5.1 Preparations for installation
For a winding temperature of approx. 25° C, the minimum insulation resistances are 20 MΩ
(U ≤ 1000 V) or 300 MΩ (U > 1000 V). The values apply for the complete winding to ground.
Twice the minimum values apply to the measurement of individual assemblies.
● Dry, new windings have an insulation resistance of between 100 and 2000 MΩ, or possibly
● Over its operating lifetime, the motor winding insulation resistance can drop due to ambient
Measuring the polarization index
1. To determine the polarization index, measure the insulation resistances after one minute
2. Express the measured values as a ratio:
For insulation resistances > 5000 MΩ, the measurement of the PI is no longer meaningful and
consequently not included in the assessment.
Damage to insulation
If the critical insulation resistance is reached or undershot, this can damage the insulation
and cause voltage flashovers.
● Contact the Service Center (Page 137).
● If the measured value is close to the critical value, you must subsequently check the
Limit values of the anti-condensation heating insulation resistance
The insulation resistance of the anti-condensation heating with respect to the machine housing
should not be lower than 1 MΩ when measured at 500 V DC.
even higher values. An insulation resistance value close to the minimum value could be
due to moisture and/or dirt accumulation. The size of the winding, the rated voltage and
other characteristics affect the insulation resistance and may need to be taken into account
when determining measures.
and operational influences. Calculate the critical insulation resistance value depending on
the rated voltage by multiplying the rated voltage (kV) by the specific critical resistance
value. Convert the value for the current winding temperature at the time of measurement,
see above table.
and ten minutes.
PI = R
insul 10 min
insul 1 min
Many measuring devices display these values automatically following the measurement.
insulation resistance at shorter intervals.
Insulation in good condition
Dependent on the complete diagnosis of the insulation
SIMOTICS TN Series N-compact 1LA8
Operating Instructions 05/2016