Danfoss VLT Series Programming Manual page 139

Parameter Description
If (Error x Gain) jumps with a value equal to what is set in
20-14 Maximum Reference/Feedb. the PID controller will try
to change the output speed equal to what is set in
4-13 Motor Speed High Limit [RPM]/4-14 Motor Speed High
Limit [Hz] but in practice of course limited by this setting.
The proportional band (error causing output to change
3 3
from 0-100%) can be calculated by means of the formula:
(
1
Proportional Gain
NOTE
Always set the desired for 20-14 Maximum Reference/Feedb.
before setting the values for the PID controller in meter
group 20-9*.
20-94 PID Integral Time
Range:
20.00 [0.01 -
s 10000.00
*
s]
20-95 PID Differentiation Time
Range:
0.00 [0.00 -
s 10.00 s]
*
138
)
× ( Max Reference )
Function:
Over time, the integrator accumulates a
contribution to the output from the PID
controller as long as there is a deviation
between the Reference/Setpoint and
feedback signals. The contribution is propor-
tional to the size of the deviation. This
ensures that the deviation (error)
approaches zero.
Quick response on any deviation is obtained
when the integral time is set to a low value.
Setting it too low, however, may cause the
control to become unstable.
The value set, is the time needed for the
integrator to add the same contribution as
the proportional for a certain deviation.
If the value is set to 10,000, the controller
will act as a pure proportional controller
with a P-band based on the value set in
20-93 PID Proportional Gain. When no
deviation is present, the output from the
proportional controller will be 0.
Function:
The differentiator monitors the rate of change
of the feedback. If the feedback is changing
quickly, it will adjust the output of the PID
Controller to reduce the rate of change of the
feedback. Quick PID Controller response is
obtained when this value is large. However, if
too large of a value is used, the frequency
converter's output frequency may become
unstable.
Differentiation time is useful is situations where
extremely fast frequency converter response
and precise speed control are required. It can
be difficult to adjust this for proper system
control. Differentiation time is not commonly
®
used in VLT HVAC Drive applications.
MG.11.CC.02 - VLT
®
VLT HVAC Drive Programming Guide
20-95 PID Differentiation Time
Range:
20-96 PID Diff. Gain Limit
Range:
5.0 [1.0 -
*
50.0 ]
®
is a registered Danfoss trademark
Function:
Therefore, it is generally best to leave this
meter at 0 or OFF.
Function:
The differential function of a PID Controller
responds to the rate of change of the feedback.
As a result, an abrupt change in the feedback
can cause the differential function to make a
very large change in the PID Controller's output.
This meter limits the maximum effect that the
PID Controller's differential function can
produce. A smaller value reduces the maximum
effect of the PID Controller's differential function.
This meter is only active when 20-95 PID Differ-
entiation Time is not set to OFF (0 s).