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3.1.3 Control Structure Closed Loop
The internal controller allows the frequency converter to become an integral part of the controlled system. The frequency
converter receives a feedback signal from a sensor in the system. It then compares this feedback to a set-point reference
value and determines the error, if any, between these two signals. It then adjusts the speed of the motor to correct this
error.
For example, consider a pump application where the speed of a pump is to be controlled so that the static pressure in a
pipe is constant. The desired static pressure value is supplied to the frequency converter as the set-point reference. A static
pressure sensor measures the actual static pressure in the pipe and supplies this to the frequency converter as a feedback
signal. If the feedback signal is greater than the set-point reference, the frequency converter will slow down to reduce the
pressure. In a similar way, if the pipe pressure is lower than the set-point reference, the frequency converter will automat-
ically speed up to increase the pressure provided by the pump.
7-30 PI
Normal/Inverse
Control
PI
Reference
Feedback
Scale to
speed
P 4-10
Motor speed
direction
To motor
control
130BB894.11
S
100%
0%
-100%
100%
*[-1]
_
+
While the default values for the frequency converter’s Closed Loop controller will often provide satisfactory performance,
the control of the system can often be optimized by adjusting some of the Closed Loop controller’s parameters.
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