Pid Functions - Danfoss VLT FCD 300 Series Design Manual

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VLT Decentral FCD 300 Design Guide
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 parameter 417 to parameter 421.
Process regulation
The PID regulator maintains a constant process mode (pressure, tem-
perature, 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 reg-

4.6.2 PID Functions

Unit of reference/feedback
Speed regulation, closed loop
When
figuration
the unit of reference/feedback is always rpm.
When Process regulation, closed loop
figuration the unit is defined in parameter 416
Feedback
A feedback range must be preset 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 will be
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 two terminals simultaneously, the
two signals will be added together.
Use the overview below to determine which terminal is to be used and
which parameters are to be programmed.
Feedback type Terminal
Pulse 29, 33
Voltage 53
Current 60
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
Min./Max scaling
group 300 .
Parameters 414/415 Minimum/Maximum feedback
to a value in the process unit corresponding to the minimum and maxi-
mum scaling values for signals that are connected to the terminal.
Reference
Maximum reference, Ref
In parameter 205
maximum reference that scales the sum of all references, i.e. the result-
ing reference.
The minimum reference in parameter 204 is an expression of the mini-
mum value that the resulting reference can assume.
Con-
is selected in parameter 100
is selected in parameter 100 Con-
Process units.
Parameters
305, 307, 327, 328
308, 309, 310
314, 315, 316
must also be preset
it is possible to preset a
MAX
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ulator by means of the output frequency being regulated 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 opti-
mised for use in process applications. This means that there are a number
of special functions available in the frequency converter.
Previously it was necessary to obtain a system to handle these special
functions by installing extra I/O modules and programming the system.
With the frequency converter the need to install extra modules can be
avoided. The parameters that are specific to the Process Regulator are
parameter 437 to parameter 444.
All references will be added together and the sum will be the reference
against which regulation will take place. It is possible to limit the refer-
ence range to a range that is smaller than the feedback range. This can
be an advantage if you want to avoid an unintentional change to an ex-
ternal reference making the sum of the references move too far away
from the optimal reference. The reference range cannot exceed the feed-
back range.
If preset references are desired, they are preset in parameters 215 to 218
Preset reference . See description Reference Function
References .
If a current signal is used as the feedback signal, it will only be possible
to use voltage as an analogue reference. Use the overview below to de-
termine which terminal is to be used and which parameters are to be
programmed.
Reference type Terminal
Pulse 29, 33
Voltage 53
Current 60
Preset
references
Bus reference 68+69
Note that the bus reference can only be preset via serial communication.
NB!
It is best to preset terminals that are not being used
No function
to [0].
Differentiator gain limit
If very rapid variations occur in an application in either the reference sig-
nal or the feedback signal, the deviation between the reference/setpoint
and the process's actual mode will change quickly. Thedifferentiator can
then become too dominant. This is because it is reacting to the deviation
between the reference and the process's actual mode, and the quicker
the variance changes the more powerful the differentiator's frequency
contribution becomes. The differentiator's frequency contribution can
therefore be limited in such a way that both a reasonable differentiation
4 Programming
4
and Handling of
Parameters
305, 307, 327, 328
308, 309, 310
314, 315, 316
215-218
93