Danfoss VLT Series Design Manual page 40

2. Introduction to VLT HVAC Drive
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 par. 20-20.
2
Multi Zone, Single Setpoint
Multi Zone Single Setpoint uses two or three feedback sensors but only one setpoint. The feed-
backs 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
applies an individual setpoint reference to each feedback. The drive's Closed Loop Controller
chooses one pair to control the drive based on the user's selection in par. 20-20. If
Max
speed. (Note that a negative value is always smaller than a positive value).
If Multi Setpoint Min
the speed of the drive.
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
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
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).
2.9.5. 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
where A1, A2 and A3 are refrigerant-specific constants.
40
[14] is selected, the setpoint/feedback pair with the smallest difference controls the drive's
[13] is selected, the setpoint/feedback pair with the largest difference controls
Multi Setpoint Maximum
Multi Setpoint Min
A
2
=
(
ln pressure
MG.11.B2.02 - VLT
[14] attempts to keep all zones at or below their
[13] attempts to keep all zones at or above their
Multi Setpoint Max
Multi Setpoint Min
A
− 3
+ 1 ) −
A
1
®
is a registered Danfoss trademark
®
VLT HVAC Drive Design Guide
Multi Setpoint
[14] is selected, Zone 1's setpoint
[13] is selected,