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/
Configuration/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% cor-
responding 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 recipro-
cating compressor 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 compres-
sor will start and follow 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 compres-
sors, the timer Delay before 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 activa-
ted when the signal is below 0%. When the Delay before stop timer has finished counting
down, the compressor will stop.
Compressor control and surveillance
Engineering manual - Unisab III 1.10
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