Long 3-wire RTD run

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jjwitty

Member
Location
Indianapolis
Occupation
Automation
I manage controls in my plant and we hired local electricians to do a project for us.

The project consists of 3 sections of heat tape that heat piping to move oil from a tanker to storage. We need the oil to be around 110 degrees to move around, but I can’t let it go beyond 140 degrees or the chemistry starts breaking down.

I will preface this with the fact the electricians ran the power for the heat tape (220) to one location that’s about 400’ from the pipe inlet. I did show them a location for 220 where the piping begins, but for whatever reason the didn’t tap off of it. Unfortunately there was no formal scope, so I had to work with what they ran. There are three sections of heat tape, two of them are daisy chained.

To control this I bought 3 Solo 4848 ( Manual ) process controllers. I am running a VFD through one controller, and heat tape through the other two ( Rough Sketch ).

Controller 1
Powerflex 525 may not be allowed to start unless the temperature setpoint is met. RTD run = 50’ (outdoors).

Controller 2
L1 of heat tape runs through a NC contact on the controller. This controllers sensor will be close to the sensor that controls the PF525. If the temperature exceeds 140’, the heat tape will lose L1 - power is cut to heat tape - which will also prevent PF525 from starting if conditions aren’t met. RTD = 400’ (outdoors)

Controller 3
L1 of heat tape runs through NC contact on controller. This controller is indoors, and will cut power to the last section of heat tape. RTD=50’ (indoors).

What I’m afraid of is signal loss with the long run of RTD wire that goes outside. Am I right to be concerned? Is there anything I can do to avoid signal loss?
 

rlundsrud

Senior Member
Location
chicago, il, USA
Make sure you get one that will work with the wiring already in place. By that, I mean a two wire versus a three wire temperature transmitter. More specifically, loop powered 2 wire (typically a sinking device) versus a 3 wire which requires a 24 volt suppply (typically a sourcing device).
 

Electromatic

Senior Member
Location
Virginia
Occupation
Master Electrician
Isn't the purpose of the '3rd' wire in a 3-wire RTD to have a reference resistance of the wiring itself that can be compared to the resistance of the sensor? I would think the controller makes that comparison/calculation.
 

tallgirl

Senior Member
Location
Glendale, WI
Occupation
Controls Systems firmware engineer
Isn't the purpose of the '3rd' wire in a 3-wire RTD to have a reference resistance of the wiring itself that can be compared to the resistance of the sensor? I would think the controller makes that comparison/calculation.
Yes. Recall that the resistance change in an RTD is fairly small, even relative to wiring resistance on long runs.
 

rlundsrud

Senior Member
Location
chicago, il, USA
Isn't the purpose of the '3rd' wire in a 3-wire RTD to have a reference resistance of the wiring itself that can be compared to the resistance of the sensor? I would think the controller makes that comparison/calculation.
Three wire RTD's can be inaccurate if the reference wire doesn't match the resistance of the signal wire. This is magnified as the cable length increases. Using a 4-20ma signal eliminates this issue.
 
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kwired

Electron manager
Location
NE Nebraska
Three wire RTD's can be inaccurate if the reference wire doesn't match the resistance of the signal wire. This is magnified as the cable length increases. Using a 4-20ma signal eliminates this issue.
I suppose the amount of acceptable error factors in, but most cases isn't that error going to be pretty minimal if using say twisted conductor and/or a shielded cable? This method would be self correcting should there be changes in ambient temperature along the cable run.
 

rlundsrud

Senior Member
Location
chicago, il, USA
I suppose the amount of acceptable error factors in, but most cases isn't that error going to be pretty minimal if using say twisted conductor and/or a shielded cable? This method would be self correcting should there be changes in ambient temperature along the cable run.
Yes but you still have EMI issues even with twisted pair on a 3 wire RTD. We just use 4-20ma for process control when accuracy is important because it eliminates these issues.
 

tallgirl

Senior Member
Location
Glendale, WI
Occupation
Controls Systems firmware engineer
Yes but you still have EMI issues even with twisted pair on a 3 wire RTD. We just use 4-20ma for process control when accuracy is important because it eliminates these issues.
Correct, because it’s not a communications signal, which is where twisted pair is important.

As regards RTDs and small errors, the resistance per degree change is small enough that for temperature control in environmental spaces it is important. The advantage of current loop is the electrons are still going to be there at the end of the loop, even if the voltage changes for any given current.
 

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
For long runs, use a four wire RTD. That will compensate for the resistance in the conductors. Then choose the transmitter for the output you want.
 

jjwitty

Member
Location
Indianapolis
Occupation
Automation
OP here,

I thought I would let everyone know the project was successful. The long RTD run was about 14ohms, well below the suggested maximum of 40 (? Not sure where that comes from). I did offset the long run by a couple degrees. There are two sensors next to one another, one run is only about 40’, and then the long one is ~400’. The short run was a couple degrees cooler so I took that as being the accurate and made the adjustment.

One issue I’m fighting now is the VFD. I have never set one up for torque control versus speed, but I need to do that for this. The product has the consistency of what seems like super glue, so trying to pump it has been a problem. Do I need any type of input into the VFD for torque control ?
 

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
OP here,

I thought I would let everyone know the project was successful. The long RTD run was about 14ohms, well below the suggested maximum of 40 (? Not sure where that comes from). I did offset the long run by a couple degrees. There are two sensors next to one another, one run is only about 40’, and then the long one is ~400’. The short run was a couple degrees cooler so I took that as being the accurate and made the adjustment.

One issue I’m fighting now is the VFD. I have never set one up for torque control versus speed, but I need to do that for this. The product has the consistency of what seems like super glue, so trying to pump it has been a problem. Do I need any type of input into the VFD for torque control ?
"Do I need any type of input into the VFD for torque control ?" Yes, you will need an analog input or a communication register input telling the VFD how much torque to produce. There will also be parameters in the VFD telling it that the input is for torque, rather than for Hz or for actual RPM or for Horsepower. However, not all VFDs has torque control. Your distributor should be able help you out with this.
 

jjwitty

Member
Location
Indianapolis
Occupation
Automation
"Do I need any type of input into the VFD for torque control ?" Yes, you will need an analog input or a communication register input telling the VFD how much torque to produce. There will also be parameters in the VFD telling it that the input is for torque, rather than for Hz or for actual RPM or for Horsepower. However, not all VFDs has torque control. Your distributor should be able help you out with this.
Powerflex 525
 
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