Functions
208
7UM62 Manual
C53000-G1176-C149-3
Figure 2-99
Curves of the Displacement Voltage U
g
, the Shunt Voltage U
Meas
and the Measurement Current I
g
Monitoring
Functions
On each polarity reversal, the charging current of the rotor earth capacitor is deter-
mined. A drop of this current allows to detect defects in the measurement circuit such
as wire breaks, poor brush contact etc. This is only possible, however, if the rotor-earth
capacitance is high enough (> 0.15 µF) and the interference from the excitation sys-
tem is low.
As an alternative, the protection function offers an external test option using a test re-
sistor (included in the 7XR6004 and 7XR6003). The test mode is activated via a binary
input, and the fault resistor is connected to a slip ring with an external relay. The pro-
tection function must be informed of the pertinent test resistance. The protection func-
tion issues appropriate indications showing the test results. It is also able to detect
one-sided interruptions (such as wire breaks or loose terminals in a coupling (see Fig-
ure 2-98)).
The evaluation logic is shown in Figure 2-100.
I
g1
= I
g2
= 0
u
Meas
(t)
i
g
(t)
5 mA
5 mA
t
R
E
∞
U
g
50 V
50 V
2
⋅
U
g
u
g
(t)
t
Displacement voltage U
g
u
Meas
(t)
i
g
(t)
I
offset
I
g1
t
I
g2
2
⋅
I
g
R
E
= 5 k
Ω
I
g2
- I
g1
= 2
⋅
I
g
I
offset
= Interference
2
⋅
U
g
2
⋅
I
g
=
R
s
2
+ R
E
, with R
M
<< R
s
