Danfoss VLT 5000 Series, Instruction Manual

Page: 204 / 240

Differentiator gain limit
If there are quick changes in reference or feedback in
a given application - which means that the error
changes swiftly - the differentiator may soon become
too dominant. This is because it reacts to changes in
the error. The quicker the error changes, the stronger
the differentiator gain is. The differentiator gain can
thus be limited to allow setting of the reasonable dif-
ferentiation time for slow changes and a suitably quick
gain for quick changes. This is done in parameter 443,
Process PID Differentiator gain limit .
Lowpass filter
If there are oscillations of the current/voltage feedback
signal, these can be dampened by means of a lowpass
filter. Set a suitable lowpass filter time constant. This
time constant represents the limitfrequency of the rip-
ples occurring on the feedback signal. If the lowpass
filter has been set to 0.1s, the limit frequency will be
10 RAD/sec., corresponding to (10/2 x
ʌ ) = 1.6 Hz.
This will mean that all currents/voltages that vary by
more than 1.6 oscillations per second will be removed
by the filter. In other words, control will only be carried
out on a feedback signal that varies by a frequency of
less than 1.6 Hz. Choose a suitable time constant in
parameter 444,
Process PID Lowpass filter .
Optimisation of the process controller
The basic settings have now been made; all that needs
to be done is to optimise the proportional gain, the in-
tegration time and the differentiation time (parameters
440, 441, 442). In most processes, this can be done
by following the guidelines given below.
1. Start the motor
2. Set parameter 440 (proportional gain) to 0.3
and increase it until the feedback signal again
begins to vary continuously. Then reduce the
value until the feedback signal has stabilised.
Now lower the proportional gain by 40-60%.
3. Set parameter 441 (integration time) to 20s
and reduce the value until the feedback sig-
nal again begins to vary continuously. In-
crease the integration time until the feedback
signal stabilises, followed by an increase of
15-50%.
4. Only use parameter 442 for very fast-acting
systems only (differentiation time). The typi-
cal value is four times the set integration time.
The differentiator should only be used when
the setting of the proportional gain and the
integration time has been fully optimised.
NOTE
If necessary, start/stop can be activated a
number of times in order to provoke a var-
iation of the feedback signal.
PID for speed control
Feedback
Use the list below to decide which terminal to use for
the feedback signal and which parameters to program.
Feedback type Terminal Parameters
Pulse 32 306
Pulse 33 307
Feedback pulse/
rev. 329
Voltage 53 308, 309, 310
Current 60 314, 315, 316
Furthermore, the minimum and maximum feedback
(parameters 414 and 415) are to be set to values in the
process unit that correspond to the actual minimum
and maximum process feedback values and units. The
minimum feedback cannot be set to a value lower than
0. Choose unit in parameter 416.
Reference
A minimum and a maximum reference can be set (204
and 205) which limit the sum of all references. The
reference range cannot exceed the feedback range.
If one or several preset references are required, the
simplest way of doing this is by setting such references
directly in parameters 215 to 218. Choose between the
preset references by connecting terminals
16,17,29,32 and/or 33 to terminal 12. Which of them
that are to be used depends on the choice in the pa-
rameters of the terminals in question (parameters 300,
301, 305, 306 and/or 307). The table below can be
used to select the preset references.
VLT
®
5000 Series
MG.51.C5.22 - VLT
p
is a registered Danfoss trademark. 201
List of functions