AK-HP 780
Heat pump controller RS8GF202 © Danfoss 2015-08
81
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
Sctrl-temperature might very well change zone for a period
of time when the controller is starting/stopping a compressor
i.e. the temperature 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 temperature 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.
Compressor timers
Time delays for cutins and cutouts
To protect the compressor against frequent restarts three time
delays can be put in.
- A minimum time to run from a compressor’s startup and until it
may be restarted.
- A minimum time (ON-time) for the compressor to operate before
it may be stopped again.
- A minimum OFF time to run from a compressor stops and until it
may be restarted
When unloaders are cut in and out, the time delays will not be
used.
Timer
The operating time of a compressor motor is registered continu-
ously. You can read out:
- operating time for the previous 24-hour period
- total operating time since the timer was last set to zero-set.
Coupling counter
The number of relay cutins and cutouts is registered continuously.
The number of starts can be read out here:
- Number during the previous 24-hour period
- Total number since the counter was last set to zero-set.