It may or may not be obvious, but based on my comments, it will be very relevant WHERE You check the resistance.
You have option to check remotely where the leads enter the remote computer interface, or you can lift the leads at the motor.
You can in theory use successive measurements closer to the RTD to try to narrow down any high resistance in the connections.
You can lift the leads at the remote measurement and then check resistance at the local (motor terminal strip) without lifting leads at terminal strip (to capture as-found condition). (one RTD at a time). In the simplest terminal strip configuration you have a remote lead connected on one side and a local/RTD connected to the same terminal on the other side of the same point (connected by the terminal strip itself). Depending on where you put your probe you are including/excluding different parts of the resistance that may reside in that terminal strip. Bearing mind in theory the high resisstance could be at the crimp (have seen that... didn't strip the insulation correctly for the lug type... this type of problem tends to shows up immediately after a rewind), you rule out the most of the potential high resistance sources near the terminal strip by measuring the barrel of the lug of the lead going back to the RTD (rtd side, not monitoring side).
For our for our 3-wire RTD's we have to do 3 resistance measurements (based on the 3 pairs of leads). It should be one low and 2 high resistance measurements (where the high resistance measurements are very close). Subtract the low from the high, and that is the resistance that you will use with your appropriate RTD table to convert to temperature. You can compare that computed temperature to your plant indication. Yes there are RTD calibrators to do it for you, if you have those available that may be easier.
An important excercize is to consider what the temperature difference is that you suspect is attributable to indication, and then convert that to a resistance reading. That gives an idea how accurately you need to measure record the resistance. We use 10 ohm copper RTDs and we're looking for big resistance differences, we don't find a need to do a 4 wire resistance measurement (the only kelvin probes are the ones we use with our ductor DLRO... those are too big for this purpose anyway)... we just uses a digital multimeter. Again we have 10 ohm copper RTD's, I'm not sure if a more accurate measurement would be needed with 100ohm since smaller contact resistance have bigger effect there.
An important thing to realize in the process is that lifting the leads and relanding often FIXES the indication problem because for whatever reason (mechanical agitation or retightening) it resolves the high resistance at that termination.
In the end the simplest procedure we follow usually end up as follows:
[ol 1]
[li]The work is done with the motor running because we have no trips associated with winding temperature, only indication. If you have a temperature trip, then that deserves extra consideration for conditions of test and precautions to prevent trip during test[/li]
[li]Notify operators / control room that temperature indication will be affected by the work and alarms may be received[/li]
[li]Either lift remote leads at the remote instrumentatio, or skip this step Apparently it's not needed for us and our measurement system (*) .[/li]
[li]Measure the resistance of all 6 RTD's (3 resistance pairs per RTD) from the local terminal strip without lifting any leads[/li]
[li]Repeat measuremtn of the suspect RTD by lifting the lead.[/li]
[li]During previous 2 steps, inspect and clean lugs as applicable while they are lifted[/li]
[li]With only a single channel available for remote monitoring, reconnect so that the hottest of the RTD's is connected to that channel[/li]
[li]Check remote computer indication of the connected channel[/li]
[li]Figure out additional actions from there (most of the time the problem was resolved by lifting/cleaning/landing the problem lead and the indication now appears correct)[/li]
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(*) I'm not sure how we get away with skipping lifting leads at the remote instrumentation since I'd think anything connected on that end would affect the measurement. I used to tell our guys to do that. They came back and told me they were getting the same resistance readings either way. Don't ask me to explain it.
We have had a few times when this procedure did not resolve the problem... all the connections were good and we still have higher indicated temeprature on one RTD. IN that case you have to look for other causes, but at least you have ruled out the cause which is by far most common at our plant (high resistance RTD terminations at the motor). In all the cases I recall with one or more indicating high temperature and several indicating normal temperature, we still ended up eventually attributing it to indication problem of some kind. Often the problematic resistance goes to infinity (open circuit) and there is presumably open circuit somewhere further into the motor. We have had rats chewing on the wire in one of those cases. Another case very fine gage RTD wire was used and ended up breaking.
For the last 23 years my work responsibilities have included health monitoring for 100+ large motors equipped with stator RTD's (all 10 ohm copper 3 wire RTD's). In all that time,
I've never encountered a problem where in the end we attributed it as a real temperature difference. So my opinion is that's a very unlikely outcome even though we have theoretical possibilities of blocked air passages (by foreign material getting past the filters, or by overly aggressive dip and bake that left extra resin... I don't ask for dip and bake but some of our shops have done it during refuribhsment anyway) or localized core hotspots. We have experienced both of those conditions affecting temperature on a motor wide basis but never on a single-rtd basis. What else could cause localized higher temperature?..
[ul]
[li]Severely unbalanced current could cause the localized higher temperature on one phase, but we have never seen it happen. At our plant, we have very stable / reliable power system for our motors so they are not subject to unbalanced voltage originating from the power system. We did have high resistance at the T-leadr lug enough to eventually burn it open and trip the motor, but we never saw any abnormal winding temperatures prior to that (perhaps we weren't monitoring the right rtd)[/li]
[li]There is one more scenario that is often brought up by our plant operators which is an electrical fault within the winding causing local high temperature. I tell them it's not possible because either entire motor would heat (if bizarre winding failure decreased torque capability) or else motor would trip in very short order. They remind me that our synchronous generator has temperature monitoring on individual coils precisely to find problems there like a strand to strand short. But there is a difference in coil construction, that generator uses Roebel bar with effectively single turn per coil. For them strand to strand short is a big deal, for multiturn diamond coil in large motor strand to strand short is insignificant. Multi turn diamond coil in large motor is also susceptible to turn to turn short, but that type of fault will inevitably escalate to trip very quickly, it is not something that will linger long enough to show on winding temeprature.[/li]
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