Danfoss VLT 2800 Series Operating Instructions Manual page 43

since the process regulator will take long to regulate
if an error has occurred.
419 Speed PID differential time
419
(SPEED DIFF. TIME)
Value:
0.00 (OFF) - 200.00 ms
Function:
The differentiator does not react to a constant error.
It only makes a contribution when the error changes.
The quicker the error changes, the stronger the gain
from the differentiator will be. The contribution is
proportional to the speed at which errors change.
Description of choice:
Quick control is obtained by a long differential time.
However, if this time is too long, it can make the pro-
cess unstable. When the differential time is 0 ms,
the D-function is not active.
420 Speed PID D- gain limit
420
(SPEED D-GAIN LIM)
Value:
5.0 - 50.0
Function:
It is possible to set a limit for the gain provided by
the differentiator. Since the D-gain increases at
higher frequencies, limiting the gain may be useful.
This enables obtaining a pure D-gain at low frequen-
cies and a constant D-gain at higher frequencies.
Description of choice:
Select the required gain limit.
421 Speed PID lowpass filter time
421
(SPEED FILT. TIME)
Value:
20 - 500 ms
Function:
Noise in the feedback signal is dampened by a first
order lowpass filter to reduce the noise's impact on
the regulation. This might be an advantage, e.g. if
there is a great amount of noise on the signal. See
drawing.
= factory setting. () = display text [] = value for use in communication via serial communication port
MG.28.A5.02 - VLT is a registered Danfoss trade mark
®
VLT 2800 Series
20.00 ms
Description of choice:
If a time constant (t) of 100 ms is programmed, the
cut-off frequency for the lowpass filter will be 1/0.1 =
10 RAD/sec., corresponding to (10 / 2 x
The PID regulator will then only regulate a feedback
signal that varies with a frequency of less than 1.6
Hz. If the feedback signal varies by a higher fre-
5.0
quency than 1.6 Hz, it will be dampened by the
lowpass filter.
423 U1 voltage
423
(U1 VOLTAGE)
Value:
0.0 - 999.0 V
Function:
Parameters 423-428 are used when in parameter
101Torque characteristic a selection has been made
of Special motor characteristic [8]. It is possible to
determine a U/f characteristic on the basis of four
definable voltages and three frequencies. The volt-
100 ms
age at 0 Hz is set in parameter 133 Start voltage.

) = 1.6 Hz.
par. 103
43