
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 par. 20-20
to option [3], Minimum, and entering the desired pressure in
. 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 par. 20-21
and par. 20-23
Setpoint 3
. By selecting
Multi setpoint minimum
, [5], in
, 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 Proc-
essCtrlU-
nit*
[ par. 20-13 - par. 20-14 Proc-
essCtrlUnit]
Setpoint 1 is used in Closed Loop Mode to enter a setpoint reference that is used by the frequen-
cy converter’s PID Controller. See the description of par. 20-20
Setpoint reference entered here is added to any other references that are ena-
bled (see meter group 3-1*).
20-22 Setpoint 2
Range:
Function:
0.000 Proc-
essCtrlU-
nit*
[ par. 20-13 - par. 20-14 Proc-
essCtrlUnit]
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
The set-point reference entered here is added to any
other references that are enabled (see meter group
3-1*).
6 How to Programme
VLT HVAC Drive High Power Operating In-
structions
126
MG.11.F3.02 - VLT
®
is a registered Danfoss trademark
6