Control Structure Closed Loop; Feedback Conversion - Danfoss VLT Compressor Drive CDS 803 Design Manual

Product Overview

2.6.3 Control Structure Closed Loop

The internal controller allows the frequency converter to become a part of the controlled system. The frequency converter
receives a feedback signal from a sensor in the system. It then compares this feedback to a setpoint reference value and
determines the error, if any, between these 2 signals. It then adjusts the speed of the motor to correct this error.
For example, consider an application where the speed is to be controlled so that the static pressure in a pipe is constant.
The desired static pressure value is supplied to the frequency converter as the setpoint reference. A static pressure sensor
measures the actual static pressure in the pipe and supplies this to the frequency converter as a feedback signal. If the
feedback signal is greater than the setpoint reference, the frequency converter slows down to reduce the pressure. In a
similar way, if the pipe pressure is lower than the setpoint reference, the frequency converter automatically speeds up to
increase the pressure provided by the pump.
+
Reference
S
_
Feedback
Illustration 2.3 Control Structure Closed Loop
While the default values for the frequency converter's closed-loop controller often provides satisfactory performance, the
control of the system can often be optimized by adjusting some of the closed-loop controller's parameters.

2.6.4 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. See
Illustration 2.4.
Ref.
signal
Ref. +
P 20-01
-
Desired
FB conversion
ow
FB
signal
Illustration 2.4 Feedback Signal Conversion
MG18N202
Design Guide
PI
*[-1]
7-30 PI
Normal/Inverse
Control
PI
FB
P
Flow
Flow
P
P
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100%
0%
Scale to
speed
100%
-100%
P 4-10
Motor speed
direction
2 2
To motor
control
13