Technical information

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
®
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 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.
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
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: Function:
0.000
ProcessCtrlUnit*
[ par. 20-13 -
par. 20-14
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
20-20 Feedback Function.
NOTE
Setpoint reference
entered here is added
to any other references
that are enabled (see
parameter group 3-1*).
20-22 Setpoint 2
Range: Function:
0.000
ProcessCtrlUnit*
[ par. 20-13 - par.
20-14
ProcessCtrlUnit]
Setpoint 2 is used in
Closed Loop Mode to
enter a setpoint reference
that may be used by the
frequency converter’s PID
Controller. See the
description of Feedback
Function, 20-20 Feedback
Function.
NOTE
The set-point reference entered here is added to any other
references that are enabled (see parameter group 3-1*).
Parameter Description
VLT
®
HVAC Drive Programming Guide
134 MG11CD02 - VLT
®
is a registered Danfoss trademark
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