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On machines that are already in operation, higher values
of insulation resistance can be measured, compared to
the initial values of commissioning.
The comparison with values obtained in previous tests on
the same machine, in similar load, temperature and
humidity conditions is as a better indication of the
insulation conditions than the value obtained in a single
test, seeing that any sudden reduction is considered
suspicious.
Table 4.2: Referential limits of the insulation resistance in electrical
machines
Insulation resistance value
2 M or lower
< 50 M
50...100 M
100...500 M
500...1000 M
> 1000 M
4.5.5
Minimum insulation resistance
If the measured insulation resistance is below 100 M
at 40 ° C, the windings must be carefully inspected
and cleaned or, if necessary, dried according to the
following procedure before the machine goes into
operation:
Disassemble the alternator by removing the rotor and
bearings;
Place the components that have the winding with low
insulation resistance in an industrial oven and heat it up
to a temperature of 130 °C and keep this temperature
for at least 08 hours.
Check if the insulation resistance achieved is within the
acceptable values, in accordance with Table 4.2,
otherwise contact WEG.
4.5.6
Polarization index
The polarization index (I.P.) is typically defined by the ratio
between the measured insulation resistance in 10
minutes and the insulation resistance measured in 1
minute at relatively constant temperature. By means of
the polarization index, the insulation conditions of the
alternator can be evaluated according to Table 4-3.
Table 4-3 - Polarization index (ratio between 10 and 1 minute)
Polarization index
1 or lower
< 1.5
1.5 to 2.0
2.0 to 3.0
3.0 to 4.0
> 4.0
DANGER
Immediately after measuring the insulation
resistance, ground the winding to avoid
accidents.
Insulation assessment
Dangerous
Bad
Regular
Good
Very Good
Excellent
Insulation assessment
Dangerous
Bad
Regular
Good
Very Good
Excellent
4.5.7
Conversion of the measured values
If the test is done at a different temperature, it is
necessary to correct the reading for 40 °C by using a
curve for the variation of the insulation resistance
considering the temperature, determined with the
machine itself. If this curve is not available, the
approximate correction provided by the curve of Figure
4.7 can be used, according to NBR 5383 / IEEE43.
In order to convert the insulation resistance
measured (Rt) to 40°C, multiply it by the
temperature coefficient (Kt)
Winding temperature °C
R
40ºC
Figure 4.7: Coefficient of insulation resistance variation according to
temperature
4.6 PROTECTIONS
4.6.1
Thermal protections
The alternators have protection devices against
temperature rise, installed on the main stator coils and
bearings, as follows:
Thermoresistance (RTD) - It is a calibrated resistance
element. Its operation is based on the principle that the
electrical resistance of a metallic conductor varies linearly
with temperature. The terminals of the detector must be
connected to a control panel, which includes a
temperature meter.
Installation, operation and maintenance manual
www.weg.net
= Rt x Kt
40ºC
|
21
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