Danfoss VLT HVAC Drive FC 102 Design Manual page 206

Basic Operating Principles ...
Feedback conversion
In some applications, it is useful to convert the feedback signal. One example 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, see Illustration 11.5.
Reference signal
Illustration 11.5 Feedback Conversion
11.2.4 Control Structure Overview
The control structure is a software process that controls the motor based on user-defined references (for example, RPM) and
whether feedback is used/not used (closed loop/open loop). The operator defines the control in parameter 1-00 Configu-
ration Mode.
11 11
The control structures are as follows:
Open-loop control structure
•
Speed (RPM)
•
Torque (Nm)
Closed-loop control structure
•
Speed (RPM)
•
Torque (Nm)
•
Process (user-defined units, for example, feet, lpm, psi, %, bar)
11.2.5 Open-loop Control Structure
In open-loop mode, the drive uses 1 or more references (local or remote) to control the speed or torque of the motor. There
are 2 types of open-loop control:
•
Speed control. No feedback from the motor.
•
Torque control. Used in VVC
limited. Open-loop torque function works only in 1 speed direction. The torque is calculated based on current
measurement within the drive. See chapter 12 Application Examples.
204
®
VLT HVAC Drive FC 102
Reference
PID
Parameter 20-01
Parameter 20-04
Parameter 20-07
FB conversion
FB
Flow
FB signal
P
+
mode. The function is used in mechanically robust applications, but its accuracy is
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P
Flow
P
MG16C302