Siemens 1PH728 Operating Instructions Manual page 42

Start-up
4.1 Preparations
Limit values
The table below specifies the measuring circuit voltage as well as the limit values for the
minimum insulation resistance and the critical insulation resistance with a rated motor
voltage of U
Table 4-1
Measuring circuit voltage
Minimum insulation resistance with new, cleaned or repaired
windings
Critical specific insulation resistance after a long operating
time
Note the following:
● Dry, new windings have insulation resistance of between 100 and 2000 MΩ, or possibly
● Over its operating lifetime, the motor winding insulation resistance can drop due to
[ID: 343.01]
40
< 2 kV:
N
Measurement voltage, minimum insulation resistance, critical specific insulation
resistance
values that are even higher.
If the insulation resistance is close to the minimum value, then the cause could be
humidity and/or fouling.
ambient and operational influences. The critical insulation resistance for a temperature of
25°C on the winding can be calculated by multiplying the rated voltage (kV) by the
specific critical resistance value (5 MΩ/kV);
Example: Critical resistance for a rated voltage (U
690 V x 5 MΩ/kV = 3.45 MΩ
NOTICE
Cleaning and/or drying the windings when reaching critical insulation resistance
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.
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
Measured value close to critical value
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.
Rated voltage U < 2 kV
N
500 V (at least 100 V)
10 MΩ
5 MΩ/kV
) of 690 V:
N
Siemens AG Operating Instructions 2.02 1PH728
A5E00177602A AD