Danfoss VLT 2800 Design Manual page 96

Programming
Lb/s (lb/s)
Lb/min. (lb/min)
Lb/hour (lb/h)
Lb ft (lb ft)
Ft/s (ft/s)
Ft/min. (ft/min)
Psi (Psi)
4 4
Function:
Select among different units to be shown on the display.
The unit is read out if an LCP control unit can be
connected, and if [2] Reference [unit] or [3] Feedback [unit]
has been selected in one of parameters 009-012 Display
read-out, and in Display mode. The unit is used in Closed
loop also as a unit for minimum/maximum reference and
minimum/maximum feedback.
Description of choice:
Select the required unit for the reference/feedback signal.
4.5.1 VLT 2800 Regulators
The VLT 2800 has 2 integrated PID regulators, one to
regulate speed and one to regulate processes.
Speed regulation and process regulation require a
feedback signal back to an input. There are a number of
settings for both PID regulators that are made in the same
parameters, but selection of regulator type affects the
selections that have to be made in the shared parameters.
In parameter 100 Configuration, it is possible to select
regulator type, [1] Speed regulation, closed loop or [3]
Process regulation, closed loop.
Speed regulation
This PID regulation is optimised for use in applications in
which there is a need to maintain a particular motor
speed. The parameters that are specific for the speed
regulator are parameters 417 Speed PID proportional gain to
421 Speed PID low pass filter time.
Process regulation
The PID regulator maintains a constant process mode
(pressure, temperature, flow, etc.) and adjusts the motor
speed on the basis of the reference/setpoint and feedback
signal.
A transmitter provides the PID regulator with a feedback
signal from the process as an expression of the process's
actual mode. The feedback signal varies as the process
load varies.
This means that there is a variance between the reference/
setpoint and the actual process mode. This variance is
compensated by the PID regulator by adjusting the output
frequency up or down in relation to the variance between
the reference/setpoint and the feedback signal.
The integrated PID regulator in the frequency converter
has been optimised for use in process applications. This
94
Design Guide
means that there are a number of special functions
[35]
available in the frequency converter.
Previously, it was necessary to obtain a system to handle
[36]
these special functions by installing extra I/O modules and
[37]
programming the system. With the frequency converter
[38]
the need to install extra modules can be avoided. The
[39]
parameters that are specific to the process regulator are
[40]
parameters 437 Process PID normal/inverse control to 444
[41]
Process PID low pass filter time.
4.5.2 PID functions
Unit of reference/feedback
When Speed regulation, closed loop is selected in parameter
100 Configuration the unit of reference/feedback is always
[3] RPM.
When Process regulation, closed loop is selected in
parameter 100 Configuration, the unit is defined in
parameter 416 Process units.
Feedback
Preset a feedback range for both regulators. At the same
time, this feedback range limits the potential reference
range so that if the sum of all references lies outside the
feedback range, the reference is limited to lie within the
feedback range. The feedback signal must be connected to
a terminal on the frequency converter. If feedback is
selected on 2 terminals simultaneously, the 2 signals are
added. Use the overview below to determine which
terminal is to be used and which parameters are to be
programmed.
Feedback type
Pulse
Voltage
Current
Table 4.6
A correction can be made for loss of voltage in long signal
cables when a transmitter with a voltage output is used.
This is done in parameter group 300 Min./Max scaling.
Parameters 414/415 Minimum/Maximum feedback must also
be preset to a value in the process unit corresponding to
the minimum and maximum scaling values for signals that
are connected to the terminal.
Reference
In parameter 205 Maximum reference, Ref
to preset a maximum reference that scales the sum of all
references, i.e. the resulting reference.
The minimum reference in parameter 204 Minimum
reference, Ref
that the resulting reference can assume.
All references are added and the sum is the reference
against which regulation takes place. It is possible to limit
the reference range to a range that is smaller than the
feedback range. This can be helpful to avoid an uninten-
Danfoss A/S © Rev. May/2014 All rights reserved.
Terminal
33
53
60
MAX
is an expression of the minimum value
MIN
Parameters
307, 327
308, 309, 310
314, 315, 316
, it is possible
MG27E402