Johnson Controls SABROE Unisab III Control User Manual

3.2.3 Special limiters
High suction pressure limitation
The effect of the high suction pressure limiting function is that the compressor capacity is
limited to an adjustable maximum value whenever the suction pressure is above the suction
pressure alarm limit.
Max. capacity at high suction pressure limitation is set in the picture: Setup/Configuration/
Plant as High limit xxx.x %. High limit can be set between 0% and 100% where a setting of
100% disables the function. The text Limiting high suction pressure appears in the Alarm pic-
ture.
For reciprocating compressors the High limit value must be set to a value corresponding to at
capacity step. E.g. for an SMC 108, the high limit should be set to 25%, 50% or 75%.
Motor torque (current) limiter (SABflex and SABCube)
When the motor torque is high the load is released by throttling the (S)SSTV valve. The level
of torque is in practice measured as the motor current.
The (S)SSTV valve is closed and opened by issuing a pulse modulated signal to it. The period
time of this signal can be set with the parameter Run Time found in the picture Control val-
ues/Suction/Internal pressure/Control.
The set point limit for motor current is set in the Set point 1 or Set point 2 selected in the
picture Control values/Motor/Current/Control.
If the motor current is above the set point + ½ neutral zone the (S)SSTV valve will start to
close. The size of the closing signal, i.e. the pulse/pause ratio, is proportional to the difference
between the set point and the actual motor current. The valve will close completely when
there is a constant ON signal to the valve, i.e. when the actual motor current has been higher
than the set point for a while.
If the motor current is below the set point - ½ neutral zone the (S)SSTV valve will start to
open. The size of the closing signal, i.e. the pulse/pause ratio, is proportional to the difference
between the set point and the actual motor current. The valve will open completely when
there is a constant OFF signal to the valve, i.e. when the actual motor current has been lower
than the set point for a while.
If the motor current is inside the neutral zone the valve closing signal, i.e. the pulse/pause
ratio, will be fixed to the value it had before the neutral zone was entered.
High internal suction pressure limiter (SABflex and SABCube)
If the internal suction pressure gets higher than the high alarm limit the load is released by
throttling the (S)SSTV valve.
The (S)SSTV valve is closed and opened by issuing a pulse modulated signal to it. The period
time of this signal can be set with the parameter Run Time found in the picture Control val-
ues/Suction/Internal pressure/Control.
The high alarm limit for the internal suction pressure is set in the picture Control values/
Suction/Internal pressure/Limits.
If the internal suction pressure is above the high alarm limit the (S)SSTV valve will start to
close. The size of the closing signal, i.e. the pulse/pause ratio, is proportional to the difference
between the alarm limit and the actual internal suction pressure. The valve will close com-
pletely when there is a constant ON signal to the valve, i.e. when the actual internal suction
pressure has been higher than the alarm limit for a while.
Compressor control and surveillance
Engineering manual - Unisab III 1.10
001930 en 2014.09
87/319

Johnson Controls SABROE Unisab III Control Specifications

General

Brand	Johnson Controls
Model	SABROE Unisab III Control
Category	Control Systems
Language	English