Specifications

2.7.4 Local (Hand On) and Remote (Auto
On) Control
The frequency converter can be operated manually via the
local control panel (LCP) or remotely via analog/digital
inputs or serial bus. If allowed in 0-40 [Hand on] Key on
LCP, 0-44 [Off/Reset] Key on LCP, and 0-42 [Auto on] Key on
LCP, it is possible to start and stop the frequency converter
by LCP using the [Hand On] and [Off/Reset] keys. Alarms
can be reset via the [Off/Reset] key.
Hand
On
Off
Reset
Auto
On
130BB893.10
Illustration 2.18 LCP Keys
Local reference forces the configuration mode to open
loop, independent on the setting of 1-00 Configuration
Mode.
Local Reference is restored at power-down.
2.7.5
Control Structure Closed Loop
The internal controller allows the frequency converter to
become an integral 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 2 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
speed up to increase the pressure provided by the pump.
7-30 PI
Normal/Inverse
Control
PI
Reference
Feedback
Scale to
speed
P 4-10
Motor speed
direction
To motor
control
130BB894.11
S
100%
0%
-100%
100%
*[-1]
_
+
Illustration 2.19 Control Structure Closed Loop
While the default values for the frequency converter’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
Illustration 2.20.
130BB895.10
+
-
PI
P
P
P
Ref.
signal
Desired
ow
FB conversion
Ref.
FB
Flow
FB
signal
Flow
P 20-01
Illustration 2.20 Feedback Signal Conversion
Product Overview
VLT
®
HVAC Basic Drive FC 101 Design Guide
26 MG18C502 - Rev. 2013-09-06
22