Siemens H-compact PLUS, Operating Instructions Manual

Page: 36 / 83

Start-up
7.1 Checking the insulation resistance
WOGeneric TEAAC / 2008
34
Siemens AG Operating Instructions 1.03 1RQ671...-Z
Limit values of the stator winding insulation resistance
The following table indicates the measuring circuit voltage and the limit values for the
minimum insulation resistance and the critical insulation resistance of the stator winding.
Table 7-1 Stator winding insulation resistance at 25°C
Rated voltage
U
N
< 2 kV
Rated voltage
U
N
≥ 2 kV
Measuring circuit voltage 500 V 500 V
Minimum insulation resistance with new,
cleaned or repaired windings
10 MΩ 100 MΩ
Critical specific insulation resistance after a
long operating time
0.5 MΩ/kV 5 MΩ/kV
Note the following:
● If the measurements are performed at winding temperatures other than 25°C, the
measured value will need to be converted to the reference temperature of 25°C in order
to be able to compare the values with the table above.
– The insulation resistance halves every time the temperature rises by 10 K.
– The resistance doubles every time the temperature falls by 10 K.
● Dry, new windings have insulation resistances of between 100 and 2000 MΩ, or possibly
even higher values. Insulation resistance close to the minimum value could be due to
humidity and/or dirt accumulation.
● Over its operating lifetime, the motor winding insulation resistance can drop due to
ambient and operational influences. The critical insulation resistance value for a winding
temperature of 25°C can be calculated, depending on the rated voltage, by multiplying
the rated voltage (kV) by the specific critical resistance value;
Example:
Critical resistance for 3.3 kV rated voltage:
3.3 kV x 5 MΩ/kV = 16.5 MΩ
NOTICE
Critical insulation resistance reached or undershot
If the critical insulation resistance is reached or undershot, this can result in damage to
the insulation or voltage flashovers.
Dry the windings, or thoroughly clean and dry them (remove the rotor first).
•
Once the clean windings have been dried and cooled to almost 25°C, a smaller
insulation resistance will be measured. The insulation resistance can only be
properly assessed after conversion to the reference temperature of 25°C. Allow the
windings to cool down to 25°C, or convert the insulation resistance to the reference
temperature of 25°C
•
If the measured value is close to the critical value, you must check the insulation
resistance at suitably frequent intervals.