates smoothly, without noise or irregular vibrations due to bearing deterioration.
- The operating data complies with that detailed on the rating plate.
- The air inlet openings are not obstructed.
- The supply cables show no signs of deterioration and connections are firmly tight.
- The electrical connections are in perfect condition (undamaged).
- Screws and nuts are firmly tightened.
For the above inspections it is not necessary to dismantle the generator, dismantling is only necessary when the bearings are cleaned
or replaced and in that occasion the following additional checks are required:
- Alignment.
- Insulation resistance.
- Tightening of all fixing bolts, screws and nuts.
SIN.UM.805.6
R
°
isol 20 c T10'
R
°
isol 20 c T1 '
INSULATION LEVEL
PI ≤1
Bad
PI <1,5 Dangerous
1,5 < PI < 2 Uncertain
2 < PI < 3 Good
PI > 3 Very good
10
ENGLISH
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