4-20mA Distance limits

[sunrays]

I need to send the speed reference from Drive A to Drive B over a distance of 800 Meters. Application is not very critical from the speed accuracy point of view. A +/- 10 RPM is good enough.

Option A: Use a field bus option but length is an issue. I am given to understand that I need to use Fiber Optic Cables to over come the distance, which add to the cost.

Option B: 4-20mA signal. I need a converter at both ends to convert the 0-10V Signal of Drive A to 4-20mA and 4-20mA back to 0- 10 V at Drive B since the drives can deliver/accept only Volts. The cable proposed to be used will be shielded one.

What is the maximum distance permissible with a 4-20mA option? Would I face any loss of signal over this distance?

Thanks!

 

[roydm]

Some of the posters seem to think this is a two wire loop. Drive A puts out 0-10 Volts, Drive B needs 0-10 Volts
In order to use a current loop
At Drive A you will need a 4 wire 0-10/0-20 transducer. Either way you will only loose a few volts e.g. 5 (not 12 as you would with a 2 wire transmitter) so assuming you just have a 500 Ohm resistor at B you have 9 V for line loss.
If you use a loop powered 0-10/4-20 transducer at A you would also need a transducer at B but this would be 250 Ohms so now you have 14 Volts for line loss. But now you can also use a higher power supply voltage e.g. 48 V giving you 38 for line losses.
Not all transducers are 250 Ohms some are 50 Ohm or less.

Another option would be fiber optics thus eliminating any noise problem and need for conduit.

http://weedinstrument.com/product_groups/fiber/analog_system.html

Roy

 

[electricpete]

Thanks rodym. When I worked with transmitters, they were all remote 2-wire pressure / flow transmitters that get all their power from remote power supply through those two signal wires. I guess if the transmitter is in a control panel like a vfd, then it is usually 4 wire with direct input from both a power supply and a transducer?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[sreid]

Did anyone notice that 18 ga wire has a resistance of about 6.4 Ohms/1000 ft, not 37 Ohms?

 

[SteveWag]

From my post of 14 Aug 08 9:44,

“800 meters should be no problem, #18 wire has a resistance of less than 7 ohms per 1000 feet, # 16 has less than 5 per 1000”

Steve

 

[electricpete]

This distance we are talking about was 1600m. Round numbers 5000 ft. 5*6.4 =32. Conservative allowance made for temperature higher than reference (20C) and other factors.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.