1746-NR8 RTD module failure? 1

[ScottI2R]

Hello all,

We have a 2300V 1500hp Siemens motor Model:CGII frame:5812S that is giving us intermittent outer(rear) bearing temperature high faults. Now our control system monitors this and the trending I found showed temperature variations from a -328 degrees to 1562 degrees F. This was with the motor not running. The wiring is straight from the motor j-box to the input on the plc. I checked that (111.4 ohms) and there are no opens which I believe would cause the 1562 degree value. The negative temperature is what has me looking at the card. What I'm wondering is, what is everybody's experience with RTD failures vs the RTD module in the plc? At present I have a new RTD module in the plc and I am running it monitoring temperature data. Any and all thoughts are welcome and thank you in advance.

Scott

I really am a good egg, I'm just a little scrambled!

 

[ScottI2R]

Continued from my O/P:

Here is the -328 temperature value shown in the trending.

Thanks again,
Scott

I really am a good egg, I'm just a little scrambled!

 

[thebard3]

I have replaced very few PLC modules. Analog module failures are usually due to some transient voltage above the specification limit finding it's way to the terminals. I immediately suspect the wiring, maybe intemittent connection, but you say you've already done that. If you don't have a resistance source to verify that the data agrees with the signal, then I would be guessing, same as you.

 

[danw2]

Wiring
Is it wired right? A-B uses the terms RTD, Sense and Return for the three terminals.
Confusing the terms can result in confused wiring.

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1746-in007_-en-p.pdf

page 12 pdf

A 2 wire RTD has the resistance between RTD-n and Return-n with a jumper from RTD-n to Sense-n

A 3 wire RTD has the resistance between RTD-n and Return-n with the 3rd wire to the Sense-n terminal

Element
Is the sensor the common 3 wire 100 Platinum (Pt) RTD? Are you sure? Other RTD have been used for motors. Nickel or copper elements.

What is the PLC configured for? Which sensing element?

A configuration for 500 or a 1000 ohm will read extremely low with a 100 ohm in the circuit because the configuration expects a higher resistance.

Quick Test
Assuming use and configuration for a Pt100, the quick test to tell you whether the problem is in the card or the field is to get a 100 ohm resistor and wire it across the RTD input with a jumper for the 3rd wire and PLC should read close to the freezing point of water, plus or minus the tolerance of the resistor to 100.00 ohms.

RTD's usually fail open, but the wiring can short with a resulting very low resistance.

Isolation
An RTD is expected to be isolated from ground by its inherent construction. Check for high resistance between each leg and ground. A short to ground will produce errors.