Danfoss VLT HVAC Drive Instruction Manual page 91

VLT ® HVAC Drive Instruction Manual
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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 vol-
ume) HVAC system must ensure a minimum
pressure at selected VAV boxes. Due to the
varying pressure losses in each duct, the pres-
sure 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
Function
and entering the desired pressure in par.
20-21. The PID controller will increase the
speed of the fan if any one feedback result is
below the setpoint, and decrease the speed of
the fan if all feedback results 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, 20-22 and 20-23. By selecting
Multi setpoint minimum
Feedback Function, the PID controller will in-
crease the speed of the fan if any one of the
feedback results is below its setpoint, and de-
crease the speed of the fan if all feedback
results are above their individual setpoints.
Sum
[0] sets up the PID controller to use the
sum of Feedback 1, Feedback 2 and Feedback
3 as the feedback.
Multi-zone, single setpoint
Multi-zone, multi setpoint
Feedback
, par. 20-20 to option [3], Minimum,
, [5], in par. 20-20,
NOTE
Any unused feedback results
No Function
must be set to
par. 20-00, 20-03, or 20-06.
MG.11.A4.22 - VLT ® is a registered Danfoss trademark
6. How to program the adjustable frequency
The sum of Setpoint 1 and any other referen-
ces that are enabled (see par. group 3-1*) will
be used as the PID controller's setpoint refer-
ence.
Difference
[1] sets up the PID controller to use
the difference between Feedback 1 and Feed-
back 2 as the feedback. Feedback 3 will not
be used with this selection. Only setpoint 1 will
be used. The sum of Setpoint 1 and any other
references that are enabled (see par. group
3-1*) will be used as the PID controller's set-
point reference.
Average [2] sets up the PID controller to use
the average of Feedback 1, Feedback 2 and
Feedback 3 as the feedback.
NOTE
Any unused feedbacks must be
No Function
set to
20-03, or 20-06. The sum of
Setpoint 1 and any other refer-
ences that are enabled (see par.
group 3-1*) will be used as the
PID controller's setpoint refer-
ence.
Minimum
[3] sets up the PID controller to
compare Feedback 1, Feedback 2 and Feed-
back 3 and use the lowest value as the feed-
back.
NOTE
Any unused feedbacks must be
No Function
set to
20-03, or 20-06. Only setpoint 1
will be used. The sum of Set-
point 1 and any other references
that are enabled (see par. group
3-1*) will be used as the PID
controller's setpoint reference.
Maximum
[4] sets up the PID controller to
compare Feedback 1, Feedback 2 and Feed-
back 3 and use the highest value as the feed-
back.
NOTE
Any unused feedback results
must be set to
par. 20-00, 20-03, or 20-06.
Only Setpoint 1 will be used. The sum of Set-
in
point 1 and any other references that are
drive
6
in par. 20-00,
in par. 20-00,
No Function in
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