Problem in RTD Singal to PLC

It sounds as though the signal wiring from the RTDs to the PLC input module may be an incorrect type, and thus picking up induced AC. I'm not sure, but I think shielded cable is needed, and maybe with twisted-pair feature also.

Who is Ellen Breadly? Never heard of her; wife of the Pillsbury Doughboy, maybe?

RTDs need to be connected to the inlet module of an Allen Bradley PLC using an intervening transmitter. The task of the transmitter is to convert the resistance measurement at the RTD into a 4-20mA signal for onward transmission to the card.

Why is the RTD lead voltage being measured? With what instrument?

Is the RTD transmitter in-circuit? What is its part number?

That module isn't a 4-20mA input type - it's for direct connection to 2- or 3-wire RTDs using screened twisted pair or 3-core (for 4-wire types, one wire is cropped off!).

See user manual pg 2-7.

Ahah! That explains why the transmitter isn't needed. Thanks.

Why is the lead-to-earth voltage perceived to be a problem?

Check the wiring guidelines here, but keep in mind that you're using 11KV motors ! you might need to take some countermeassures to overcome the fields surrounding these motor windings

Whre's the RTD installed ?

Yahlasit

If you see such a big voltage variation, and specially if the mere impedance of your meter makes it change, then your meassuring system is "floating"; tie the metal sheathing or shields (or both, if they exist) of your RTDs to ground.

Yahlasit

Exactly.

• If the system is floating, then connecting the RTD to an earthed structure will make no difference, and the voltage while the system is floating is irrelevant. The RTD can then be connected to the PLC with impunity.
• If the voltage is "hard", connecting the RTD to an earthed structure will operate the circuit protective device, in which case the fault that is supplying the voltage needs to be cleared.

End of.

Ok. I see what you mean, but it would apply to hand-held devices, but hard-wired meassuring systems (and this is something I forgot to mention), need some sort of shield & drain of EMI.

About your second point I don't argue that.

Question for Guest: How high is the resistance range of your RTDs? if it is too high, then you might be having stray currents induced into the RTD wiring, if shielding is not an option, then try the lowest resistance RTDs allowed by your input module specs.

Yahlasit

Eh? Shield and drain of EMI? Why? It's an RTD, not a radio receiver! EMI will appear on all leads at once, and will cancel out in the bridge on the PLC input when the resistance gets converted to a measurement.

Try changing to a cheap moving coil voltmeter [MCV], instead of using the latest whizz-bang, high input impedance DVM. If the problem goes away with the MCV, then there isn't anything to worry about. If the circuit protective device can be operated by connecting the RTD leads to earth, then the fault needs clearing before connecting them to the inputs (as stated earlier).

Every piece of wire is a radio receiver and if the cable is long it will be a very good receiver. However if you ground one leg of the RTD and it is not the same ground as the PLC you will get ground loops. You need to fit a ground loop isolator. They are not available for RTD so you have to get one that converts the RTD to a 4-20mA loop.

Please follow the link I gave in #4.

The PLC module used here is designed for direct connection to RTDs. Correct wiring (including screening/shielding and earthing/grounding) will give correct results.

If this were not the case Allen-Bradley would not sell them.

Is the PLC cards Isolated or Non Isolated?

Post #4 indicates that it is not necessary to use a transmitter with the type of input card available.

The two wires of this radio antenna are connected together with a resistance of 100Ω at 0deg C and any pick-up will be the same on both wires. Any effect will therefore be cancelled out at the input terminals.

The input is not sensitive to radio frequencies. It is a bridge circuit.

In all probability RTDs in question are for measurement of Stator winding and core temperature. These RTDs should be be taken out form the motor frame through shielded wires and the shield screen should be earthed on to the motor frame. It seem that shield is picking the stray voltage (assuming that the outlet leads are shielded). This can be confirmed by measuring the voltage signal at reduced load (the voltage would be proportional to the load current) or at no load when this voltage would be negligible ( as only the magnetizing current would be taken by the motor). Also measure insulation resistance between RTD lead and the shield screen.

Ensure that the screen extending from the panel is not getting grounded anywhere other than at the panel end. (may be outer sheathing of the screened field cable is damaged at some place, on the cable tray and getting earthed.

Ensure none of the leads of the RTDs are having earth leakage (very rare at the generator end).

As the measuring channels of the card are not isolated, any one RTD having earth fault will affect all the 8 measuring channels.

Disconnect each RTD and check its field cable for earth resistance, should be quite high.

If all the above points are OK the system should work.

You need to isolate the signal. It is a very common problem. You need to fit an RTD to 4-20mA signal conditioner with isolation. It is very important to have the galvanic isolation. Have a look at a DAT4135AC Isolator. You should be able to buy them from www.Datexel.com.