Pid For Process Control - Danfoss VLT 4000 VT Instruction Manual

PID for process control

The PID controller maintains a constant process
condition (pressure, temperature, flow, etc.) and
adjusts motor speed on the basis of a reference/
setpoint and the feedback signal.
A transmitter supplies the PID controller with a
feedback signal from the process to indicate its actual
state. The feedback signal varies with the process load.
This means that deviations occur between the
reference/setpoint and the actual process state. Such
deviations are evened out by the PID regulator, in
that it regulates the output frequency up or down
in relation to the deviation between the reference/
setpoint and the feedback signal.
The integral PID regulator in VLT 4000 VT units
have been optimised for use in water applications.
This means that a number of specialised functions
are available in VLT 4000 VT units.
Using the VLT 4000 VT, there is no need for extra
modules to be installed. For example, only one
required reference/setpoint and the handling of
feedback need to be programmed.
There is a built in option for connecting two
feed-back signals to the system.
Correction for voltage losses in long signal cables
can be carried out when using a transmitter
with a voltage output. This is done in parameter
group 300 Min./Max.scaling.
Feedback
The feedback signal must be connected to a terminal
on the VLT AFD. Use the list below to decide which
terminal to use and which parameters to program.
Feedback type Terminal
Pulse 33
Voltage 53, 54
Current 60
Bus feedback 1 68+69
Bus feedback 2 68+69
Please note that the feedback value in parameter
535/536 Bus feedback 1 and 2 can only be set via
serial communication (not via the control unit).
Furthermore, the minimum and maximum feedback
(parameters 413 and 414) must be set to a value in
the process unit that corresponds to the minimum
and maximum scaling value for signals connected
MG.40.A7.22 - VLT is a registered Danfoss trademark
to the terminal. The process unit is selected in
parameter 415 Process units.
Reference
In parameter 205 Maximum reference, Ref
a maximum reference that scales the sum of all
references, i.e. the resulting reference, can be set. The
minimum reference in parameter 204 indicates the
smallest value that the resulting reference can assume.
The reference range cannot exceed the feedback range.
If Preset references are required, set these in parameters
211 to 214 Preset reference. See Reference type.
See also Reference handling.
If a current signal is used as a feedback signal,
volt-age can be used as analog reference. Use
the list below to decide which terminal to use
and which parameters to program.
Reference type
Pulse
Voltage
Current
Preset reference
Setpoints
Bus reference
Please note that the bus reference can only be
set via serial communication.
Inverse regulation
Normal regulation means that the motor speed
increases when the reference/setpoint is higher than
Parameters
307
the feedback signal. If there is a need for inverse
308, 309, 310 or
regulation, in which the speed is reduced when the
311, 312, 313
feedback signal is lower than the reference/setpoint,
314, 315, 316
Inverse must be programmed in parameter 420
535
PID normal/inverse control.
536
Anti Windup
The process regulator is factory preset with an active
anti-windup function. This function ensures that
when either a frequency limit, current limit or voltage
limit is reached, the integrator will be initialised for
a frequency that corresponds to the present output
frequency. This avoids integration on a deviation
between the reference/setpoint and the actual state
®
VLT 4000 VT
Terminal
17 or 29
53 or 54
60
68+69
NOTE
Terminals that are not in use may preferably
be set to No function [0].
,
MAX
Parameters
301 or 305
308, 309, 310 or
311, 312, 313
314, 315, 316
211, 212, 213, 214
418, 419
101