WEG G truck Serie Manual De Instalación, Operación Y Mantenimiento página 13

Table 2.2: Insulation resistance referential limits in electric
machines
Polarization index
1 or lower
< 1,5
1.5 to 2.0
2.0 to 3.0
3.0 to 4.0
> 4.0
The minimum allowable value for the Rm insulation
resistance is given by:
Rm = Un + 1
Where:
Rm - recommended minimum insulation resistance in
Mega ohm with the winding temperature of 40 ºC.
Un - rated voltage of the machine, in kV.
If the test is done at a temperature different from 40°C, 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 following curve can be used:
Insulation assessment
Bad
Dangerous
Fair
Good
Very Good
Excellent
10258257 – Installation, Operation and Maintenance Manual
To convert the insulation resistance
measured (Rt) to 40 ° C multiply by the
temperature coefficient (Kt)
Winding temperature
Figure 2.1: Insulation resistance variation coefficient according to
the temperature
The insulation resistance is generally measured with a
MEGOHMMETER.
If the insulation resistance is less than the values obtained
by the above formula, the alternators must be submitted
to a drying process mentioned in item 8.6.
ATTENTION
Disconnect all voltage regulator wires before
measuring insulation resistance.
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