Basics of temperature control / measurement
Page 6
RESM-5
resistance value for each temperature based on its
alloy-specific TCR (temperature coefficient of
resistance).
This value may be altered by external impacts such as
partial short-circuits, contacting problems, changed
cross sections etc., i.e. the controller measures an
incorrect resistance and responds by increasing or
reducing the heating current, depending on the
direction of the change, in an attempt to "follow" this
false set point. Major deviations from the temperature
set point are often attributable to faults such as these.
Whereas an increase in the measured resistance
(narrower cross section due to physical damage,
detached or corroded clamps, worn copper plating)
simply leads to a harmless reduction in the actual
heatsealing band temperature, a partial short-circuit
(ground fault, conductive foreign particles, physical
contact between two heatsealing bands) can cause the
temperature of the band sections which are still active
to rise significantly. Dangerous levels involving a risk of
fire or explosion can then occur very quickly.
Depending on the design, execution, and wiring of the
heatsealing tools and the interconnection of the
heatsealing bands, numerous potential fault causes
can thus be identified.
3.2
Basics
The diagram below is an electrical schematic of a
sealing bar showing the heatsealing band and the
power and measurement cables.
Note:
The term "measured resistance" or "resistance
measured by the controller" is used in this document to
refer to the resistance between two connection points
A - A' on the voltage measurement cable U
R
.
In the diagram on page 6 this is the resistance R.
Only resistance faults which occur between the
measuring points
A - A' are relevant for the
overheating problems described here. External faults
are not visible to the controller and are therefore not
important for our purposes.
R
r
R
Voltage measurement cable U
R
Controller
RES
U
R
I
R
R
Current transformer
A
D'
CURRENT CONDUCTING PARTS
or TRANSITIONS
POWER CONNECTION
EXPANSION JOINT
HEATSEALING BAND
CLAMP
COPPERED END
HEATSEALING BAND
B
C'
C
B'
D
A'
Δ
R
Δ
r