VLT
®
6000 HVAC Series
■ PID for process regulation, cont.
I
nverse regulation
Normal regulation means that the motor speed
increases when the reference/setpoint is higher than
the feedback signal. If there is a need for inverse
regulation, in which the speed is reduced when
the reference/setpoint is higher than the feedback
signal, inverse must be programmed in parameter
420 PID normal/inverse control.
A
nti 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 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
the process, the controller of which is not possible
means of a speed change. This function can be
disabled in parameter 421 PID anti windup.
S
tart-up conditions
In some applications, optimum setting of the process
regulator will mean that it takes an excessive time
for the required process state to be reached. In
such applications it might be an advantage to fix an
output frequency to which the frequency converter
is to bring the motor before the process regulator
is activated. This is done by programming a PID
start-up frequency in parameter 422.
D
ifferentiator gain limit
If there are very quick variations in a given application
with respect to the reference/setpoint signal
or the feedback signal, the deviation between
reference/set-point and the actual process state will
quickly change. The differentiator may thus become
too dominant. This is because it reacts to the deviation
between the reference/setpoint and the actual
process state. The quicker the deviation changes,
the stronger the resulting differentiator frequency
contribution. The differentiator frequency contribution
can thus be limited to allow the setting of a reasonable
differentiation time for slow changes and a suitable
frequency contribution for quick changes. This is done
in parameter 426, PID Differentiator gain limit.
L
owpass filter
If there are ripple currents/voltages on the feedback
signal, these can be dampened by means of a
built-in lowpass filter. Set a suitable lowpass filter
time constant. This time constant represents the
limit frequency of the ripples occurring on the
feedback signal. If the lowpass filter has been set
to 0.1s, the limit frequency will be 10 RAD/sec.,
correspondingto(10/2xπ ) = 1.6 Hz. This means
that all currents/voltages that vary by more than 1.6
oscillations per second will be removed by the filter.
In other words, regulation 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 427, PID Lowpass filter time.
O
ptimisation of the process regulator
The basic settings have now been made; all that
remains to be done is to optimise the proportional
gain, the integration time and the differentiation time
(parameters 423, 424 and 425). In most processes, this
canbedonebyfollowingtheguidelinesgivenbelow.
1. Start the motor.
2. Set parameter 423 PID proportional gain to 0.3
and increase it until the process shows that the
feedback signal is unstable. Then reduce the
value until the feedback signal has stabilised. Now
lower the proportional gain by 40-60%.
3. Set parameter 424 PID integration time to 20 s
and reduce the value until the process shows
that the feedback signal is unstable. Increase the
inte-gration time until the feedback signal stabilises,
followed by an increase of 15-50%.
4. Parameter 425 PID differentiation time is only
used in very fast-acting systems. The typical
value is 1/4 of the value set in parameter 424 PID
Integration time. The differentiator should only be
used when the setting of the proportional gain and
the integra-tion time have been fully optimised.
NB!:
If necessary, start/stop can be activated
a number of times in order to provoke an
unstable feedback signal.
✭ = factory setting. () = display text [] = value for use in communication via serial communication port
MG.61.A5.02 - VLT is a registered Danfoss trademark
122
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