Insulation; Measuring Insulation Resistance - ABB DC Machines DMI Operating And Maintenance Instructions Manual

Measuring of

insulation

resistance
Insulation resistance
The measurement should be done with a 500 V
megger.
It is not practical to state a definite minimum
value of the insulation resistance. In some cas-
es the machines can operate satisfactorily with
lower values than the recommended
minimum value. The recommended minimum
insulation resistance can be obtained from the
following formula (1):
R = kV+1 (1)
m
R = the lowest recommended insulation
m
resistance in MΩ at 40 °C
kV = the rated voltage of the machine in kV
For recalculation of the measured insulation
resistance to 40 °C, the following must be taken
into consideration: Temperature dependence
of the insulation resistance. When measuring
the insulation resistance, the temperature of the
winding should be measured at the same time
to enable the insulation resistance to be
recalculated to correspond to a temperature of
40 °C. This can be accomplished by using the
following formula (2):
(2)
R = K x R
40 °C t40 °C t
R = insulation resistance corrected to 40 °C
40 °C
R = measured insulation resistance in MΩ
t
at t °C
K = correction factor according to the
t40 °C
diagram below.
Correction factor to 40° C
for insulation resistance
Fig. 11 Correction factor.
Example
In the winding of a 750 V motor, the insulation
resistance has been measured as 100 MΩ at
6 °C. We obtain from the diagram:
K at 6 °C = 0.1
t40 °C
The recommended minimum insulation
resistance R
R = 0.750 + 1.0 = 1.75 MΩ
m
According to formula (2):
R = 0.1 x 100 = 10 MΩ
40 °C
Conclusion
R is larger than R
40 °C
resistance is approved.
K
t40 °C
100
50
1 0
5
1. 0
0 . 5
0 . 1
0 . 05
-1 0 0 1 0 2 0 30 4 0 50 60 7 0 80 90 1 00 °C
according to formula (1) is:
m
and the insulation
m
Winding temperature at
the time of measurement
19