what do we require 4-20 mA current out in process control

[kaus9952083382]

I am acquiring temp.& Humidity from RTD and HIH-4000 sensors... now i need to control the temperature & humidity accordingly using actuators(Fan,Humidifiers and Dehumidifiers). Some one told me that i need a current output module(4-20mA) for this... can someone please explain what is use of 4-20 mA current output...

 

[FvM]

4-20 mA interface is the most popular option to control actuators with analog setpoint, e.g. speed of motor VFD, position of proportional valves or servo drives, setpoint of temperature controllers etc. These days it's more and more obsoleted by field busses.

 

[kaus9952083382]

Thank you sir, but can you explain with the help of a practical example.... i read about vfd drives and found that they require 4-20mA or 0-10V for the operation... In my case i am using single phase induction motors (0.5 H.P) and as far as i know VFD drives are not there for single phase .. you need to use some thyristor based circuits to control it .... moreover explaining my application in more detail..... I have circular chamber ... which is to be rotated at a speed of 2 R.p.m. which i have achived through reduction gears and worm wheel... Now there are two fans on either side... one is for bring the air in and the other is an exhaust for taking the air out .... by acquiring the temperature using rtd i need to control the speed of the fans to maintain desired temperature... please help me ..... i am handling such big project for first time...

 

[FvM]

You have to find a suitable motor controller for the fans with analog input or a different remote control option. That's beyond the question about 4-20 mA.

 

[pinout]

Just to add that 4-20mA is very good for noise immunity as it is a low impedance loop, hence good for long distance use.
The loop current can also power remote sensors at up to 4mA with careful design.
Loop breaks are easy to detect, hence you don't try to control on a zero or noisey analogue voltage input.

Make sure you check how designers have generated their loop current and how you are measuring wrt ground. I have seen some incompatable interfaces where the loop current source is destroyed by the loop current measuring circuit!

 

[kaus9952083382]

You have to find a suitable motor controller for the fans with analog input or a different remote control option. That's beyond the question about 4-20 mA.

Thanks sir ... sir I am just placing a link which i read on OMRON Automation website .. https://www.ia.omron.com/support/guide/18/overview.html
Please go through the SSR Part and help me..........................

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[pinout]

The SSR is nothing to do with the actual 4 to 20mA loop, it is just an alternative to having a relay in the output circuit.
They may be current driven, but that's not (normally) related to loop current signaling.