Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.


Should I replace damaged stator RTD in 350hp motor

Should I replace damaged stator RTD in 350hp motor

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?

RE: Should I replace damaged stator RTD in 350hp motor

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.  


RE: Should I replace damaged stator RTD in 350hp motor

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.

"Why not the best?"
Jimmy Carter

RE: Should I replace damaged stator RTD in 350hp motor

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.  

Eng-tips forums: The best place on the web for engineering discussions.

RE: Should I replace damaged stator RTD in 350hp motor

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.

Eng-tips forums: The best place on the web for engineering discussions.

RE: Should I replace damaged stator RTD in 350hp motor

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.


RE: Should I replace damaged stator RTD in 350hp motor

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.

RE: Should I replace damaged stator RTD in 350hp motor

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close