6.14.2 25-2* Bandwidth Manager
Parameters for setting the bandwidth within which the
pressure/temperature will be allowed to operate before
staging/destaging fixed speed compressors. Also includes
various timers to stabilize the control.
25-20 Staging Bandwidth [%]
Range:
Function:
10%
*
[1 -
100 %
]
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
parameter 3-03 Maximum Reference
and
parameter 3-02 Minimum Reference
. For example, if
parameter 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.
Figure 6.26 Staging Bandwidth
25-21 Override Bandwidth [%]
Range:
Function:
100% =
Disabled
*
[1 –
100%]
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
(
parameter 25-23 SBW Staging Delay
/
parameter 25-24 SBW Destaging Delay
) for
immediate response.
The OBW must always be programmed to
a higher value than the value set in
parameter 25-20 Staging Bandwidth
(SBW).
The OBW is a percentage of
parameter 3-03 Maximum Reference
.
25-21 Override Bandwidth [%]
Range:
Function:
SBW
SBW
Setpoint
Override Bandwidth
175ZA673.10
Figure 6.27 Override Bandwidth
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
parameter 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.
25-22 Fixed Speed Bandwidth [%]
Range:
Function:
10%
*
[1 -
100%]
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 [Off] or [Hand On] 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
Parameter Descriptions
VLT
®
Compressor Drives CDS 302/CDS 303
114
Danfoss A/S © Rev. 2013-07-03 All rights reserved.
MG34M422
6
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Summary of Contents for VLT CDS 303
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