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pittengineer (Electrical) (OP)
4 Dec 09 13:45
The motor that I am referencing is a 350hp 4.16kV induction motor started with a DOL starting method.

The motor came equipped with 6 stator rtds which are connected to a motor protection relay for monitoring.  It was discovered recently that one of the rtd's was damaged (failed open) and was not reading anymore.  

This motor is very difficult to take out and send out to have this rtd replaced.  I was debating weather or not the motor would be fine just using the other 5 stator rtd's for monitoring?  Should I be o.k. with just leaving the 5 rtd's in place or is it absolutely necessary to go about pulling the motor and replacing the damaged one?
dpc (Electrical)
4 Dec 09 14:38
You will be OK with only five RTDs.  There are two RTDs per phase.  Back in simpler times, we typically only connected the hotter of the two to a monitoring system.  

There is no reason to consider repairing a single winding RTD, in my opinion.

The RTDs are useful for monitoring long-term heating and overloads.  They are generally not fast enough to help much with motor starting, which is usually rotor-limited anyway.  

 
waross (Electrical)
4 Dec 09 14:45
Are you sure that the devices are RTDs and not thermistors?
Running with 5 devices when many motors run with three and many many more run with none is a question for the loss prevention department.
If you have never or almost never had a valid temperature trip you are probably safe using five devices.
If the motor is pushed to the limit, run hot and frequently overloaded the question becomes more complex.
If the RTDs usually show the same temperature (even heating) that's a vote to go with five devices.
If the RTDs show different temperatures, (hot spots) and the RTD that failed was the device that ran the hottest, it becomes more worrysome to run with one less device.
How well is the motor protected by the conventional overloads? Are the conventional overloads set properly or are they set high in the expectation that the RTDs will provide all the needed protection?
What is the estimated cost of repairing the RTD and how does that compare with the cost of a rewind?
Bottom line, for most applications I would run with one device missing.
If the motor is frequently overloaded and abused and overheat trips are common I may decide differently, but without seeing the actual site conditions it depends.

Bill
--------------------
"Why not the best?"
Jimmy Carter

electricpete (Electrical)
4 Dec 09 16:01
Bill has a valid point, the answer can be judged by looking at spread of existing RTD's.  For all of our motors the spread of the 6 RTD's is 10F or less, I wouldn't think twice about losing one.

One thing I have observed, when one RTD goes, some others may follow within a period of several years.  We have one motor missing 3 / 6 right now.  I think the problem is sometimes that very thin wiring like AWG32 is used (to save money?... not sure why) and if not treated with care at various terminations and stress points within the motor, it is easy to break mechanically.  

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electricpete (Electrical)
4 Dec 09 16:04
Maybe the shop guys can comment on causes of open RTD's (in the RTD itself or the wiring?).  I have seen that very flimsy wiring once, but not sure how common that is.

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edison123 (Electrical)
4 Dec 09 19:24
pete - More than the leads, it's the RTD element itself that's often the culprit for open RTD's.

Bill has a valid point about the temperature distribution around the machine when loaded. It's not always the same and the failed RTD could be reading the highest temp.

Having said that, normally 5/6 RTD's is good enough temp protection for the motor.

Muthu
www.edison.co.in

Hoxton (Electrical)
7 Dec 09 8:02
Yes, it is the sensor part which is flimsey.  The greatest risk is during the stator winding operation, when you are driving in the top slot wedge.  That is why the manufacturer fits an additional set (so if you specify one per phase, you will get two).  Then if one fails during manufacture, the whole winding has not to be taken apart.

Thermocouples are much more robust, I am told.
esee135 (Electrical)
12 Dec 09 16:21
In general for large machines, changing a slot RTD risks more damage to the winding than what the RTD is worth.

The suggestions above are good with respect to judging between RTD's.  Alot of 1930's generators have less than 6 RTD's on a 300-inch stator bore.

If you are concerned, you could look into back iron RTD's.  These are relatively inexpensive, but then you have to determine a suitable trip value.

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