Moeller DF5 series - Page 88

09/01 AWB8230-1412GB
PID controller
85
The example in the following figure shows a fan control:
P: Proportional component
This component ensures that the output frequency and the system
deviation are subject to a proportional relationship. Using
PNU A72, the so-called proportional gain (K
p
), expressed in %,
can be defined.
The following figure illustrates the relationship between system
deviation and output frequency. A large value for K
p
results in a
quick reaction to a change of the system deviation. If, however, K
p
is too large, the system becomes unstable.
The maximum output frequency in the above figure is defined as
100 %. K
p
can be set between 0.2 and 5.0 under PNU A72.
I: Integral component
This component results in a correction of the output frequency by
integration of the system deviation. In the case of purely proporti-
onal control, a large system deviation causes a large change in the
output frequency. It follows, then, that if the system deviation is
very small, the change in the output frequency is also very small.
The problem is that the system deviation cannot be completely
eliminated. Hence the need for an integral component.
The integral component causes a continuous adding up of the
system deviation so that the deviation can be reduced to zero. The
reciprocal value of the integration gain is the integration time
T
i
=1/K
i
.
With the DF5 series frequency inverters, set the integration time
(T
i
). The value may be between 0.5 s and 150 s. To disable the
integral component, enter 0.0.
D: Differential component
This component causes a differentiation of the system deviation.
As pure proportional control uses the current value of the system
deviation and pure integral control values from previous actions, a
certain delay in the control process always occurs. The D compo-
nent compensates for this behaviour.
Differential control corrects the output frequency using the rate of
change of the system deviation. The output frequency can there-
fore be compensated very quickly.
K
d
can be set between 0 and 100 s.
The PID controller
A PID controller combines the P, I and D components described in
the previous sections. In order to achieve the optimum control
characteristics, each of the three PID parameters must be set.
Uniform control behaviour without large steps in the output
frequency is guaranteed by the proportional component; the inte-
gral component minimizes the existing system deviation the
steady-state and the differential component ensures a quick
response to a rapidly changing actual value signal.
As differential control is based on a differentiation of the system
deviation, it is very sensitive and also responds to unwanted
signals, such as interference, which can result in system instability.
Differential control is normally not required for flow, pressure and
temperature control.
Figure 82: Example of a fan control
G1:DF5 series frequency inverters
w: Setpoint value
x: Actual value
P1: Controlled variable
B1: Measured value converter
a Fan
Figure 83: Proportional gain K
p
x: System deviation
w
B1
P1
x
0... +10 V H ; 4 – 20 mA
G1
a
M
3
~
100
10050 75250
75
50
25
[%]
f
x [%]
Kp = 1
Kp = 0.75
Kp = 2
Kp = 0.5
Kp = 0.25
0.2 F Kp F 0.5