Engineering manual - UniSAB III 1.10.8
001930 en 2021.06
73/346
Compressor control and surveillance
Outer loop
If PID is selected, it is not recommended to adjust the P band further down than 10°C and T.int. further
down than 30 seconds. Higher values may very well occur. T.diff. is usually always 0.
The neutral zone can be set for 0.4°C or higher if it is not required that the suction pressure is kept as
narrow as possible around the set point. If the neutral zone is too high, the regulator may fluctuate too
much.
If I is selected, the outer loop is working more moderately than when PID is selected. A P band of less
than 10°C may therefore be used. Requirements for the neutral zone are the same as when PID is se-
lected. As already mentioned, T.int. and T.diff. are not accessible when PID = No.
Inner loop
As regards the inner loop it is not recommended to use any other values than the default values for Neu-
tral zone (Nz = 0.0) and Proportional band (P band = 200%).
The parameter Min Pulse determines the pulse duration up or down every time the solenoid valve is acti-
vated. If the value is high, a correspondingly large capacity change is required before the regulator re-
acts with an up or down signal. This is a kind of neutral zone. Default value is 0.5 seconds and should
not be higher than 2 seconds.
The parameter Run Time defines how long it will take to run the slide from 0-100% at constant up signal
or from 100-0% at constant down signal. Usually slide speed is always adjusted on the oil needle valves
positioned on the capacity regulating block so that the slide speed is approximately 60 seconds when
the oil is warm from operation. This adjustment is usually carried out by the service engineer who starts
up the compressor unit.
If the slide speed for some reason cannot be adjusted for 60 seconds, the Run Time parameter must be
changed so that run time corresponds to the actual operating conditions. If this is not done, it will appear
as if the PID regulator is out of control and it may cause hunting.
Example 1
A screw compressor with a combination of slide regulation and frequency regulation has a slide speed
at 60 seconds from 0-100% slide movement. Depending on min. and max. frequency this is shown as e.
g. 0-42% capacity on the UniSAB III display. When the slide has reached 100%, the frequency converter
will start to increase the frequency to the max. frequency set. This will take 60 seconds and be shown as
42-100% in the UniSAB III display. Thus the Run time parameter must be set for 60+60=120 seconds
for correct regulation. However, the Run time parameter only corresponds to the slide regulation and
should therefore be set to 60 seconds.
For screw compressors connected to frequency converters see also the UniSAB III sequence manual.
Example 2
PID = Yes
Values for outer loop:
Nz = 0.0
P band = 10.0
T. int. = 60.0
T. diff. = 0.0
If the compressor is running very unsteadily during regulation or two compressors cannot find one an-
other in parallel operation, raise the P band to e.g. 50°C.
If the running of the compressor has steadied, but the regulator seems slow to reach its set point, reduce
the P band a little at a time until the running of the compressor becomes unsteady again. Now raise the
P band to the last applicable value. If the suction pressure does still not reach its set point (depending
on the setting of Nz) a reduction in the integral time may be necessary to fine-tune the regulator.
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