Specifications

2.8.5 Control Structure Closed Loop
The internal controller allows the frequency converter to become a part of the controlled system. The frequency converter
receives a feedback signal from a sensor in the system. It compares this feedback to a setpoint 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 controlled so that the static pressure in a pipe is
constant. The desired static pressure value is supplied to the frequency converter as the setpoint reference. A static pressure
sensor measures the actual static pressure in the pipe and supplies this value to the frequency converter as a feedback
signal. If the feedback signal is greater than the setpoint reference, the frequency converter slows down to reduce the
pressure. Likewise, if the pipe pressure is lower than the setpoint reference, the frequency converter speeds up to increase
the pump pressure.
P 20-81
PID Normal/Inverse
Control
PID
Ref.
Handling
Feedback
Handling
Scale to
speed
P 4-10
Motor speed
direction
To motor
control
(Illustra-
tion)
(Illustra-
tion)
130BA359.12
Σ
100%
0%
-100%
100%
*[-1]
_
+
Illustration 2.21 Block Diagram of Closed Loop Controller
While the default values for the closed loop controller often provide satisfactory performance, the control of the system can
often be optimised by adjusting some of the closed loop controller parameters. It is also possible to auto tune the PI
constants.
2.8.6 Feedback Handling
Feedback handling can be configured to work with
applications requiring advanced control, such as multiple
setpoints and multiple feedbacks. Three types of control
are common.
Single Zone, Single Setpoint
Single zone single setpoint is a basic configuration.
Setpoint 1 is added to any other reference (if any, see
2.8.8 Reference Handling ) and the feedback signal is
selected using CL-20 Feedback Function.
Multi-Zone, Single Setpoint
Multi zone single setpoint uses 2 or 3 feedback sensors but
only 1 setpoint. The feedbacks can be added, subtracted
(only feedback 1 and 2) or averaged. In addition, the
maximum or minimum value could be used. Setpoint 1 is
used exclusively in this configuration.
If [13] Multi Setpoint Min is selected, the setpoint/feedback
pair with the largest difference controls the speed of the
frequency converter. [14] Multi Setpoint Maximum attempts
to keep all zones at or below their respective setpoints,
while [13] Multi Setpoint Min attempts to keep all zones at
or above their respective setpoints.
Example:
A 2 zone 2 setpoint application in which the Zone 1
setpoint is 15 bar and the feedback is 5.5 bar. Zone 2
setpoint is 4.4 bar and the feedback is 4.6 bar. If [14] Multi
Setpoint Max is selected, the setpoint and feedback of
Zone 2 are sent to the PID controller, since this has the
smaller difference (feedback is higher than setpoint,
resulting in a negative difference). If [13] Multi Setpoint Min
is selected, the setpoint and feedback of Zone 1 are sent
to the PID controller, since this has the larger difference
(feedback is lower than setpoint, resulting in a positive
difference).
Introduction VLT® HVAC Drive FC 102 Design Guide
MG16C102 - Rev. 2013-08-20 33
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