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Reference manual RS8ED102 © Danfoss 03-2006
EKC 316A
Details on the controller algorithm and settings
Kp factor and Kp min
The Proportional Gain is dependant on the value of the measured
superheat SH relative to Reference superheat SH ref. The Propor-
tional Gain has the following values relative superheat SH:
If SH is more than 2.5K greater than SH ref then Gain equals Kp
factor.
If SH is within the range 2.5K and -0.5K from SH ref, then Gain
equals Kp factor times Kp min.
The reason for this variable Gain is to give stabile superheat for
values near the superheat reference.
Note the value of Gain does not change suddenly but gradually
when SH gets close to SH ref.
Initial start-up “Kick start”
In general the valve opening degree is controlled by the measured
value of the superheat SH. This means that during certain situa-
tions during start up, the valve will be slow to open due to the
built of superheat from a small value. To prevent this happening,
the valve is given an initial opening degree dependant on the Kp
factor, the measured superheat SH and SH closes as given in the
following relationship:
Initial OD% = kp factor*(SH – SH close)
The regulation modes for controlling superheat
There two different ways of controlling superheat i.e. control-
ling according to the minimum stable superheat (MSS) and Load
Defined superheat.
The parameter SH mode selects the controlling form where it can
be set to MSS when set to 1, or Load Defined superheat when set
to 2.
SH mode = 1
The superheat reference SH ref is adaptive adjusted
The diagram shows how the SH ref is decreased 0.1 K every 30
seconds until instability in SH is detected. The SH ref will continue
increase at 0.1 K every 30 seconds until stability is achieved again.
More less the same behaviour will continue as shown where the
operation is attempting to find the minimum stable superheat.
When using this form of control, there are three settings that have
major affect on this mode of control
Finding the optimum settings
SH ref
Kp factor multiplied by Kp min
Kp factor
Proportional Gain
-0.5
2.5
Kp factor * (SH – SH Close)
OD %
Time
START
When Kp factor = 3, SH = 12, SK Close = 2
Initial OD % = 30%
0
2
4
6
8
10
12
14
16
18
SH
SH_ref
