Siemens 1PH728 Operating Instructions Manual page 32

Startup
4.1 Preparations
The insulation resistance of the winding to the motor enclosure can only be measured when
the winding temperature is between 20 and 30°C:
When making the measurement, you must wait until the final resistance value is reached
(approx. 1 minute).
Limit values
The table below specifies the measuring circuit voltage and the limit values for the minimum
insulation resistance and the critical insulation resistance with a rated motor voltage of U
kV:
Measuring circuit voltage
Minimum insulation resistance with new, cleaned or repaired
windings
Critical specific insulation resistance after a long operating
time
Note the following points:
• Dry, reconditioned windings have insulation resistances between 100 and 2000 MΩ,
possibly even higher values.
If the insulation resistance is close to the minimum value, then the cause could be either
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 for a 25°C winding
temperature can be calculated, depending on the rated voltage, by multiplying the rated
voltage (kV) by the specific critical resistance value (5 MΩ/kV);
e.g. critical resistance for a rated voltage (U
690 V x 5 MΩ/kV = 3.45 MΩ
Notice
If the critical insulation resistance is less than or equal to this value, the windings must be
dried or, if the fan is removed, cleaned thoroughly and dried.
Please note that the insulation resistance of dried, clean windings is lower than that of
warm windings. The insulation resistance can only be evaluated accurately when
measured on a winding that has been cooled down to room temperature (approx. 20 to
30°C).
Notice
If the measured value is close to the critical value, the insulation resistance should be
subsequently checked at appropriately frequent intervals.
(Values apply for measurement at a winding temperature of 25°C)
4-2
Rated voltage UN < 2 kV
500 V (min. 100 V)
10 MΩ
5 MΩ/kV
) of 690 V:
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Siemens AG A5E00177602A
Operating Instructions V 2.0 1PH728
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