Danfoss VLT FC 102 Series Design Manual page 43

Introduction to VLT® HVAC D...
Feedback handling can be configured to work with
applications requiring advanced control, such as multiple
setpoints and multiple feedbacks. Three types of control
are common.
Single Zone, Single Setpoint
Single Zone, Single Setpoint is a basic configuration.
Setpoint 1 is added to any other reference (if any, see
Reference Handling) and the feedback signal is selected
using 20-20 Feedback Function.
Multi-zone, Single Setpoint
Multi-zone, Single Setpoint uses two or three feedback
sensors, but only one setpoint. Feedback can be added,
subtracted (only feedback 1 and 2) or averaged. In
addition, the maximum or minimum value may be used.
Setpoint 1 is used exclusively in this configuration.
If [5] Multi Setpoint Min is selected, the setpoint/feedback
pair with the largest difference controls the speed of the
adjustable frequency drive. [6] Multi Setpoint Max attempts
to keep all zones at or below their respective setpoints,
while [5] Multi Setpoint Min attempts to keep all zones at
or above their respective setpoints.
Example
A two-zone two setpoint application Zone 1 setpoint is
15 bar and the feedback is 5.5 bar. Zone 2 setpoint is
4.4 bar and the feedback is 4.6 bar. If [14] Multi Setpoint
Max is selected, Zone 1's setpoint and feedback are sent to
the PID controller, since this has the smaller difference
(feedback is higher than setpoint, resulting in a negative
difference). If [13] Multi Setpoint Min is selected, Zone 2's
setpoint and feedback is sent to the PID controller, since
this has the larger difference (feedback is lower than
setpoint, resulting in a positive difference).
MG11BC22
Design Guide
2.8.8 Feedback Conversion
In some applications, it may be useful to convert the
feedback signal. One example of this is using a pressure
signal to provide flow feedback. Since the square root of
pressure is proportional to flow, the square root of the
pressure signal yields a value proportional to the flow. This
is shown in Figure 2.28.
Figure 2.28 Feedback Conversion
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