Engineering manual - UniSAB III 1.10.8
001930 en 2021.06
85/346
Compressor control and surveillance
3.1.12
Capacity control with user input 1
UniSAB III must be set in
Remote
mode in the picture
Setup/Compr. Control
. Select
Setup/Configura-
tion/External input
, and choose
capacity set point control
.
Ex.: The set point of the capacity slide on a screw compressor is to vary from 0 to 100% corresponding
to a change in the current signal from 4-20 mA.
Move the cursor to 4 mA, press
and change the value to 0 (skip this if the value is already 0). Move
the cursor to 20 mA, press
and change the value 100. (The value can be set to 102 to make sure that
the capacity can always be increased to 100%). Move the cursor to the second line, press
and select
Capacity SP
with
or
. Finish by pressing
. The picture will now look like this:
The capacity slide will now move from 0% to 100% corresponding to a change in the current from 4-20
mA.
The capacity on a
reciprocating compressor
can be controlled the same way. The reciprocating com-
pressor changes one capacity stage up or down depending on whether the signal is above or below the
percentage corresponding to the capacity stage in question. The timers
Delay up
and
Delay down
,
which are used during the loading/unloading of capacity stages, are also active when capacity regulation
takes place with an external 4-20 mA signal.
This means that for an SMC 108 without total unloading the following changes apply:
0%
< = Signal
< 25%
= 0% capacity
25%
< = Signal
< 50%
= 25% capacity
50%
< = Signal
< 75%
= 50% capacity
75%
< = Signal
< 100%
= 75% capacity
100%
< = Signal
= 100% capacity
This works as follows:
Start
When the signal is above 5% (4.8 mA) and
Auto start
=
Yes
has been configured, the timer
Delay before
start
will begin to count down (screw compressors).
For reciprocating compressors, the timer
Delay before start
will start once the signal is above the lowest
capacity stage. When the timer
Delay before start
has counted down, the compressor will start and fol-
low the signal up/down. Note, however, that various limiters may reduce the compressor capacity.
Stop
When the signal is below 5% (4.8 mA) and
Auto stop
=
Yes
has been configured, the timer
Delay before
stop
will begin counting down (screw compressors). For reciprocating compressors, the timer
Delay be-
fore stop
will start once the signal is below the lowest capacity stage. For reciprocating compressors with
total unloading, the timer
Delay before stop
will be activated when the signal is below 0%. When the
De-
lay before stop
timer has finished counting down, the compressor will stop.
If the current signal drops to -10% (2.4 mA), the chosen form of regulation will change (e.g.
suction pres-
sure regulation, brine regulation
, etc). If the signal exceeds -10% once more, regulation will change back
to
capacity remote control
. When UniSAB III is configured to
capacity set point
, the start number
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