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R7426A,B,C,D TEMPERATURE AND UNIVERSAL INPUT CONTROLLERS
Table 14. Throttling Range and Reset Time Reference
Application
R7426A
Main Temperature Control
High or Low Limit Temperature
Control
Master Cascade Control
Submaster Cascade Control
R7426B,C (Main Temperature Sequence Control)
Mixed Air Damper
Energy Recovery
Heating
Cooling
R7426B,C (Temperature Cascade Sequence Control)
Master
Submaster
- Mixed Air Dampers
- Energy Recovery
- Heating
- Cooling
R7426D
Main Temperature Control
High or Low Limit Temperature
Control
Master Cascade Control
Submaster Cascade Control
SETTING RESET TIME GUIDELINES FOR PROPORTIONAL, P+I, AND
P+I+D CONTROL
Adjust reset time (tr) to 2 to 3 times the response time (Tu). Tu
is the time interval between the beginning of a sustained
disturbance (such as a rapid step change of valve position) and
the instant when the resulting change in the output signal
reaches a specified fraction of its final steady-state value, either
before or in the absence of overshoot.
In discharge air control, Tu normally ranges from 0.1 to 0.6
minutes. This allows adjustments of tr in a range of 0.2 to 2
minutes. In room control, Tu ranges from 0.5 to 5 minutes,
which results in a setting of 1 to 15 minutes.
To estimate the throttling range (Xp) for stable control under
all different load conditions, you must know the control or
correcting range (Xh).
With the R7426A,B,C, Xh is the maximum air temperature
increase (heating) or decrease (cooling) produced by the coil,
with a fully open control valve. With the R7426D, Xh is the
maximum difference in the controlled variable between the
fully closed and fully open position of the final controlling
element (such as a valve).
Calculate Xp for discharge air control using the following:
X
h
----- -
X
=
p
5
63-2565
I01 x
x
I02
x
x
I01 x
x
I02
x
x
I01 x
I01 x
I01
x x
I01
x
x
I01 x
x
I02
x
I02
x
I02
x
x
I02
x
x
I01 x
x
x
I02
x
x x
I01 x
x
I02
x
x x
14
For room temperature control use one of the following:
∆t
X
h
max
----- -
------------- -
X
=
or
p
10
10
NOTE: ∆ tmax (Xh) of the discharge air for mixed air damper
control is the maximum difference between OAT and
return air temperature.
The often specified accuracy for room control of ±2°F (where
Xp is 3.6°F) allows a discharge air alteration of 68°F. P+I and
proportional control can use the same throttling range. Use
the following formula for P+I control:
X
h
----- -
X
=
p
x
• Discharge air control: x ranges from 4 to 5.
• Room control: x ranges from 8 to 10.
The same formula is used for P+I+D control.
• Discharge air control: x ranges from 5 to 6.
• Room control: x ranges from 10 to 12.
Reset Time: tr1/tr2 (P.12/P13)
With P+I control, reset time (tr) is defined as the required time
after which the integral part is equal to the change due to the
proportional action for a predetermined step change in the
input variable. See Fig. 11.
The control parameter tr1 sets the reset time of the P+I main
temperature control loop.
For limit or submaster cascade control the control parameter
tr2 sets the reset time of these control loops (such as
discharge temperature I02. See Table 14.)
NOTE: If only proportional control is required, set parameter
tr to Off.
SETTING GUIDELINES FOR P+I CONTROL RESET TIME
Adjust reset time (tr) to 2 to 3 times the response time (Tu).
NOTE: Tu is the time interval between the beginning of a
sustained disturbance (such as a rapid step change
of valve position) and the instant when the resulting
change in the output signal reaches a specified
fraction of its final steady-state value, either before
overshoot or in the absence of overshoot.
• Discharge air control: Tu normally ranges from 0.1 to 0.6
minutes. This allows tr adjustment to range from 0.2 to 2
minutes.
• Room control: Tu ranges from 0.5 to 5 minutes. This
results in a tr setting of 1 to 15 minutes.
Minimum Position: MINPOS (P.14)
Adjustment of parameter MINPOS is available only with
R7426B,C controllers. MINPOS determines the outdoor air
damper actuator minimum position. It maintains the minimum
outdoor air damper setting even when temperature conditions
call for a fully closed position.
NOTE: External plant/system On/Off input for plant/system
shut off overrides minimum position. When switched
to Off, actuators are driven fully-closed.
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