the frequency converter goes into Hand Mode and follows
(as default) the Local reference set by using [
▲
] and [
▼
].
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
Auto
LCP Keys
Reference Site
3-13 Reference Site
Active Reference
Hand
Linked to Hand/
Auto
Local
Hand
⇒
Off
Linked to Hand/
Auto
Local
Auto
Linked to Hand/
Auto
Remote
Auto
⇒
Off
Linked to Hand/
Auto
Remote
All keys
Local
Local
All keys
Remote
Remote
Table 2.7 Conditions for Either Local or Remote Reference
shows under which conditions either the Local
Reference or the Remote Reference is active. One of them
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.
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 frequency converter as the set-
point 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 set-point reference, the
frequency converter slows down to reduce the pressure. In
a similar way, if the pipe pressure is lower than the set-
point reference, the frequency converter automatically
speeds up to increase the pressure provided by the pump.
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
controller’s parameters. It is also possible to autotune the
PI constants.
Introduction
VLT
®
Refrigeration Drive Design Guide
MG16G102 - VLT
®
is a registered Danfoss trademark
21
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