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

Product Overview
and ramp-down times, and the direction of motor rotation.
The reference is then passed on to control the motor.
Reference
handling
Remote
reference
Auto mode
Linked to hand/auto
Hand mode
Local
reference
scaled to
RPM or Hz
LCP Hand on,
o and auto
on keys
Illustration 2.11 Open-loop Mode Block Diagram

2.4.2 Control Structure Closed Loop

In closed-loop mode, an internal PID controller allows the
frequency converter to process system reference and
feedback signals to act as an independent control unit. The
converter can provide status and alarm messages, along
Ref.
+
Handling

(Illustra-
tion)
_
Feedback
Handling
(Illustra-
tion)
Illustration 2.12 Block Diagram of Closed-loop Controller
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 (see Illustration 2.12). The frequency
converter receives a feedback signal from a sensor in the
system. It 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.
The desired static pressure setpoint is the reference signal
to the frequency converter. A static pressure sensor
measures the actual static pressure in the pipe and
provides this information to the frequency converter as a
feedback signal. If the feedback signal is greater than the
setpoint reference, the frequency converter slows to
MG20N602
Design Guide
P 4-13
Motor speed
high limit [RPM]
P 4-14
Motor speed
high limit [Hz]
Remote
Reference
Local
P 4-11
Motor speed
low limit [RPM]
P 3-13
Reference P 4-12
site Motor speed
low limit [Hz]
PID
*[-1]
P 20-81
PID Normal/Inverse
Control
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100%
P 3-4* Ramp 1
P 3-5* Ramp 2
0%
Ramp
100%
-100%
with many other programmable options, for external
system monitoring while operating independently in
closed loop.
100%
0%
100%
-100%
P 4-10
Motor speed
direction
reduce the pressure. In a similar way, if the pipe pressure is
lower than the setpoint reference, the frequency converter
speeds up to increase the pressure provided by the pump.
While the default values for the frequency converter in
closed loop often provide satisfactory performance, system
control can often be optimised by tuning the PID
parameters. Auto tuning is provided for this optimisation.
To motor
control
P 4-10
Motor speed
direction
To motor
Scale to
control
speed
21
2 2