Operating instructions

3.23.2 25-2* Bandwidth Settings
Parameters for setting the bandwidth within which the
pressure will be allowed to operate before staging/destaging
fixed speed pumps. Also includes various timers to stabilize
the control.
25-20 Staging Bandwidth
Range: Function:
10
%
*
[Applica-
tion
depend-
ant]
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
par. 20-13 Minimum Reference/Feedb. and
par. 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
%
*
[Applica-
tion
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 require-
ment. The override bandwidth (OBW) is program-
med to override the staging/destaging timer
(par. 25-23 SBW Staging Delay and par. 25-24 SBW
Destaging Delay) for immediate response.
The OBW must always be programmed to a higher
value than the value set in Staging Bandwidth
(SBW), par. 25-20 Staging Bandwidth. The OBW is a
percentage of par. and par. .
25-21 Override Bandwidth
Range: Function:
depend-
ant]
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 par. 25-25 OBW Time.
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:
Applica-
tion
depend-
ent
*
[Applica-
tion
depend-
ant]
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.CA.02 - VLT
®
is a registered Danfoss trademark 169
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