Engineering manual - UniSAB III 1.10.8
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001930 en 2021.06
Compressor control and surveillance
Motor current limiter (reciprocating compressors)
When one stage is disengaged because of the limiter, the resulting motor current drop will be measured.
It is assumed that if the disengaged stage is re-engaged, the motor current will increase by the size of
the drop. To avoid disengaging and re-engaging in turn, the limiter will ensure that the stage is not re-en-
gaged until the resulting current is below 95% of the limit for the limiter zone.
Note that the alarm limit for
Motor current
is
Set point 1
or
Set point 2
selected in the picture
Control val-
ues/Motor/Current/Control
. If Set point 1 = 1000 Amp, the limit for the limiting zone will be 1000 – 5% =
950 Amp. The same limitation is used when measuring
Motor power (kW)
. For reciprocating compres-
sors with VSD, high motor current will reduce the motor speed.
Motor current limiter (screw compressors)
Capacity cannot be increased if the motor current is within the limiter zone. If the motor current set point
is set to 1000 Amp, the limiting zone will start at 1000 - 5% = 950 Amp. If the motor current becomes
higher than 1000 Amp, the limiter decreases capacity. The size of the decrease signal will be calculated
according to the difference between the set point and the actual motor current.
Note that the alarm limit for motor current is set in
Set point 1
or
Set point 2
selected in the picture
Con-
trol values/Motor/Current/Control
. The same limitation is used when measuring
Motor power (KW)
.
Motor power limiter (screw and reciprocating compressors with VSD)
When the motor power is high the load is released by reducing the motor speed.
The set point limit for motor power is set in the
Set point 1
or
Set point 2
selected in the picture
Control
values/Motor/Power/Control
.
If the motor power is above the set point, then the limiter will start to reduce the motor speed. The size of
the decrease signal is proportional to the difference between the set point and the actual motor power.
If the motor power is inside the neutral zone it will not be possible to increase the motor speed.
If the motor power is below the set point + neutral zone the speed regulation will be set free and it is
again possible to increase the speed in automatic and manual operation
High difference pressure limiter (reciprocating compressors)
This limiter protects HPO, HPC and HPX compressors from running at a too high differential pressure
(meaning too much difference between the discharge and the suction pressure).
The high differential pressure limiter uses a fixed alarm limit, a fixed shutdown limit and a fixed limiting
zone of 0.2 bar. The limiter is
active
when the difference pressure is above alarm limit,
passive
when the
difference pressure is in the limiting zone and
watching
when the difference pressure is at or below the
limiting zone.
See Table 15 for information about alarm and shutdown limits.
Summary of Contents for SABROE Unisab III Control
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