Control Structure Closed-Loop; Feedback Conversion - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

Product Overview

2.7.5 Control Structure Closed-loop

2 2
The internal controller allows the adjustable frequency drive to become an integral part of the controlled system. The
adjustable frequency drive 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 two signals. It then adjusts the speed of the motor
to correct this error.
For example, consider a pump application where the speed of a pump is to be controlled so that the static pressure in a
pipe is constant. The desired static pressure value is supplied to the adjustable frequency drive as the setpoint reference. A
static pressure sensor measures the actual static pressure in the pipe and supplies this to the adjustable frequency drive as
a feedback signal. If the feedback signal is greater than the setpoint reference, the adjustable frequency drive slows down
to reduce the pressure. In a similar way, if the pipe pressure is lower than the setpoint reference, the adjustable frequency
drive automatically speed up to increase the pressure provided by the pump.
Reference
Feedback
Figure 2.19 Control Structure Closed-loop
While the default values for the adjustable frequency drive'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.

2.7.6 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 Figure 2.20.
Ref.
signal
Desired
ow
Figure 2.20 Feedback Signal Conversion
28
Design Guide
+
S
_
*[-1]
7-30 PI
Normal/Inverse
Control
Ref. +
PI
P 20-01
-
FB conversion
FB
Flow
P
FB
signal
P
Danfoss A/S © Rev. 2014-01-14 All rights reserved.
100%
0%
PI
100%
-100%
P
Flow
To motor
Scale to
control
speed
P 4-10
Motor speed
direction
MG18C522