Version 2.02 15.05.2007 Function of VS 3000
26
3.25 Monitoring low pressure
All compressors are shut down if low pressure drops below a definable limit (Menu 3-4 t
0
Comp. OFF). As previ-
ously described, the compressors are loaded stagewise when low pressure rises to the level proportional to the
t
0
setpoint + NZ/2.
Alarm is signalled after a set delay. The delay (Menu 3-4 Del. t
0
OFF) and priority are definable. The value of the
limits is entered in °C, from which the controller then calculates an absolute proportional pressure value. Addi-
tionally the LP cutout is evaluated. Forced shutdown of all compressors takes place if pressure drops below the
limit set on the LP cutout.
HP monitoring is shown diagrammatically in the following illustration:
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VS 3000 LP monitoringcontrol algorithm
neutral zone (NZ)
t
Time
0
Suction pressure
t
0 Setpoint
LP-Limit
Compressor shutdown
Alarm is signaled
Compressor load
Alarm delay
3.26 Monitoring compressor starts
The number of compressor starts per hour is limited as a means of preventing short cycling of compressors.
C1
Time
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VS 3000 Starts monitoring
C2
C3
Start time delay = 6 Min.
Off time
Start time delay = 6 Min.
Off time
Start time delay = 6 Min. No off time
The Compressor Starts/h parameter (Menu 3-4) determines the minimum time intervals at which a compressor
can be started. If, for example, the parameter is set to 10 starts per hour, the compressor cannot be started
earlier than every 6 minutes.
With variable-speed compressors, limiting of compressor starts is deactivated. A High Starts alarm will be gen-
erated in occurrence of a system fault due to short-cycling of variable-speed compressors if the maximum num-
ber of starts set by the Compressor Starts/h parameter is exceeded.
Compressor starts limiting deactivated for variable-speed compressors will be re-activated in the event
of a frequency changer fault.
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