Note that some screw compressors do not have the 'Forced Speed' function nor any capacity
slide. In that case the compressor will start at minimum speed and then the VSD will be re-
leased for speed control between min. and max. speed.
In auto/remote operation the compressor starts on demand and the drive increases to
'Forced Speed' to achieve oil pressure. At the same time the capacity slide valve starts to
move up to the Manual zero point. When main oil pressure is achieved and there is a positive
demand. Expect the drive to stay on the 'Forced Speed' value until the capacity slide valve
gets to 100%. Then the drive is released to increase speed. This initial phase depends on de-
mand, and if it means that the capacity gets too high because of the capacity increase from
the slide valve moving up, the drive will decrease speed to compensate. Otherwise, during
normal operation, the capacity slide will stay at 100% and capacity is controlled by speed. At
very low capacity with minimum speed, the capacity slide will move back.
Note that some screw compressors do not have the 'Forced Speed' function nor a capacity
slide. In that case the compressor will start at minimum speed and the VSD will be released
for speed control between min. and max. speed.
If during operation the VSD/motor is overloaded, Unisab III will reduce the capacity slide po-
sition to unload the compressor and reduce the torque/current. Torque reduction cannot be
done by reducing speed. Some screw types without capacity slide are equipped with a relief
valve which in case of overload will unload the compressor and reduce the torque/current.
3.3.25 Oil heating
Oil heating is not a configuration point, but it has various functions depending on whether the
compressor in question is configured as a reciprocating or a screw compressor.
For screw compressors the heating element will be connected while the compressor is not in
operation. Except for SAB 120-151, SAB 157HR and SAB 193-233-283-355 there is no tem-
perature regulation.
SAB 120-151, SAB 157HR, SAB 193-233-283-355
When the compressor is not in operation, Unisab III will control the heating element in the oil
separator. If the temperature is lower than the set point the heating element will be activa-
ted. The set point and neutral zone can be set in the in the menu Control values/Oil/Separator
temp/Control.
Normal setting for SAB 120-151, SAB 157HR and SAB 193-233-283-355
Set point 1 = 50°C
Neutral zone = 2°C
Regardless of oil separator temperature, the heating element will be deactivated when the
compressor is running.
Compressor control and surveillance
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Engineering manual - Unisab III 1.10
001930 en 2014.09
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