Danfoss VLT FC 100 Series Programming Manual page 144

Parameter Description
The difference between the two is illustrated by the following examples:
Example 1: Multi-zone, single setpoint
In an office building, a VAV (variable air volume) VLT
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 20-20 Feedback
Function to option [3], Minimum, and entering the desired pressure in 20-21 Setpoint 1. The PID controller will increase the
speed of the fan if any one feedback is below the setpoint, and decrease the speed of the fan if all feedbacks are above the
setpoint.
FC 102
Damper
Cooling/
heating coil
Damper
Damper
Figure 3.47
Example 2: Multi-zone, multi setpoint
The previous example can be used to illustrate the use of multi-zone, multi-setpoint control. If the zones require different
pressures for each VAV box, each setpoint may be specified in 20-21 Setpoint 1, 20-22 Setpoint 2 and 20-23 Setpoint 3. By
selecting Multi-setpoint minimum, [5], in 20-20 Feedback Function, the PID controller will increase the speed of the fan if any
one of the feedbacks is below its setpoint and decrease the speed of the fan if all feedbacks are above their individual
setpoints.
20-21 Setpoint 1
Range:
0.000 [ par. 20-13 -
ProcessCtrlUnit par. 20-14
*
ProcessCtrlUnit]
®
VLT HVAC Drive Programming Guide
®
HVAC Drive system must ensure a minimum pressure at selected VAV
Supply
air fan
Return air fan
Function:
Setpoint 1 is used in
Closed-loop mode to enter
a setpoint reference that is
used by the adjustable
frequency drive's PID
controller. See the
description of
20-20 Feedback Function.
MG11CD22 - VLT ® is a registered Danfoss trademark
P P
VAV
Box
P P
VAV
Box
P P
VAV
Box
20-21 Setpoint 1
Range:
Zone 1
Zone 2
Zone 3
Function:
NOTE!
Setpoint reference
entered here is added
to any other references
that are enabled (see
parameter group 3-1*).
3-117
3 3