Danfoss VLT HVAC Basic Drive FC 100 Series Design Manual page 25

Introduction to VLT HVAC Ba...
2.6.3 Control Structure Closed Loop
2 2 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.
Reference
Feedback
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.
2.6.4 Feedback Conversion
In some applications it may be useful to convert the feedback signal. One example of this is using a pressure signal to
provide flow feedback. Since the square root of pressure is proportional to flow, the square root of the pressure signal
yields a value proportional to the flow. This is shown below.
Ref.
signal
Desired
flow
24
VLT HVAC Basic Drive Design Guide
+
S
_
*[-1]
7-30 PI
Normal/Inverse
Control
Ref. +
PI
P 20-01
-
P 20-07
FB conversion
FB
Flow
P
FB
signal
P
MG.18.C2.02 - VLT
100%
0%
PI
100%
-100%
P
Flow
®
is a registered Danfoss trademark
To motor
Scale to
control
speed
P 4-10
Motor speed
direction