Control Structure Closed Loop - Danfoss VLT AQUA Drive FC 202 Design Manual

Introduction

2.8.4 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 set-point 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 in which the speed of a pump is controlled so that the static pressure in a pipe is
constant. The desired static pressure value is supplied to the frequency converter as the set-point reference. A static
pressure sensor measures the actual static pressure in the pipe and supplies this information to the frequency converter as a
feedback signal. If the feedback signal is greater than the set-point reference, the frequency converter slows to reduce the
pressure. In a similar way, if the pipe pressure is lower than the set-point reference, the frequency converter speeds up to
increase the pressure provided by the pump.
Ref.
+
Handling

(Illustra-
tion)
_
Feedback
Handling
(Illustra-
tion)
Illustration 2.12 Block Diagram of Closed Loop Controller
While the default values for the closed loop controller often provides satisfactory performance, the control of the system
can often be optimised by adjusting some of the parameters of the closed loop controller. It is also possible to autotune the
PI constants.
®
VLT AQUA Drive FC 202 Design Guide
PID
*[-1]
P 20-81
PID Normal/Inverse
Control
®
MG20Z102 - VLT is a registered Danfoss trademark
100%
0%
Scale to
speed
100%
-100%
P 4-10
Motor speed
direction
2 2
To motor
control
23