Danfoss VLT, Instruction Manual

Page: 105 / 179

NOTE!
If only two feedback signals are used, the feedback that is not to be used must
be set to
No Function in par.20-00 Feedback 1 Source , par.20-03 Feedback 2
Source or par.20-06 Feedback 3 Source . Note that each setpoint reference will
be the sum of its respective parameter value (par.20-21 Setpoint 1 , par.
20-22 Setpoint 2 and par. 20-23 Setpoint 3 ) and any other references that are
enabled (see par. group 3-1*).
NOTE!
Any unused feedback must be set to “No function” in its Feedback Source parameter: par.20-00 Feedback 1 Source , par.20-03 Feedback
2 Source or par.20-06 Feedback 3 Source .
The feedback resulting from the function selected in par.20-20
Feedback Function
will be used by the PID controller to control the output frequency of
the adjustable frequency drive. This feedback can also: be shown on the adjustable frequency drive’s display, be used to control an adjustable frequency
drive's analog output, and be transmitted over various serial communication protocols.
The adjustable frequency drive can be configured to handle multi-zone applications. Two different multi-zone applications are supported:
• Multi-zone, single setpoint
• Multi-zone, multi setpoint
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
Feedback Function to option [3], Minimum, and entering the desired pressure in par.
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.
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
Setpoint 1
, par.20-22
Setpoint 2
and par. 20-23
Setpoint 3
. By selecting
Multi-setpoint minimum
, [5], in par.
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.
VLT ® HVAC Drive Instruction Manual
6 How to program the adjustable frequency
drive
MG.11.A9.22 - VLT ® is a registered Danfoss trademark
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