Danfoss VLT HVAC Drive FC 100 Series Design Manual page 38

2. Introduction to VLT HVAC Drive
Multi Zone, Single Setpoint
Multi Zone Single Setpoint uses two or three feedback sensors but only one setpoint. The feedbacks 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.
Multi Zone Multi Setpoint
2
applies an individual setpoint reference to each feedback. The frequency converter's Closed Loop Controller chooses one pair to control the frequency
converter based on the user's selection in par. 20-20. If
the frequency converter's speed. (Note that a negative value is always smaller than a positive value).
If Multi Setpoint Min [13] is selected, the setpoint/feedback pair with the largest difference controls the speed of the frequency converter.
Maximum [14] attempts to keep all zones at or below their respective setpoints, while
respective setpoints.
Example:
A two zone two setpoints application Zone 1 setpoint is 18°C and the feedback is 19°C. Zone 2 setpoint is 22°C and the feedback is 20°C. If
Max
[14] 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
larger difference (feedback is lower than setpoint, resulting in a positive difference).
2.8.9. 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 below.
Another application that may benefit from feedback conversion is compressor control. In such applications the output of a pressure sensor may be
converted to the refrigerant temperature using the equation:
Temperature
=
( ln (
where A1, A2 and A3 are refrigerant-specific constants.
38
Multi Setpoint Max
Multi Setpoint Min
[13] is selected, Zone 2's setpoint and feedback is sent to the PID controller, since this has the
A
2
A
− 3
pressure + 1 ) − A 1 )
MG.11.B7.02 - VLT
[14] is selected, the setpoint/feedback pair with the smallest difference controls
Multi Setpoint Min [13] attempts to keep all zones at or above their
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VLT HVAC Drive Design Guide
Multi Setpoint
Multi Setpoint