Fault Tracing Of Load Cells, Junction Boxes And Wiring - ABB PFTL 101 User Manual

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Pressductor PillowBlock Load Cells, Horizontal Measuring PFTL 101, User Manual
universal joints. Longitudinal expansion of the drive shaft should also be taken into account. Since
such expansion is often taken up by splines, these must also be lubricated. The symptoms are
instability of the signal, for instance jumps in the signal during slow running.
Fault Tracing of Load Cells, Junction Boxes and
wiring
The load cell is very robust and can withstand high overloads. The data of a Pressductor load cell
does not change slowly, but in steps, usually in connection with an event in the mill. Excessive
overloading usually results in permanent shifting of the zero point.
Poor contact in the junction box causes intermittent faults. Both sensitivity and zero point may vary.
Check all screw terminals. Do not use pins crimped to the connecting wires, as these often work
loose after a time.
The cabling, especially the cable to the load cell, is the part that is most exposed to damage.
Since the resistance of the load cell windings is low, it is easy to check the load cells and cabling
from the control unit.
Typical readings are 1 Ω for the resistance of the primary winding and 1-3 Ω for the output impe-
dance of the secondary winding.
Insulation faults in the cabling or the load cell may cause incorrect sensitivity or unstable zero point.
When the load cell circuits have been isolated from earth and from the control unit at the discon-
nectable terminals, it is easy to measure the insulation from the control unit.
If the cables are not routed correctly, they may pick up interference from other cables.
Load cell A
B 3
C 2
D
4
A 1
Load cell B
B 3
2
C
D 4
A
1
Figure 16. Typical load cell cabling
Junction box
6 Fault Tracing
Control unit
EXC+
EXC-
+
INPUT A
-
+
INPUT B
-
3BSE009965R0401 en Rev E