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Installation, operation and maintenance instructions
High voltage flameproof motors for explosive atmosphere
AMD Rg 355-400-450-500
Medium voltage windings maintenance
Maintenance of the windings
The maintenance of medium voltage windings (i.e.
rated voltage of the motor between 1,5 and 12 kVolt)
requires the following actions (see relevant Sheet):
regular check of the winding temperature by
means of the temperature detectors
between the coils.
An abnormal difference between the detected
temperatures can indicate an incipient
problem of poor dissipation of the heat generated by
the circulation of current. When such a situation
would arise, further inspections and diagnosis could
be necessary to clarify the problem.
Measurement
(suggested every three months, but at least one
time per year).
The measurement technique is described on the
relevant Sheet, together with some indication of the
acceptable values. It is important to remember that
the insulation resistance values are not correlated in
any way with the actual dielectric strength of the
insulation, but only supply information on wetness
and degrees of contamination of the insulation itself.
The a.m. information is very useful for scheduling the
possible cleaning and drying actions. In any case the
most important parameter for the judgment is the
comparison of the measured value with the previous
measurements. Remember, for the comparison
purpose, to take always into account the effect of the
temperature on the measured insulation resistances,
so take and record every time the winding
temperature.
In case the disconnection of the motor terminals from
the feeding cables would be a problem, it is possible
to carry out the measurement including even the
insulation resistance of the cables, if their dielectric
condition can be considered safe and sound.
It is also common to take into account the
"Polarization Index" that is the ratio between the
insulation resistance value taken after 1 minute from
the application of the direct voltage to the object
under test and the same value taken after 10
minutes from the application of the voltage.
inserted
fault or a
of
the
insulation
resistance
R
1min
PI = ---------
R
10min
Sheet No
3AAM101046 E
Rev. B
Page 1 of 4
The advantage given by the Polarization Index is its
independence from the temperature, at least for
windings temperature lower than 50°C (122°F). The PI
normally ranges from 1 to 46;
When the winding is humid and dirty the PI is near 1,
whereas when the winding is in normal condition the PI
is at least around 22,5. It is important to note that
occasionally the PI can give misleading values;
sometime very low PI values were measured on modern
epoxy-mica insulation, although the windings have been
dry and the insulation resistance very high (thousands of
MΩ). So when estimating the situation of a winding, the
insulation resistance value has to be considered more
important than the PI.
Internal visual inspection of the winding at overhaul.
A dielectric condition diagnosis is suggested at
overhaul.
Visual inspection
The winding visual inspection reveals information about:
The rate of contamination for presence of dirt and
humidity.
The condensation of humidity inside the motor.
The mechanical stability of the winding.
The possible overheatings (generalised or local)
It is strongly suggested that the results of every
inspection should be recorded.
When examining the winding the following points have to
be remembered:
The dirt accumulated on the windings' overhangs can
close the spaces between the coils, diminishing the
cooling capacity of the machine. As a result the winding
temperature
on load increases,
acceleration of the winding's ageing.
Mechanical strain, vibration and shocks may produce
cracks near the retaining support of the coils or around
the core slot edges. Loose supports or moving slot
wedges are sign of deterioration too. Abrasion marks or
presence of yellow powder near the retaining supports
and slot ends must be checked and investigated.
Complete loosening of slot wedges or bent coils are
serious problems, that must be rectified immediately. On
the contrary, the loosening of few magnetic wedges (if
any) is not a serious problem, providing that the loosing
process would not be continuous; the motor can work
without problems even with some loosen magnetic
wedges. The replacement of the loosen wedges can
even be scheduled at the following overhaul.
Humidity on the winding is shown by the marks it leaves
behind, like rust on iron components, drop marks, and
wetting marks on dirt layers. Signs of tracking current on
the coils' surface with "bush" patterns and charred
residues are sign of probable incipient failure and their
reason must be investigated and resolved.
Very noticeable are the effects of short period strong
overloading or fault condition that can produce
01.2023
with consequent
Issued by: MOLM
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