After this start-up period where oil, suction and internal pressure are inside the alarm limits,
the compressor is now in normal operation. The relief valve will be closed and there will be no
restrictions on speed regulation.
Alarm surveillance
During start the following alarms are delayed:
Low oil pressure: 45 sec.
High filter diff. pressure: 600 sec.
Low superheat: 300 sec.
High superheat: 300 sec.
Low/high oil temperature: 300 sec.
The delayed alarms are described in section 6.5. Timers.
Running
After the starting sequence and during normal running operation the following relief valve lim-
iters will be active:
• low oil pressure
• low main oil pressure
• high suction pressure
• high internal pressure
• high torque
• low Vi ratio
See subsection 3.2.3. Special limiters for more information.
Stopping sequence
At normal stop, the speed will be reduced to minimum speed which will stop the frequency
converter. At shutdown, the frequency converter is stopped immediately.
Vi ratio regulation
Unisab III calculates the Vi ratio according to refrigerant and the actual pressure ratio. The Vi
ratio is shown in the /Capacity picture. There is no Vi slide and therefore no Vi regulation, but
the relief valve will be activated if the compressor is running in Vi conditions which is signifi-
cantly less than the configured fixed Vi ratio. This works in two steps:
• If speed is 100% the relief valve will be unloaded when the calculated Vi ratio is less
than 65% of the fixed Vi ratio. This is to protect the compressor.
• If speed is less than 95% the relief valve can be unloaded if the calculated Vi ratio is
less than 80% of the fixed Vi ratio. This is to optimise efficiency. To be activated this
function must be configured, i.e. the configuration point VI comp. plug valve must be
set to Yes.
The compressor can be delivered with a number of fixed Vi ratios which can be configured,
see subsection 6.4.3. Compressor block, VI comp. plug valve.
Compressor control and surveillance
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Engineering manual - Unisab III 1.10
001930 en 2014.09
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