2 effect of the cable capacitance, Effect of the cable capacitance -8, Us l – Rockwell Automation Low-Voltage Switchgear and Controlgear User Manual
Page 160
5.3.4.2 Effect
of
the cable capacitance
With AC controls with long control lines, low coil power ratings of the contactors and high control
voltage, depending on the topography of the circuit, the capacitance of the control line can be in
parallel to the controlling contact and practically bypass it when it is open. This can mean that
when the control contact has opened sufficient current continues to flow via the cable capaci-
tance causing the contactor not to drop out. An example may be a contactor that is controlled by
a distantly located sensor (for example limit-switch).
Fig. 5.3-6
When the control contact switches off the cable to the contactor, the capacitance of the line causes at
most a slight drop-off delay.
Fig. 5.3-7
If the long control line to the contactor stays live when the control contact is open, the current via the
cable capacitance can prevent the contactor from dropping out. With pulse contact control, the capaci-
tance of the lines acts twice, whereby the permissible line length is halved.
A worked example would be
I
H
=
0.25
I
CN
U
H
=
0.6
U
C
cos
φ = 0.3
I
H
Holding current of the contactor
I
CN
Rated current the contactor coil
U
H
Drop-out voltage of the contactor
U
C
Control voltage
cos
φ Power factor of the contactor coil (on-state)
The permissible cable capacitance is calculated at 50 Hz approximately to be
C
Z
≈ 500 · S
H
/U
C
2
[
μF]
C
Z
Permissible cable capacitance [
μF]
S
H
Holding power at U
C
[VA]
U
C
Control voltage [V]
At a typical cable capacitance of 0.3
μF/km the permissible line length for maintained contact
control is
2
3
3
.
0
10
500
c
H
z
U
S
l
⋅
⋅
=
[m]
LVSAM-WP001A-EN-P - April 2009
5-8