Siemens RDF302 - Control Outputs

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Siemens RDF302 Basic documentation CE1P3079en
Building Technologies 07 Dec 2011
3.8 Fan control
The fan operates in automatic mode or at the selected speed with manual mode.
In automatic mode, the fan speed depends on the setpoint and the current room
temperature. When the room temperature reaches the setpoint, the control valve
closes and the fan switches off or stays at fan speed 1 (parameter P60; factory
setting: 0 = fan speed 1 in dead zone).
Only one fan output at a time is on, either Q1, Q2 or Q3.
The fan speed and mode can be changed via Modbus.
The fan speed and mode can be monitored via Modbus.
The individual switching points for ON of each fan stage can be adjusted via control
parameters P55…P57. The fan speed switch off point is 20% below the switch on
point. The diagrams below show fan speed control for modulating PI control.
w Room temperature setpoint
Q Fan speed
Y
H
Control demand “Heating”
Y
C
Control demand “Cooling”
XpH Proportional band “Heating”
XpC Proportional band “Cooling”
X
dz
Dead zone
w
XpH
Y
C
[%]
Q
Q3
Q2
Q1
Y
H
[%]
0
0
XpC
100
100
x
dz
The diagram only shows the PI thermostat’s proportional part.
On applications with 2-position control:
1) The switching point for low fan speed (Q1) is synchronized to the heating /
cooling output. Parameter “Switching point fan speed low” P57 is not relevant.
2) The maximum switching range of the fan (XpH
Fan
/ XpC
Fan
) is defined by the
switching differential (SDH/SDC) via a look-up table.
T[°C] Room temperature
w Room temperature setpoint
Q Fan speed
Y Control command “Valve”
SDH Switching differential “Heating”
SDC Switching differential “Cooling”
X
dz
Dead zone
XpH
Fan
Switching range for fan “Heating”
Cool
T[°C]
w
XpH
Fan
Q
Q3
Q2
Q1
T[°C]
Y
1
0
w
XpC
Fan
x
dz
SDC
SDH
XpC
Fan
Switching range for fan “Cooling”
SDH/SDC [K] 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 >4.5
XpH
Fan
/XpC
Fan
[K] 2 3 4 5 6 7 8 9 10
Fan command value
Fan operation
Fan output
3-speed fan control
with modulating
heating / cooling
control
Note:
3-speed fan control
with ON/OFF heating /
cooling control
Look-up table with
ON/OFF control