Siemens SIMOTICS SD Operating Instructions Manual page 68

Commissioning
7.1 Insulation resistance
Measuring the insulation resistance
1. Before you begin measuring the insulation resistance, please read the operating manual
for the insulation resistance meter you are going to use.
2. Disconnect any connected main-circuit cables from the terminals before measuring the
insulation resistance.
3. Where possible, measure the insulation resistance of the winding with respect to the
motor enclosure when the winding temperature is between 20 ... 30 °C. Different
insulation resistance values apply for other temperatures.
4. When measuring, wait until the final resistance value is reached. This is reached after
approximately one minute. Then read off the insulation resistance.
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
Measuring circuit voltage
Minimum insulation resistance for new, cleaned or
repaired windings
Critical specific insulation resistance after a long operating
time
Note the following:
● If the measurements are performed at winding temperatures ≠ 25 °C, convert the
measured value 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 a typical insulation resistance of more than 100 ... 2000 MΩ
depending on the winding size, design and rated voltage. An insulation resistance value
close to the minimum value could be due to moisture and/or dirt accumulation.
● During operation, the insulation resistance of the windings can fall to the critical insulation
resistance due to ambient and operational influences. The critical insulation resistance
value for a winding temperature of 25 °C can be calculated by multiplying the rated
voltage (kV) by the specific critical resistance value (0,5 MΩ / kV).
68
Insulation resistance of the stator unwinding at 25 °C
500 V
10 MΩ
0,5 MΩ / kV
Operating Instructions, 05/2014, 5 610 00000 02 000
Low-voltage motors