Thermocouple noise

[kchap]

I'm running into a similar issue. I am using an RTD to read the temperature of a large kettle. We recently installed a new mixer in the kettle...a setup I have not seen before. The mixer motor (30 HP) is wired for 240V, but it is driven by a 480V VFD. The RTD is about three feet away from the mixer. Any time the mixer runs, the temperature value decreases by four degrees. We are using shielded cable with the drain wire grounded on the PLC end.

No one in our maintenance or engineering departments has been able to figure out what is causing the drop in temperature value, but it obviously has something to do with the new mixer. Does anyone have any insights as to what might be going on?

 

[twocozz]

hi kchap,

you say the RTD is about three feet away from the mixer. What about the cable run? where is that in relation to the motor cable? do they enter the cabinet at a similar location?

We use RTD's extensively (hundreds per year) and often have problems where the commissioning guys wire the motor and the RTD cable in close proximity to each other giving the same results as you mentioned.

Personally I prefer RTD sensors with a 4-20mA transmitter at the head of the sensor. We never have a problem with interference using this method.

 

[widelto]

Taken from "The Engineer’s Guide to Industrial Temperature Measurement 2013 EDITION" from rosemount.
Virtually every plant environment contains electrical interference sources like pumps, motors, Variable Frequency Drives (VFD’s) and radios as well as sources of electrostatic discharge and other electrical transients. Low level sensor signals from RTDs and T/Cs are very susceptible to Electromagnetic Interference (EMI), Electrostatic Discharge (ESD), and Radio Frequency Interference (RFI).
Sensor leads act like an antenna for noise interference causing potentially very large errors in the measurement. The longer the leads (The antenna) the greater will be the noise pickup. A transmitter is designed to reject common mode and normal mode interference as well as provide a high degree of immunity to EMI, ESD and RFI. Where possible and practical, transmitters should be mounted close to the measurement point to minimize potential noise pickup by the sensor leads. This is especially important for low level T/C signals which are especially susceptible to noise.

 

[danw]

Emerson Rosemount's statement is true for their industrial grade temperature transmitter, but it is not true for the breath of the temperature transmitter market.

There a hundreds of temperature transmitters in the market, which break into two groups - isolated and non-isolated. One has to pay for isolation and isolation increases the cost of the transmitter.

It's very easy to get a temperature transmitter that has no isolation, converts the incoming signal to 4-20mA that reflects ever bit of common mode offset and EMI/RFI/ESD noise that is on its input. That gets you 4-20mA, but it doesn't solve the noise/offset problem.

An isolated transmitter is likely to solve the problem.

 

[osmanmom]

others info

 

[James Fillmore]

What was the make/model of the VFD used for the heater controller the original poster spoke of? Was it actually a VFD or was a it a sophisticated power controller?

Also, another way of preventing overtemp of a heater is a photo sensor. I've use Sylvania air process heaters that have a photo sensor built in.

 

[Ron Beaufort]

just as a troubleshooting step – can you remove the probe from the system – and just "hang it in the air" nearby for a temporary test? ... or (better yet) if you have an exact replacement (spare) unit, connect it to the leads – leaving the original unit to plug the hole ...

we did this test once upon a time with "grounded" thermocouple probes – and as soon as the probe lost contact with the water in our tank, the "noise" problem went away ... then we replaced the probes with the "ungrounded" type – and solved the problem completely ...

side story:

we had specified UNGROUNDED probes when we placed the order – but our finance department (bless their little hearts) sent the specifications out for competitive bids ... a vendor gave them a cheaper price – but nobody noticed that he had substituted cheaper GROUNDED type probes to keep the price down ...

we installed the probes that we THOUGHT we had ordered – then wasted a lot of time tracking down the cause of the "noise" problem ...

 

[kchap]

Thank you for your responses.

The PLC and VFD (Automation Direct DuraPulse) are in separate enclosures located about forty feet away from each other. The RTD cable is run in conduit that contains only low-voltage control wires.

I don't know whether the RTD is isolated or not...most likely not, given the fact that we do everything on the cheap here.

Ron - I will try removing the probe and see if that eliminates the interference.

 

[danw]

An RTD is, by design, isolated from its sheath.

An RTD is 'excited' by a constant current source from the RTD input circuit. The input reads the voltage drop across the resistance. The current has to be a known constant in order for the IR drop to be meaningful.

In order to ensure that the only current running through the RTD element is sourced from the input circuit, the element is isolated from the sheath/environment to eliminate a ground loop.

If the sensor in question is an RTD (OP says it's a T/C), it might pay to measure for resistance from either RTD leg to the sheath, because a grounded RTD element is a failed RTD assembly.

Thermocouples can be either isolated or not, depends, as Ron has pointed out.

Dan