Current Loops; Current Loop Characteristics; Wind Speed Loop - FT Technologies FT702LT/D Manual

2.2 Current Loops

2.2.1

Current Loop Characteristics

The FT702LT/D wind sensor incorporates two galvanically isolated 4-20mA current loop outputs, one loop for
wind speed and one loop for wind direction. These current loop outputs can be converted into measurable
output voltages with the addition of external resistors.
Each loop should be powered from a DC supply in the range 20V to 30V. A positive supply must be provided to
one of the current loop wires. The other wire must then be connected through a current meter to ground. This is
because the FT702LT/D current loops sink current, they do not source it. A common supply can be used if
required. Loop connections are polarity insensitive so that the +ve or –ve loop supply connection can be made
to either of the current loop connection pins. See example wiring in Figure 21.
The current loops are able to operate over long cable distances; however the overall loop resistance should not
exceed the values given in Figure 3. It is recommended that twisted pair interconnection cabling should be
used. The cable should also include an overall braided screen.
The wind sensor's current loop outputs are updated at a rate of 5 times per second. An average of several
readings should always be used for any calculations or control decisions because single readings can
accidentally become corrupted (see section 2.2.5).
The sensor has its own internal filtering enabled by default, which dampens the speed and wind direction
outputs by averaging the previous 1.6 seconds of data. It is possible to disabled the internal filter (however not
recommended) by using the FT702LT/D Set-Up and Test Program (See Section 4 and 5).
2.2.2

Wind Speed Loop

The default wind speed scaling is such that a change from 4 to 20mA represents 0m/s-50m/s. This corresponds
to a scaling factor of 0.32mA per m/s. The measurement scaling factor for the wind speed current loop is set at
the factory. Figure 4 shows the wind speed calibrations that are available.
The models with scaling factors above 50m/s are intended for applications such as wind turbine controllers
where the wind speed scaling is preset and cannot easily be changed. Because the maximum wind speed
range of the FT702LT/D is 50m/s for models with full scale settings above 50m/s the maximum output current is
limited to a value equivalent to 50m/s (see Figure 4).
Wind Speed Scaling references
30
40
50
60
70
Custom
From build version 22 the wind speed loop can also be calibrated with non-linear scaling functions. See
example customisable calibration in Figure 5
10
Loop Supply
20V
24V
30V
Figure 3: Maximum Current Loop Resistance
4mA 20mA Scaling Factor (mA per m/s)
0m/s 30m/s
0m/s 40m/s
0m/s 50m/s
0m/s 60m/s
0m/s 70m/s
0m/s XX
Figure 4: Wind Speed Scaling Factors
.
FT702LT/D Flat Front Wind Sensor Manual
Section 2 Functional Description
Maximum Loop Resistance
500 Ω
700 Ω
1000 Ω
0.5333
0.4000
0.3200
0.2666 output limited to 17.33mA (50m/s)
0.2286 output limited to 15.43mA (50m/s)
Variable Only available from build version 22