PID for process control
The PID controller maintains a constant process con-
dition (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 feed-
back signal from the process to indicate its actual
state. The feedback signal varies with the process
load.
This means that deviations occur between the refer-
ence/ 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 6000 HVAC units has
been optimised for use in HVAC applications. This
means that a number of specialised functions are
available in VLT 6000 HVAC units.
Formerly, it was necessary to get a BMS (Building
Management System) to handle these special func-
tions by installing extra I/O modules and by program-
ming the system.
Using the VLT 6000 HVAC, there is no need for extra
modules to be installed. For example, only one re-
quired reference/setpoint and the handling of feed-
back need to be programmed.
There is in-built a option for connecting two feedback
signals to the system, making two-zone regulation
possible.
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 frequency converter. Use the list below to de-
cide which terminal to use and which parameters to
program.
Feedback type Terminal Parameters
Pulse 33 307
Voltage 53, 54 308, 309, 310 or
311, 312, 313, 314
Current 60 315, 316
Bus feedback 1 68+69 535
Bus feedback 2 68+69 536
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 to the
terminal. The process unit is selected in parameter 415
Process units.
Reference
In parameter 205 Maximum reference, Ref
MAX
, a max-
imum 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 val-
ue that the resulting reference can assume.
The reference range cannot exceed the feedback
range.
If Preset references are required, set these in parame-
ters 211 to 214 Preset reference. See Reference type.
See also Reference handling.
If a current signal is used as a feedback signal, voltage
can be used as analogue reference. Use the list below
to decide which terminal to use and which parameters
to program.
Reference type Terminal Parameters
Pulse 17 or 29 301 or 305
Voltage 53 or 54 308, 309, 310 or
311, 312, 313
Current 60 314, 315, 316
Preset reference 211, 212, 213,
214
Setpoints 418, 419
Bus reference 68+69
Please note that the bus reference can only be set via
serial communication.
NB!
Terminals that are not in use may pref-
erably be set to No function [0].
VLT
®
6000 HVAC Series
= factory setting, () = display text, [] = value for use in communication via serial communication port
146 MG.61.B5.02 - VLT
®
is a registered Danfoss trademark
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