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 com-
pletely when there is a constant OFF signal to the valve, i.e. when the internal suction pres-
sure has been lower than the alarm limit for a while.
Low oil pressure limiter (SABflex and SABCube)
If the oil pressure is lower than the low 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 low alarm limit for the oil pressure is set in the picture Control values/Oil/Pressure/Limits.
If the oil pressure is below 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 oil pressure. The valve will close completely when there is a con-
stant ON signal to the valve, i.e. when the actual oil pressure has been lower than the alarm
limit for a while.
If the oil 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 oil pressure. The valve will close completely when there is a con-
stant OFF signal to the valve, i.e. when the actual oil pressure has been higher than the alarm
limit for a while.
Start-up RPM limiter (SAB 157HR)
This limiter will limit the speed during compressor start-up. If the suction pressure is higher
than the High alarm limit for suction pressure, this limiter will during the start-up phase limit
speed to minimum speed, default 1,000 [rpm].
To leave the Start-up RPM limiter the suction pressure must be well below the suction pres-
sure alarm limit. The neutral zone used is 1.0 Bar.
The start-up limiter can maximum be active for the time set in the timer Start RPM limiter
(default 300 seconds). When the limiter is active the Start RPM limiter timer is running and a
Start up RPM limiter alarm is shown. If the Start RPM limiter timer runs out while the limiter is
still active the compressor is stopped by a shutdown Start RPM limiter error.
Start-up limiter (SABflex and SABCube)
This limiter will throttle the (S)SSTV valve and limit the speed during compressor start-up, if:
• The internal suction pressure is higher than the High alarm limit for internal suction
pressure or the oil pressure is lower than the low alarm limit curve Y', then this limiter
will, during the start-up phase, limit the SABCube speed to 1,000 [rpm].
• The external suction pressure is higher than the High alarm limit for internal suction
pressure or the oil pressure is lower than the low alarm limit curve Y, then this limiter
will too, during the start-up, keep the (S)SSTV valve closed.
To leave the “Start-up limiter”, and then slowly open (S)SSTV valve, the external suction
pressure must be well below the internal suction pressure alarm limit and the oil pressure
must be well over the oil pressure alarm curve Y.
The start-up limiter is left when both these two conditions are fulfilled
P
suct
< ‘High Alarm limit’ – NZ
suct,int
for P
suct,int
and P
oil,rel
> ‘Low Alarm curve’ Y + NZ
oil,rel
for
P
oil,rel
Compressor control and surveillance
88/319
Engineering manual - Unisab III 1.10
001930 en 2014.09
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