Control Structure Closed Loop - Danfoss VLT FC51 Design Manual

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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 2 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 slows down to reduce the
pressure. In a similar way, if the pipe pressure is lower than the set-point reference, the frequency converter automatically
speed up to increase the pressure provided by the pump.
+
Reference
S
_
Feedback
Illustration 3.3 Control Structure Closed Loop
While the default values for the frequency converter's closed loop controller often provides satisfactory performance, the
control of the system can often be optimised by adjusting some of the closed loop controller's parameters.
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VLT Micro Drive FC 51 Design Guide
PI
*[-1]
7-30 PI
Normal/Inverse
Control
MG02K302 - Rev. 2013-12-03
100%
0%
Scale to
speed
100%
-100%
P 4-10
Motor speed
direction
3 3
To motor
control
19