Danfoss VLT CDS302 Operating Instructions Manual page 110

Parameter Descriptions
25-20 Staging Bandwidth [%]
Range:
10% [1 -
*
100
%]
6 6
25-21 Override Bandwidth [%]
Range:
100% = [1 –
Disabled 100%]
*
110
Function:
Set the staging bandwidth (SBW) percentage to
accommodate normal system pressure fluctuation.
In cascade control systems, to avoid frequent
switching of fixed speed compressors, the desired
system pressure is typically kept within a
bandwidth rather than at a constant level.
The SBW is programmed as a percentage of the
numerically larger value of 3-03 Maximum
Reference and 3-02 Minimum Reference. For
example, if 3-03 Maximum Reference is 10 bar and
the SBW is set at 10%, a system pressure between
4.0 and 6.0 bar is tolerated if the setpoint is 5 bar.
No staging or destaging will occur within this
bandwidth.
Illustration 6.27
Function:
When a large and quick change in the
system demand occurs, the system pressure
rapidly changes and an immediate staging
or destaging of a fixed speed compressor
becomes necessary to match the
requirement. The override bandwidth (OBW)
is programmed to override the staging/
destaging timer (25-23 SBW Staging Delay/
25-24 SBW Destaging Delay) for immediate
response.
The OBW must always be programmed to a
higher value than the value set in
25-20 Staging Bandwidth (SBW). The OBW is
a percentage of 3-03 Maximum Reference.
MG34M302 - VLT
®
Operating Instructions VLT CDS302 and CDS303
25-21 Override Bandwidth [%]
Range:
25-22 Fixed Speed Bandwidth [%]
Range:
10%
*
100%]
®
is a registered Danfoss trademark/Commercial Compressor
Function:
Illustration 6.28
Setting the OBW too close to the SBW
could defeat the purpose with frequent
staging at momentary pressure changes.
Setting the OBW too high might lead to
unacceptably high or low pressure in the
system while the SBW timers are running.
The value can be optimized with increased
familiarity with the system. See 25-25 OBW
Time.
To avoid unintended staging during the
commissioning phase and fine tuning of the
controller, initially leave the OBW at the
factory setting of 100% (Off). When the fine
tuning is completed, the OBW should be set
to the desired value. An initial value of 10%
is suggested.
Function:
[1 - When the cascade control system is running
normally and the frequency converter issues a
trip alarm, it is important to maintain the system
head. The Cascade Controller does this by
continuing to stage/destage the fixed speed
compressor on and off. Due to the fact that
keeping the head at the setpoint would require
frequent staging and destaging when only a
fixed speed compressor is running, a wider Fixed
Speed Bandwidth (FSBW) is used instead of SBW.
It is possible to stop the fixed speed
compressors, in case of an alarm situation, by
pressing the LCP OFF or HAND ON keys or if the
signal programmed for Start on digital input
goes low.
In case the issued alarm is a trip-lock alarm then
the Cascade Controller must stop the system
immediately by cutting out all the fixed speed
compressors. This is basically the same as
Emergency Stop (Coast/Coast inverse Command)
for the Cascade Controller.