Examples 1 and 2 illustrate the difference between the 2:
Example 1 – Multi-zone, single setpoint
In an office building, a VAV (variable air volume) VLT
®
HVAC Drive system must ensure a minimum pressure at
selected VAV boxes. Due to the varying pressure losses in
each duct, the pressure at each VAV box cannot be
assumed to be the same. The minimum pressure required
is the same for all VAV boxes. This control method can be
set up by setting
parameter 20-20 Feedback Function
to
[3]
Minimum
, and entering the desired pressure in
. If any feedback is below the
setpoint, the PID controller increases the fan speed. If all
feedbacks are above the setpoint, the PID controller
decreases the fan speed.
P
P
P
P
P
P
VAV
Box
Zone 1
Damper
Cooling/
heating coil
Supply
air fan
Return air fan
Damper
Damper
Zone 2
Zone 3
VAV
Box
VAV
Box
130BA353.10
Illustration 3.48 Example, Multi-zone, Single Setpoint
Example 2 – Multi-zone, multi-setpoint
The previous example illustrates the use of multi-zone,
multi-setpoint control. If the zones require different
pressures for each VAV box, each setpoint may be specified
in
•
.
•
.
•
.
By selecting
[5] Multi-setpoint minimum
in
parameter 20-20 Feedback Function
, the PID controller
increases the fan speed if any one of the feedbacks is
below its setpoint. If all feedbacks are above their
individual setpoints, the PID controller decreases the fan
speed.
20-21 Setpoint 1
Range:
Function:
0
ProcessCtrlUnit
*
[-999999.999 -
999999.999
ProcessCtrlUnit]
Setpoint 1 is used in closed-
loop mode to enter a setpoint
reference that is used by the
frequency converter’s PID
controller. See the description
of
.
NOTICE
The setpoint reference
entered here is added to
any other references that
are enabled (see
parameter group
3-1*
References
).
Parameter Descriptions
Programming Guide
MG11CE02
Danfoss A/S © 03/2015 All rights reserved.
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