48
VLT
®
3500 HVAC
MG.35.B1.02 – VLT is a registered Danfoss trademark
It can be optimized for min. frequency as follows:
100
×
(%) < reference (%)
⇒
parameter 125 = 100%
100
×
(%) > reference (%)
⇒
new reference
= f
max
(%) - f
min
(%) + f
min
(%)
2
⇒
parameter 125 = new reference (%)
×
100%
old reference (%)
Min. freq.
Max. freq.
Regulating accuracy
PID (closed loop)
±0.1%
5-50 Hz: (-140 - +140% load change)
Open loop (digital)
±0.01%
0,5-120 Hz
(frequency stability)
±0.05%
Frequency resolution (digital)
■
Feedback factor
Parameter 125 makes it possible to scale the feed-
back signal to adapt to the desired setpoint.
The formulae shown below must be modified if a
scaling for the transmitter is needed as well.
Min. freq.
Max. freq.
PID optimization
Parameters 121, 122 and 123 are set to factory set-
tings, see page 86.
1. Start the frequency converter.
2. Set parameter 121 (proportional gain) for 0,3 sec.
and increase the value, until the feedback signal
(FB) oscillates constantly. Reduce the value until
the oscillations stop. Carry out further reduction
(0.4-0.6 times).
3. Set parameter 122 (integration time) for 20 sec.
and reduce the value, until the feedback signal
(FB) oscillates again. Increase the value until the
oscillations stop. Subsequently, increase again
(1.15-1.5 times).
4. Parameter 123 (differentiation time) is only used in
fast reacting systems. The typical value is the inte-
gration time divided by 4.
Not used in HV-AC applications and only to be
used if the integrator is fully optimized.
5. Reduce the control range if necessary
(parameter 120) to reduce overshooting.
There must however be correlation between the feed
back and the motorspeed.
Note:
Note:
Note:
Note:
Note:
Activate start/stop repeatedly if necessary to
generate oscillations.
■
Programming the PID controller
Activating the PID controller
Set parameter 101 to CLOSED LOOP to choose PID
control. The PID controller is activated, when the P, I
and D have been programmed .
The transmitter
Which connection terminal in the VLT
®
frequency
converter is to be used for the transmitter will depend
on the transmitter type. Connect the transmitter
(feedback signal) as follows and set the parameter
according to the transmitter signal data.
Then programme parameter 114 feedback signal.
The reference
If for example a current transmitter is used as feed-
back signal and an external adjustment of the refe-
rence (setpoint) is required parameter 412 must be
set for 0-10 VDC and the potentiometer must be
connected to terminal 53.
It can be seen from the table, that the reference signal
cannot be of the same type as the feedback signal.
The reference (setpoint) can also be set internally. This
can be done in parameters 205-208, digital reference
1 - 4.
Par. 405 must be set for digital reference and terminal
29 must be connected to terminal 12.
A selection between the digital references can be
made on terminal 32/33 if parameter 406 is program-
med for speed select.
If an internal reference is used terminals 17, 53 and
60 should be set for no operation (terminal 17 =
Freeze reference) if not used for the feedback signal
or other purposes.
The reference with minimum frequency
To select a reference independently of minimum
frequency setting, change parameter 411 to PRO-
PORTIONAL. If the minimum frequency as a percent-
age of f
max
is larger than the reference the minimum
frequency will function as minimum reference.
This will call for an adjustment of the feedback factor
(par. 125), as shown in the formulae below.
Ramp times
The ramp times are set in parameter 215-216 accor-
ding to the application, but only on start and stop. On
start and stop the PID controller will regulate internally
on ramp time, but the actual ramp time on the motor
may be shorter or longer, than the programmed
value.
3.5.1 PID contr
oller