Carel MPXPRO User Manual page 35

During the day most of the load of the showcase is due to the warm
air that enters from the outside and mixes with the cool air inside.
Control based on the intake probe, due to high temperature outside
the showcase and the mixing of the air, may not manage to reach the
set point. Displaying the intake temperature would show a temperature
that is too high. Setting a set point that is too low for the intake probe Sr
may cause the food to freeze. On the other hand, displaying the outlet
temperature would show a temperature that is too low. Consequently,
the display of the control probe, set point or virtual probe can be
confi gured using parameters /t1 and /t2.
ON/OFF control on the outlet probe is defi ned by:
• set point;
• diff erential.
These values determine the control request and consequently, allowing
for the protection times, disabling functions or activation/deactivation
delays, the activation and deactivation of the compressor.
Par. Description
St Set point
rd Set point diff erential St
R
ON
OFF
St
Key
St set point
rd diff erential
ON/OFF control depends on the capacity of the produce to absorb and
release heat, as well as on the evaporator cooling time. The temperature
therefore fl uctuates above and below the set point, and this may cause
a decline in the quality of food conservation. Decreasing the diff erential
to make control more precise increases the frequency at which the
compressor starts and stops and therefore additional wear.
Precision of the measurement is in any case limited by the tolerance of
both the controller and the probe.
Night-time operation
During night-time operation the curtain on the display case is closed
and consequently less cold inside air is mixed with warm outside air.
The thermal load decreases. The temperature of the air that cools
the produce is near the outlet temperature, and therefore to avoid
excessively low temperatures and reduce energy consumption, the set
point needs to be increased at night, by setting parameter r4. Parameter
r6 can then be used to possible the virtual probe Sv or intake probe Sr as
the control probe. Naturally, the change to night-time operation must
be signalled externally. This is usually done using the curtain switch, set
with the parameters relating to the digital inputs (A4, A5, A10, A11, A12),
signalling that the curtain has been lowered, or by setting the time bands
(parameters tS1 to tS8 and tE1 to tE8), from the supervisor, or from the
Master controller via the Master/Slave network. Fot time bands setting,
see par. 3.4.
Par. Description
r4 Automatic night-time set point variation
r6 Probe for night-time control
0 = virtual probe Sv;
1 = intake probe Sr
tS1 to 8 Start time band 1 to 8 day
tE1 to 8 End time band 1 to 8 day
Def Min Max
50 r1 r2
2 0.1 20
Tab. 5.n
Sreg
rd
Fig. 5.f
Sreg control probe
R control request
Def Min Max
0 -50 50
0 0 1
- - -
- - -
Tab. 5.o
Variable
Control probe (Sreg)
Set point
day
night
During the daytime:
• Setpoint= St
• light on
• control on virtual probe Sv
During the nighttime:
UoM
• Setpoint= St++r4
°C/°F
• light off
°C/°F
• control on Sr (if r6= 1) or on Sv (if r6= 0)
"Weighted control" and "double thermostat" can be used for automatic
changeover to night-time operation without an external signal.
Weighted control
This function compensates for the disadvantages of control based solely
on the outlet probe or the intake probe. The control probe becomes the
virtual probe:
Sv =
The weighted average of the outlet and intake probes is used to
compensate for the mixing of air from outside the display case. Normally
the weight of /4 is set to 50% and the value of the virtual probe can be
chosen for both display and temperature recording. The value of the
virtual probe thus becomes the mean value of the outlet and intake
probes and the measurement that best corresponds to the temperature
of the produce. Another advantage is automatic adaptation to night-time
operation with the curtain closed, without needing an external signal.
When the curtain is open there is immediately an increase in load on the
evaporator, consequently the outlet temperature is lowered so as to keep
the average temperature constant.
night
T
Sr
Sv
Sm
Key
T temperature
t time
Sr intake probe
Double thermostat and control with electronic valve
See paragraph 6.5.
UoM
°C/°F
-
-
-
35
Night-time control
Daytime control
r6= 0
Virtual probe (Sv) Virtual probe (Sv) Intake probe (Sr)
St St+r4
tS1 tE1 tS2 tE2
Fig. 5.g
Sm Sr
day
Fig. 5.h
Sv virtual probe
Sm outlet probe
MPXPRO - + 0300055EN rel. 1.1 30/08/10
ENG
r6=1
Tab. 5.p
t
tS8 tE8
t