General Considerations; Minimum Values For Insulation Resistance - ABB AMA Series Manual

7.6.4.2 General considerations

7.6.4.3 Minimum values for insulation resistance

3BFP 000 050 R0101 REV E
R40 = 0.25 x 30 MΩ = 7.5 MΩ
Table 7-4. Temperature values in degrees Celsius (ºC) and degrees Fahrenheit (ºF)
°C 0 10 20
°F 32 50 68
The following consideration should be noted, before deciding any actions based upon the
insulation resistance tests:
• If the measured value is considered too low the winding must be cleaned and/or dried. If
these measures are not sufficient, expert help should be acquired
• Machines, that are suspected to have moisture problem, should be dried carefully
independent of the measured insulation resistance value
• The insulation resistance value will decrease when the winding temperature rises
• The resistance is halved for every 10 ... 15 K temperature rise.
NOTE: The insulation resistance indicated in the test report is normally considerably higher
than the values measured on site.
Criteria for windings in a normal condition:
Generally, the insulation resistance values for dry windings should exceed the minimum values
significantly. Definite values are impossible to give, because resistance varies depending on the
machine type and local conditions. In addition, the insulation resistance is affected by the age
and usage of the machine. Therefore, the following values can only be considered as guidelines.
The insulation resistance limits, which are given below, are valid at 40 °C, and when the test
voltage has been applied for 1 minute or longer.
• Rotor
For induction machines with wound rotors: R(
NOTE: Carbon dust on slip rings and naked copper surfaces lower the insulation resistance
values of the rotor.
• Stator
For new stators: R(
1-10 min at 40 °C
and humid, R(
1-10 min at 40 °C
For used stators: R(
1-10 min at 40 °C
NOTE: If the values given here are not reached, the reason for the low insulation resistance
should be determined. A low insulation resistance value is often caused by excess
humidity or dirt, although the actual insulation is intact.
Manual for Induction Motors and Generators
30 40 50 60
86 104 122 140 158 176 194 212 230
1-10 min at 40 °C
) > 1000 MΩ. If the measuring conditions are extremely warm
) values above 100 MΩ can be accepted
) > 100 MΩ
70 80 90 100 110
) > 5 MΩ
Maintenance - 81