25-20 Staging Bandwidth
Range:
Function:
10
%
*
[ 1 -
par.
25-21
%]
Set the staging bandwidth (SBW) percentage to
accommodate normal system pressure fluctuation.
In cascade control systems, to avoid frequent
switching of fixed speed pumps, the desired system
pressure is typically kept within a bandwidth rather
than at a constant level.
The SBW is programmed as a percentage of
20-13 Minimum Reference/Feedb.
and
20-14 Maximum Reference/Feedb.
. For example, if
the set-point is 5 bar and the SBW is set to 10%, a
system pressure between 4.5 and 5.5 bar is
tolerated. No staging or de-staging will occur
within this bandwidth.
25-21 Override Bandwidth
Range:
Function:
100
%
*
[ par.
25-20
- 100
%]
When a large and quick change in the system
demand occurs (such as a sudden water demand),
the system pressure rapidly changes and an
immediate staging or destaging of a fixed speed
pump becomes necessary to match the
requirement. The override bandwidth (OBW) is
programmed to override the staging/destaging
timer (
and
) for immediate response.
The OBW must always be programmed to a higher
value than the value set in
Staging Bandwidth
(SBW),
. The OBW is a
percentage of
and
3-03 Maximum Reference
.
Setting the OBW too close to the SBW could defeat
the purpose with frequent staging at momentary
25-21 Override Bandwidth
Range:
Function:
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
.
To avoid unintended staging during the commis-
sioning 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:
Size
related
*
[ par.
25-20 -
par.
25-21
%]
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 pump 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 pump is running, a wider
Fixed Speed Bandwidth (FSBW) is used
instead of SBW. It is possible to stop the fixed
speed pumps, 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 pumps. This is basically the same
as Emergency Stop (Coast/Coast inverse
Command) for the Cascade Controller.
Parameter Description
VLT
®
HVAC Drive Programming Guide
MG.11.CC.02 - VLT
®
is a registered Danfoss trademark
179
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