Fridgewatch 2000 Controller Mk5
Section 3 Publication 3-59
Issue 3.2 : 05.23 Page 176 of 222
Fig 86 Liquid Injection Settings
S220 Discharge Temperature (P16) Set Point (units: °C)
The measured temperature is sensed at the compressor discharge outlet
by P16 (ANIN 18).
This is the discharge temperature value which the Fridgewatch P + I
control algorithm attempts to control to by opening and closing the
electronic liquid injection valve.
S221 Start Setting Liquid Injection (units: °C)
This is the discharge temperature value which Fridgewatch uses to
enable the electronic liquid injection valve for opening.
S222 Stop Setting Liquid Injection (units: °C)
This is the discharge temperature value which Fridgewatch will use to
close the electronic liquid injection valve.
S223 Proportional Band Liquid Injection (units: °C)
This is the deviation from the set point resulting in the maximum possible
change to valve opening/discharge temperature.
Using too small a proportional band setting will cause instability as small
changes in the measured value give rise to large changes in discharge
temperature.
On the other hand, making the proportional band very large makes the
control system very slow to respond to changes in the demand for cooling
capacity.
Before making alterations to S223, S224 and S226, it is essential to have
an understanding of 15.2. Proportional and Integral (P + I) Control and
the possible impact to the system on any changes made to these
settings.
S224 Integral Action Time Liquid Injection (units: second)
This is the time, in seconds, required for the integral action to duplicate
the proportional action.
The integral term in the P + I algorithm is used to remove the steady state
error which occurs with large and therefore stable proportional bands,
once electronic liquid injection valve capacity is as close as possible to
that required.
A low value for the integral action time will produce a greater integral
effect and therefore quickly remove any steady state error, however, this
will lead to hunting if the time period is too short.
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