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 in-
crease by the size of the drop. To avoid disengaging and re-engaging in turn, the limiter will
ensure that the stage is not re-engaged 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 values/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 pow-
er (kW). For reciprocating compressors 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 cur-
rent 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 Control 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 pic-
ture 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)
Note in particular that this limiter only applies to HPO and HPC compressors and that it has a
fixed alarm limit of 25.2 bar and a fixed shutdown limit of 26 bar. The limiter is released at 25
bar and is passive between 25 and 25.2 bar.
Compressor control and surveillance
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Engineering manual - Unisab III 1.10
001930 en 2014.09
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