Danfoss VLT FC 102 Series Design Manual page 45

Introduction to VLT® HVAC D...
The remote reference is comprised of:
•
Preset references.
•
External references (analog inputs, pulse
frequency inputs, digital potentiometer inputs
and serial communication bus references).
•
The preset relative reference.
•
Feedback controlled setpoint.
Up to eight preset references can be programmed in the
adjustable frequency drive. The active preset reference can
be selected using digital inputs or the serial communi-
cations bus. The reference can also be supplied externally,
most commonly from an analog input. This external source
is selected by one of the three Reference Source
parameters (3-15 Reference 1 Source, 3-16 Reference 2 Source
and 3-17 Reference 3 Source). Digipot is a digital potenti-
ometer. This is also commonly called a Speed Up/Slow
Control or a Floating Point Control. To set it up, one digital
input is programmed to increase the reference, while
another digital input is programmed to decrease the
reference. A third digital input can be used to reset the
digipot reference. All reference resources and the bus
reference are added to produce the total external
reference. The external reference, the preset reference or
the sum of the two can be selected to be the active
reference. Finally, this reference can be scaled by using
3-14 Preset Relative Reference.
The scaled reference is calculated as follows:
Y
Reference = X + X ×
100
Where X is the external reference, the preset reference or
the sum of these and Y is 3-14 Preset Relative Reference in
[%].
If Y, 3-14 Preset Relative Reference is set to 0%, the
reference is affected by the scaling.
MG11BC22
Design Guide
2.8.10 Example of Closed-loop PID Control
Figure 2.30 Closed-loop Control for a Ventilation System
In a ventilation system, the temperature is to be
maintained at a constant value. The desired temperature is
set between 23–95 °F [-5–+35 °C] using a 0–10 V potenti-
ometer. Because this is a cooling application, if the
temperature is above the setpoint value, the speed of the
fan must be increased to provide more cooling air flow.
The temperature sensor has a range of 14–104 °F
[-10–+40 °C] and uses a two-wire transmitter to provide a
4–20 mA signal. The output frequency range of the
adjustable frequency drive is 10 to 50 Hz.
1.
2.
3.
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Start/Stop via switch connected between
terminals 12 (+24 V) and 18.
Temperature reference via a potentiometer
(23–95 °F [-5–+35 °C], 0 to 10 V) connected to
terminals 50 (+10 V), 53 (input) and 55 (common).
Temperature feedback via transmitter (14–104 °F
[-10–+40 °C], 4–20 mA) connected to terminal 54.
Switch S202 behind the LCP set to ON (current
input).
2 2
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