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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 pres-
sure in the system while the SBW timers are running. The value
can be optimized with increased familiarity with the system. See
Override Bandwidth Timer
, par. 25-25.
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 comple-
ted, 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 main-
tain 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 re-
quire 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 Control-
ler.
25-23 SBW Staging Delay
Range:
Function:
15 sec.
*
[0-3000 sec.]
Immediate staging of a fixed speed pump is not desirable when
a momentary pressure drop in the system exceeds the Staging
Bandwidth (SBW). Staging is delayed by the length of time pro-
grammed. If the pressure increases to within the SBW before
the timer has elapsed, the timer is reset.
VLT
®
AQUA Drive Programming Guide
2. Parameter Description
MG.20.O2.02 - VLT
®
is a registered Danfoss trademark
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