10/05 AWB8230-1541GB
PID control
135
Output limitation (PNU A078)
The PID controller has an automatic output limit function. It
monitors the percentage deviation of the manipulated variable
(output frequency) from the control difference (e = reference value
– actual value). You can specify the limit value with PNU A078.
This setting can be made only at the lower and upper operating
frequency limits (a section “Minimum and maximum operating
frequency”, page 95).
• If the system deviation (reference value – actual value) is less
than or equal to the value set with PNU A078, the controller
works within its normal, linear range.
• If the system deviation is greater than the value set with
PNU A078, the controller changes the output frequency so that
the limit value is not reached.
• The polarity of the system deviation can be set PNU A077.
Inversion (PNU A077)
In typical control applications, such as heating and ventilation
control systems, an increase in power consumption is the result of
a rising actual value (system deviation = reference value minus
actual value); in cooling systems, increased power consumption
results from a falling (negative) actual. With PNU A077 you can
set the polarity of the system deviation.
Display factor (PNU A075)
When PID control is enabled (PNU A071 = 01), the reference and
actual values become process variables and are automatically
converted into percentages. The specified reference value (0 –
100 %) corresponds with, for example, a volume flow rate (0 –
50 m
3
/h). The process variable (PV) is the reading (m
3
/h) from a
sensor and is again evaluated as a percentage (0 100 %).
With PNU A075 you can scale the displayed value so that the
process data is displayed in its physical units, in this case m
3
/h. The
reference value is output at PNU F001 and the actual value at
PNU d001.
Figure 139:PID control – limitation of minimum output frequency
a Limitation active
b System deviation (reference value – actual value)
A078: Percentage limit value.
A078
A078
t
%
a
b
a
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