ABB DC Machines DMI User Manual

19
Measuring of
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
m
= kV+1 (1)
R
m
= the lowest recommended insulation
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):
R
40 °C
= K
t40 °C
x R
t
(2)
R
40 °C
= insulation resistance corrected to 40 °C
R
t
= measured insulation resistance in MΩ
at t °C
K
t40 °C
= correction factor according to the
diagram below.
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
t40 °C
at 6 °C = 0.1
The recommended minimum insulation
resistance R
m
according to formula (1) is:
R
m
= 0.750 + 1.0 = 1.75 MΩ
According to formula (2):
R
40 °C
= 0.1 x 100 = 10 MΩ
Conclusion
R
40 °C
is larger than R
m
and the insulation
resistance is approved.
Winding temperature at
the time of measurement
Correction factor to 40° C
for insulation resistance
Fig. 11 Correction factor.
0
.
1
0
.
05
0
.
5
100
-1
0
0
1
0
2
0
30
4
0
50
60
7
0
80
90
1
00
°
C
1.
0
5
1
0
50
Insulation resistance
K
t40 °C

ABB DC Machines DMI Specifications

General

Brand	ABB
Model	DC Machines DMI
Category	Engine
Language	English