Danfoss VLT Design Manual page 22

Introduction
the frequency converter goes into Hand Mode and follows
(as default) the Local reference set by using [
After pressing the [Auto On] key, the frequency converter
goes into Auto mode and follows (as default) the Remote
reference. In this mode, it is possible to control the
frequency converter via the digital inputs and various serial
interfaces (RS-485, USB, or an optional fieldbus). See more
about starting, stopping, changing ramps and parameter
set-ups etc. in parameter group 5-1* (digital inputs) or
parameter group 8-5* (serial communication).
Illustration 2.13
Hand Off Reference Site
Auto 3-13 Reference Site
LCP Keys
Hand Linked to Hand/
Auto
Hand ⇒ Off
Linked to Hand/
Auto
Auto Linked to Hand/
Auto
Auto ⇒ Off
Linked to Hand/
Auto
All keys Local
All keys Remote
Table 2.7 Conditions for Either Local or Remote Reference
Illustration 2.14 Block Diagram of Closed Loop Controller
While the default values for the frequency converter's
Closed Loop controller will often provide satisfactory
performance, the control of the system can often be
optimized by adjusting some of the Closed Loop
®
VLT Refrigeration Drive Design Guide
Table 2.7 shows under which conditions either the Local
▲ ▼
Reference or the Remote Reference is active. One of them
] and [ ].
is always active, but both can not be active at the same
time.
Local reference will force the configuration mode to open
loop, independent on the setting of 1-00 Configuration
Mode.
Local Reference will be restored at power-down.
2.8.4 Control Structure Closed Loop
The internal controller allows the frequency converter to
become an integral part of the controlled system. The
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.
Active Reference
For example, consider a pump application where the
speed of a pump is to be controlled so that the static
Local
pressure in a pipe is constant. The desired static pressure
value is supplied to the frequency converter as the set-
Local
point reference. A static pressure sensor measures the
actual static pressure in the pipe and supplies this to the
Remote
frequency converter as a feedback signal. If the feedback
signal is greater than the set-point reference, the
Remote
frequency converter slows down to reduce the pressure. In
a similar way, if the pipe pressure is lower than the set-
Local
point reference, the frequency converter automatically
Remote
speeds up to increase the pressure provided by the pump.
controller's parameters. It is also possible to autotune the
PI constants.
®
MG16G102 - VLT is a registered Danfoss trademark
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