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Dynamic extension of the neutral zone
All refrigeration systems have a dynamic response time when
starting and stopping compressors. In order to avoid that the
controller will start/stop compressors shortly after each other, the
controller must be allowed some extra time after a compressor
start/stop to see the effect of the previous change in running
capacity.
In order to achieve this, a dynamic extension of the zones is
added.
The zones will be extended for a short period of time when
starting or stopping a compressor. By extending the zones the
PI controller will be slowed down in a short period of time after a
change in compressor capacity.
The amplitude of the zone extension depends upon the actual
running compressor capacity and upon the size of the compressor
step which is being stopped/started. The amplitude of the zone
extension is bigger when running with low compressor capacity
and when starting/stopping big compressor capacity steps.
However the time period for the zone extension is constant – after
a fixed time period after a compressor start/stop the dynamic
zone extension is reduced to 0.
Via the "Minimize number of couplings" setting it is possible to
influence how big the amplitude of the dynamic zone extension
should be in order to minimize the cycling of the compressors.
By setting "Minimize number of couplings" to "No reduction" there
will be no dynamic extension of the zones.
By setting "Minimize number of couplings" to "Low", "Medium" or
"High" the dynamic extension of the zones will be activated. The
amplitude of the zone extension will be highest when "Minimize
number of couplings" is set to "High". Please refer to the next
sketch which shows an example with 6 compressor steps and with
"Minimize number of couplings" set to "High". Please also note that
the dynamic extension of the zones is highest at low compressor
capacity.
"Minimize number of couplings" = "High"
Actual band
As a consequence of the dynamic extension of the zones the
suction pressure might very well change zone for a period of
time when the controller is starting/stopping a compressor i.e.
the suction pressure is in the +Zone, but as the controller starts
a compressor, the zones are extended for a period of time and
during this period of time the suction pressure will be in the NZ.
In the controller the readout "Actual band" will show in which zone
the PI controller is operating – this includes the extension of the
zones.
The dynamic expansion of the neutral zone is not used when one
of the compressors in the group is speed-regulated.
AK-PC 781
Capacity controller RS8GG602 © Danfoss 2015-03
95
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