Calculation of lead lenghts for motor stator RTD's

[Jbbz]

Hi everybody, I'm new at this so please bare with me. I am interested in how to calculate the lead lengths for motor stator RTD's (100 ohm platium) and for motor bearing thermocouples Type K. I have a motor that is approximately 2000 feet from the motor starter and protection relay (Multilin 469).

Thanks in advance.

Jbbz

 

[electricpete]

Do you want max lead length?

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[Jbbz]

Yes. I want maximum lead length. I also have some more information. The RTD is 3-wire, the thermocouple is 2-wire. And according to Multilin the 469 only accepts RTD's and not thermocouples.

 

[edison123]

Normally, 3-wire RTD's are used to nullify the lead resistance and to measure only RTD resistance by the temp scanner/controller. So, lead length should pose a problem.

 

[DougMSOE]

Jbbz,
What edison123 said...
But you need to be knowlegeable as to what the complyance voltage is from the 469.
That is, as the (usually) the constant current source supplies the RTD, and the 3rd wire measures the volt drop across the RTD, will the volt drop by putting the constant current through the wire supplying the RTD be higher than the "source voltage". Usually this is not a problem, and can be calculated easily.

 

[Jbbz]

The specification sheet for the 469 indicates the following:

RTD Sensing current: 5mA

Lead Resistance: 25 ohm Max per lead for Pt and Ni type
3 ohm Max per lead for Cu type.

I've done the following to get the distance but just want to be sure it's right.

I've used Belden cable #1099A which is 16 AWG with Nom. Conductor DC resistance @ 20 deg C of 3.67 ohm/1000 ft. I converted the resistance to a 75 deg C rating to get 4.322 ohms/1000 ft.

The Multilin requires the leads be less than 25 ohms. I do not know what the resistance of the RTD is so, I limited the lead length to 15 ohms or less. Dividing the 15 ohms by the lead resistance ohms/1000 ft gives me a lead length of approximately 3470.6 ft.

Any comments would be appreciated

Jbbz

 

[edison123]

jbbz,

Why would you correct the lead resistance to 75 deg C (unless you expect to run the leads thru' really hot zones). Temp Correction to your ambient temp will suffice.

Correction to my earlier post,

"So, lead length should not pose a problem."

 

[jbartos]

Suggestion: The cable may be of bigger AWG size. The calculation result should leave some margin for temperature increase, contact resistance, cable resistance tolerance due to manufacturing, etc.
The resistance data, e.g. 25&[ignore]Omega;[/ignore] lead, are available at:

http://www.geindustrial.com/products/manuals/469/469-db_2.pdf

 

[uno]

Hi
Just for information

From the 'Engineering talk Weekly product information Guide' -

[5] RTD CONNECTOR TAKES TRANSMITTER ONBOARD
(Omega Engineering, 25 January 2004)

The Omega SPRTX Series are inline high-performance low-cost industrial RTD
connector-transmitters. They feature a hermetically sealed micro miniature
signal conditioner built into a 3-pin connector housing. The design converts
the resistive change of a 100ohm, 0.00385 RTD sensor or probe across a
dedicated temperature range into an industry standard, loop powered, 4-20mA
analogue output. This analogue output can be sent hundreds of metres away
from the location of the sensor to an indicating device or controller.

 

[uno]

Apologies

This may not be applicable for motors.

 

[jbartos]

Suggestion to the previous postings: There is an expected run of 2000feet, which is an equivalent to 2000feet x .3048 m/ft = 609.6 m