Danfoss VLT FC 103 Programming Manual page 153

Parameter Description
25-20 Neutral Zone [unit]
Range:
3 3
25-21 +Zone [unit]
Range:
3.00
*
[0-9999.99]
25-22 -Zone [unit]
Range:
3.00
*
[0-9999.99]
152
®
VLT
Function:
of fixed speed compressors, the desired
system pressure is typically kept within a
zone rather than at a constant level.
The NZ is programmed in the same unit as
selected in 20-12 Reference/Feedback Unit. It
places a zone above and below the set-point
in which staging and destaging will not
occur. For example, if the set-point is -
æ20æ˚C and the NZ is set to 4æ˚C, a suction
pressure equivalent to a temperature
between -æ24æ˚C and -æ16æ˚C is tolerated.
No staging or destaging will occur within
this zone.
Function:
When a large and quick change in the system
demand occurs, the system pressure rapidly
changes and a quicker staging or destaging
of a fixed speed compressor becomes
necessary to match the requirement. The
+Zone defines the range where the + zone
delay is active.
Setting the +Zone too close to zero could
defeat the purpose with frequent staging at
momentary pressure changes. Setting the
+Zone too high might lead to an
unacceptably high or low pressure in the
system while the +Zone Delay timer (25-24 +
Zone Delay) is running. The +Zone value can
be optimized with increased familiarity with
the system. See 25-26 ++ Zone Delay.
To avoid unintended staging during the
commissioning phase and fine tuning of the
controller, initially set the +Zone to a large
value beyond any expected pressure peak.
This implicitly disables the override function
for pressure peaks. When the fine tuning is
complete, the +Zone should be set to the
desired value. An initial value of 3æ˚C is
suggested.
Function:
When a large and quick change in the
system demand occurs, the system pressure
rapidly changes and a quicker staging or
destaging of a fixed speed compressor
becomes necessary to match the
requirement. The -Zone defines the range
where the - zone delay is active.
Setting the -Zone too close to zero could
defeat the purpose with frequent staging at
momentary pressure changes. Setting the -
Zone too high might lead to an unacceptably
MG16H102 - VLT
Refrigeration Drive Programming Guide
25-22 -Zone [unit]
Range:
25-23 Fixed Speed Neutral Zone [unit]
Range:
10% [1 -
*
100%]
25-24 +Zone Delay
Range:
120 [0-3000
sec. sec.]
*
25-25 -Zone Delay
Range:
60 sec. [0-3000
*
sec.]
®
is a registered Danfoss trademark
Function:
high or low pressure in the system while
the25-25 - Zone Delay is running. The -Zone
value can be optimized with increased
familiarity with the system. See 25-27 -- Zone
Delay.
To avoid unintended staging during the
commissioning phase and fine tuning of the
controller, initially set the -Zone to a large
value beyond any expected pressure drop.
This implicitly disables the override function
for pressure drops. When the fine tuning is
complete, the -Zone should be set to the
desired value. An initial value of 3æ˚C is
suggested.
Function:
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 [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
Emergency Stop (Coast/Coast inverse Command)
for the Cascade Controller.
Function:
Immediate staging of a fixed speed
compressor is not desirable when a
momentary pressure increase in the system
exceeds the neutral zone (NZ). Staging is
delayed by the length of time programmed.
If the pressure drops to within the NZ
before the timer has elapsed, the timer is
reset and no staging occurs.
Function:
Immediate destaging of a fixed speed
compressor is not desirable when a
momentary pressure drop in the system
exceeds the neutral zone (NZ). Destaging is