Engineering manual - UniSAB III 1.10.8
001930 en 2021.06
93/346
Compressor control and surveillance
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 picture.
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 values/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 values/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 completely 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.
If the internal suction pressure is below the high alarm limit 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 alarm limit
and the actual internal suction pressure. The valve will open completely when there is a constant OFF
signal to the valve, i. e. when the internal suction pressure has been lower than the alarm limit for a
while.
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