Modulation In Neutral Zone; Auxiliary Control; Control With Backup Probes And/Or Probes Not Working - Carel PRK300L3FK User Manual

In the situation shown in the fi gure, the request sent by the controller
decreases/increases slowly as soon as the controlled value is outside of
the Neutral zone, while it decreases/increases quickly the further the
controlled value moves away from the Neutral zone; in this way the
response of the system is faster when further from steady conditions.
Note: When using fi xed times, the maximum and minimum must
be set to the same value. In this case, the request sent by the
controller decreases/increases constantly inside the deactivation/
activation diff erential.
6.2.3

Modulation in Neutral zone

pRack pR300 can activate a specifi c function inside the Neutral zone if
modulating devices are used (e.g.: inverters).
This function can be enabled in main menu branch C.a.g/C.b.g or D.a.g/
D.b.g.
Modulation in Neutral zone is used to vary the request sent by the
controller inside the Neutral zone proportionally so as to enter the
decrease zone with the minimum request and the increase zone with the
maximum request, meaning a device can be immediately deactivated/
activated when exiting the Neutral zone.
This makes it possible to remain longer inside the neutral zone without
starting or stopping any device.
An example of this operation is shown in the fi gure:
Regulation request
Neutral zone
100%
P1
Fig. 6.g
When entering the Neutral zone, the pRack pR300 software calculates
how the request needs to change in order to exit the Neutral zone at
minimum or maximum output, and applies one of the two values
according to the trend in variation in the control variable. For example, at
point P1 in the fi gure, the trend of the two requests is represented by the
segments with thin lines, and the request 'reverses' because at that point
the control variable has started increasing in value again.
Note: When exiting the Neutral zone, it is possible that the request
is not at the minimum or maximum value, where limitation is
enabled for of the modulating device variation speed.
6.2.4 Control with backup probes and/or probes not
working
pRack pR300 can use backup control probes that are activated when the
normal control probes are not working.
The backup probes must be enabled in main menu branch C.a.g/C.b.g
or D.a.g/D.b.g.
When diff erent pRack boards are used to manage the suction and
condenser lines, the backup suction pressure probe must be connected
to the board that manages the suction line, while the backup condensing
pressure probe can be connected either to the board that manages the
suction line or the board that manages the condenser line.
If the main control probes are not working and no backup probes are
fi tted, or the backup probes are also not working, or the corresponding
temperature probes are also not working, fi xed values are used for the
control request, set in main menu branch C.a.g/C.b.g or D.a.g/D.b.g.
Regulation probe value
35
6.2.5

Auxiliary control

pRack pR300 off ers the possibility to control the compressors on a single
suction line (or L1 for double lines) using an auxiliary probe. Normal
control based on the suction pressure probe reading (or converted
temperature) can be replaced by control based on a diff erent probe.
This solution can manage the secondary refrigerant in "water chiller"
or "pumped" systems, off ering greater system stability and at the same
time guaranteeing compressor safety via the suction probe, which must
always be installed.
Auxiliary control is enabled under "Compressors -> Control", selecting the
type of control (by temperature or pressure) and the type of refrigerant,
which may diff er from the main one.
The function does not need to be enabled if "water chiller" is set in the
installation Wizard
as auxiliary control by temperature probe is enabled automatically.
Choosing a "pumped" system, on the other hand, automatically enables
auxiliary control by pressure probe (see Appendix A.2)
Once the auxiliary probe is enabled on screen Cab20, the associated
universal input, the probe type, the correct limits (for pressure probes)
and calibration (if necessary) can be selected under "Inputs Outputs ->
Status -> Analogue inputs"
The type of control, band limits or diff erentials and the set point need
to be confi gured under "Compressors -> Control", in the same way as for
traditional control.
The limits for the auxiliary probe alarms, are set under "Compressors ->
Alarms" and need to be confi gured depending on the type of probe and
refrigerant. When an alarm goes off , this is saved in the log, with a special
screen shown when pressing the alarm bell.
Note: when auxiliary control is enabled, it is recommended to also
enable the prevent low suction pressure function, see paragraph 8.3.4
(prevent low suction pressure). In case of pumped systems, don't
confi gure the second condensing line.
ENG
pRack +0300025EN rel. 1.3 - 17.12.2015