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RTD Pt100
4

RTD Pt100

RTD Pt100

(OP)
Hi Folks
RTD Pt 100, fixed on the bearings of 1.8MW motor . Due to failure in the lubrication system . the bearing were melted and the motor stopped manually before the RTD detected the over temperature.
The RTD removed to be tested in heating bat, the temperature take 8min to rise form 20c to 100c.
Is this normal ? Are there different RTD time responses?

RE: RTD Pt100

That does seem slow. Is there a heavy housing around the RTD? RTDs are known for being slower than thermisters, but that is on the order of 7-10 seconds vs 1 second to reach 90% of steady state, not 8 minutes.

It may also be the case for that bearing that the failure was such that no warning would have prevented serious damage, but it would be good if it didn't melt.

RE: RTD Pt100

It seems that you were testing your heating bat (whatever that is) rather than the RTD. Boiling water and an ice bath are more appropriate for testing an RTD.

RE: RTD Pt100

2
Dear Mr. Emadshaaban1987 (Electrical)(OP)14 May 22 19:19
"...RTD Pt 100, fixed on the bearings of 1.8MW motor.... the bearing were melted and the motor stopped manually before the RTD detected the over temperature. #1. The RTD removed to be tested in heating bat, the temperature take 8min to rise form 20c to 100c. Is this normal ? #2. Are there different RTD time responses?....".
#1. Assuming it is a platinum RTD 100. Its resistance is 100 Ohm at 0deg C. It will reach 135 Ohm at 100deg C. Field test (approx value): dip it in [ice cold bath] for say 5 minute, the reading should be around 100 Ohm. Remove it from tie cold bath, clean the icing water. Wait for say 5 minutes exposing it to the ambient temperature. Dip it into a boiling 100deg C hot water bath. Attention: dip it into boiling hot bath. NOT slowing hearting up the water. The resistance is expected to reach around 135 Ohm within say <10 second.
Note: resistance measurement may be carried out with a low resistance ohm meter. Take spot reading and remove the meter connection once the read is taken.
#2. Different RTD may have different respond time. Generally, < 10s to reach 100% temperature.
3. FYI: a) Check the resistance of RTD including the long wiring and the monitoring device.
b) Check that the (oil well/RTD pocket) is [filled (not dry) with oil] for good thermal contact between the bearing housing and the RTD.
Che Kuan Yau (Singapore)

RE: RTD Pt100

In our motor rewind shop, we routinely check many PT 100 RTD's for bearings and windings at room temperature and then at boiling water. The instrument reads 100 deg C typically in 30 to 45 seconds of dipping into the boiling water. Your 8 minutes indicates defective RTD confirmed by the destroyed bearing.

Also, many OEM's don't bother to insert bearing RTD's all the way to the bearing OD and stop at somewhere in the bearing housing. We modify their fixing so that RTD's are in actual contact with the bearings.

Muthu
www.edison.co.in

RE: RTD Pt100

There is nothing that can go wrong with an RTD that would cause a time delayed response except poor thermal contact with the temperature to be measured.

RE: RTD Pt100

The metal of the detector will react to temperature just as any other metal. More mass takes longer to react. Most detectors have an insulating jacket (of some sort) - it usually is not a good thermal conductor, so it slows down the response. For bearing detectors, the "insulator" is typically either an epoxy or air; the detecting element is suspended inside the "tube" which surrounds it and is the visible portion.

As mentioned earlier - the detector should change state (by some amount) in a few seconds. The wider the change (say, room temperature to 100 C) the longer it takes - but it is still on the order of < 10 seconds.

Converting energy to motion for more than half a century

RE: RTD Pt100

I assume that this is a sleeve bearing with a separate shell to hold the white metal?

In the olden days they would apply the white metal direct to the bearing housing, then line bore it.

Someone had the bright idea of having a separate shell, so it was easier to manufacture and replace.

As Edison123 said, you must make sure that the RTD is is contact with the shell, do not leave an air gap. I once had to ship a new gas turbine from Spain to the repair shop in Houston because the RTD showed varying teperatures. A stripdown revealed that there was some swarf (USA: Chips) between the RTD and the bearing!

Also check that the RTD is NOT at the bottom dead center of the bearing. This the wrong place!

An oil lubricated sleeve bearing is a hydrodynamic bearing and in operation the shaft essentially climbs up the oil film to about an angle of 30 degrees, the RTD shouls be positioned a little way beyond this.

My view on RTDs is that they are good at monitoring trends, but may not pick up a sudden event quickly enough to avoid damage, its operation is 'slugged' by the mass of material around it.

There is lots on the web eg: see attachment

RE: RTD Pt100

There are a lot of other things that you can safely use to improve the thermal coupling also.
There are special heat sink greases, and the old favorite MgO (milk of magnesia) works well.
A very slow temp sensor makes me think that the element is not in contact with the case internally.
This is a defect for this application.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

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