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50
VLT
®
3500 HVAC
MG.35.B1.02 – VLT is a registered Danfoss trademark
120 Control range (CONTRL RANGE)
Value:
0 - 100%
★
100%
Function:
The regulator range (bandwidth) limits the output from
the PID regulator as a % of f
MAX
.
Description of choice:
A desired % value of f
MAX
can be selected. If the regu-
lator range is reduced, speed variations will become
smaller during initial adjustment.
121 Proportional gain (PROPRT/L GAIN)
Value:
OFF - 10.00
★
0.01
Function:
The proportional gain indicates how much the error
(the deviation between the feedback signal and the
setpoint) must be amplified.
Description of choice:
Fast regulation is obtained at a high amplification, but
if the amplification is too high, the process may
become unstable and cause overshooting.
122 Integral time (INTEGRAL TIME)
Value:
0.01 - 9999 secs. (OFF)
★
OFF
Function:
The integral time determines how long the PID regula-
tor takes to correct the fault. The integral time results
in a delay of the signal, and thus has a damping
effect.
Description of choice:
Fast regulation is achieved by a short integral time.
However, this time may be too short, which can make
the process unstable. When the integral time is long,
regulation becomes slow.
Off
means that the function
is inactive.
★
= factory setting. Text in ( ) = display text. Figures in [ ] are used when communicating with the bus.
123 Differential time (DIFFERENTIAL TIME)
Value:
OFF - 10.00 secs.
★
OFF
Function:
Fast control is obtained by a short differentiation time.
However, this time can be too short, which makes the
process unstable.
When the differentiation time is 0 sec., the D-function
is not active.
Description of choice:
In typical pump and fan systems, the differentiation
time is not used.
124 Lowpass filter (LOWPASS FILTER)
Value:
0.0 - 10.00 secs.
★
0.0 sec.
Function:
The feedback signal is damp by means of a lowpass
filter with a time constant (
τ
) of 0-10 secs.
0 sec. means not-activated.
Description of choice:
If a time constant (
τ
) is programmed to be 0.1 sec.,
the cut-off frequency for the lowpass filter will be 1/
0.1 = 10 Hz. The PID regulator will thus regulate a
feedback signal which varies by a frequency of less
than 10 Hz. If the feedback signal varies by a higher
frequency than 10 Hz, the PID regulator will not react.
125 Feedback factor (FEEDBACK FACTR)
Value:
0 - 500%
★
100%
Function:
The feedback factor is used if the transmitter cannot
be selected optimally for the scale range of the set-
point.
Description of choice:
A value can be programmed which scales the feed-
back signal to fit the desired setpoint. If 50% is used
as setpoint, for example, and the feedback signal is
only 25%, parameter 125 enables programming of
200%, which means that the feedback signal is
recorded as 50% (25% x 200% = 50%).
3.5.1 PID contr
oller