Frequent failing of RTDs in stator windings

[eeprom]

Hello,
I am investigating a series of RTD failures in stator windings. All the RTDs are 100 ohm PT, and the motors are mostly 250 to 300 Hp. All RTDs are wired to multilin 469s. From what I have checked, the wiring is correct. All RTD failures were checked by techs. They are open circuits.

I have been working around large motors for about 15 years, and up until now I have only seen a couple of failed RTDs. With respect to this investigation, I have seen 5 failures in one year. This is in a group of about 12 motors.

I don't have a lot more info yet. I just wanted to check in advance if anyone had experience with anything similar.

thanks
EE

 

[itsmoked]

One runs current thru RTDs to get the info. You can send too much and toast them. The hotter their environment the more likely this can happen.

There are a couple of ways to read an RTD. One is to pulse it very briefly with a relatively large current that would otherwise skew the reading by self-heating but during the short reading time it doesn't. The high-ish current helps raise the measurement way above the noise. If something goes wrong that same current could cook the RTD.

Could the multilin be set up wrong?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[catserveng]

I have ran into this issue on generators from a particular manufacturer. Units manufactured up until about 2004 rarely had RTD failures, after that time we saw an increasing number of RTD failures, all going open. Mostly in Prime and continuous operating units. After a few years of "we didn't change anything" we found that thru a cost reduction effort they went to a cheaper RTD. From what we learned someone decided since they installed extras anyway and the sensors really weren't that important (in someone's opinion), it wasn't a big deal. Or so they said. Was to us.

We were primarily using GE SR489 protection relays before and after, GE claimed no change in how they managed RTD circuits, and other units installed in the same time period had no issues with the same relays.

Maybe a similar issue, not sure "value engineering" always results in a better value to the end user.

MikeL.

 

[eeprom]

I haven't yet investigated the circuit, so it is possible that the wiring is wrong. But the ML appears to be set up for RTD - 100 ohm pt. I also have not gotten enough field data to learn if all of these are from the same motor manufacturer. I know they are all from one of two facilities, put in by the same engineering firm, at about the same time. So the possibility of getting "a bad batch" is possible.