Manual for Induction Motors and Generators
Maintenance -
81
3BFP 000 050 R0101 REV E
R40 = 0.25 x 30 M
Ω
= 7.5 M
Ω
Table 7-4. Temperature values in degrees Celsius (ºC) and degrees Fahrenheit (ºF)
7.6.4.2 General considerations
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.
7.6.4.3 Minimum values for insulation resistance
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(
1-10 min at 40 °C
) > 5 M
Ω
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
) > 1000 M
Ω
. If the measuring conditions are extremely warm
and humid, R(
1-10 min at 40 °C
) values above 100 M
Ω
can be accepted
For used stators: R(
1-10 min at 40 °C
) > 100 M
Ω
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.
°C
0
10
20
30
40
50
60
70
80
90
100
110
°F
32
50
68
86
104
122
140
158
176
194
212
230
