Version 2.02 15.05.2007 Function of VS 3000
18
Standstill time is always maintained after the first compressor has been shut down by the suction pressure con-
troller or fault conditions. With a capacity-controlled compressor the first stage is only loaded again when the
start delay and the minimum standstill time have both elapsed. A variable-speed compressor starts on elapse of
the minimum standstill time and runs at minimum speed for the duration of the start delay.
3.15 Load shedding
Forced shutdown of compressors may be needed as a means of preventing energy consumption from rising above a
specified level. Three digital inputs are provided for load shedding on the VS 3000 Pack Controller. Compressors are
shut down directly. The maximum number of compressors disabled by load shedding is equal to the number of acti-
vated load shedding inputs
Regardless of the load shedding signals, a minimum refrigeration capacity must be maintained, which means that a
minimum number of compressors must be enabled. At minimum, one compressor remains enabled at all times. The
compressor cannot be disabled by load shedding in a system equipped with only one compressor.
3.16 Emergency power mode
Emergency power mode can be activated by the Emerg. working parameter (Menu 3-1). Emergency working mode is
a variant of load shedding and serves to reduce power line load in electric power supply fault conditions (e.g. store
power supply by emergency generator).
When emergency power mode is activated by the above parameter, the additional entry No.emerg.stages is shown in
the operating screen (Menu 3-1). This is used to define the maximum number of compressor stages allowed to oper-
ate in emergency power mode. The parameter can be selected between the following limits:
- Minimum of one compressor stage (provision of minimum refrigeration capacity)
- Maximum of one compressor stage less than the maximum configuration
When emergency working mode is selected, it is activated through Digital Input 19 (Emerg. working / Load rejection
stage 3). See Section 5 - Pin and Terminal Connections.
3.17 Compressor base load rotation
Running time of each compressor is internally monitored. So as to ensure uniform run time of all compressors, the
compressor having the longest running time is disabled and that having the shortest running time is enabled at the end
of a definable cycle time.
Base load rotation is only performed with capacity-controlled compressors when the base load stage of a compressor
is available. With base load rotation, the compressor having the longest run time is disabled and the compressor hav-
ing the shortest running time is loaded.
In base load rotation with capacity-controlled compressors the control state of the capacity stage(s) is also applied for
the new compressor.
Base load rotation makes allowance for compressors disabled by load shedding. The number of compressor stages
operating is not changed by compressor base load rotation. Compressor base load rotation is active only w hen the
following conditions are satisfied:
- When all defined compressors are enabled, base load rotation takes place only at rising pressure within the neu-
tral zone.
- If compressors have been disabled by load shedding, base load rotation only takes place at rising pressure.
Compressor base load rotation can be set with the CycleTimeC.parameter (Menu 3-7). Especially when the system
is equipped with screw compressors, which have an oil return feature that does not necessitate base load rotation, this
parameter can be set to --, thereby deactivating base load rotation.
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