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Cracked Rotor End Ring
2

Cracked Rotor End Ring

Cracked Rotor End Ring

(OP)

I wanted to share some pictures of the rotor we recently has removed from a 1250hp 4.16kV motor.  This motor was sent away to a motor shop for scheduled inspection and it was found that it had a cracked rotor end ring.  This is a new one to me so I found these pics quite interesing.  

Was curious to see if anyone else ever seen this before and what the causes were.  

I'm guessing that we should have seen some electrical losses associated with this crack.

RE: Cracked Rotor End Ring

Looks like a crack in the copper casting. Did this reveal while in service like swinging stator current or did you find out only on the routine inspection ?

RE: Cracked Rotor End Ring

I will offer a guess.  As we know the most common source of stress for rotors is differential expansion during the rapid heating during motor starting.

Through repeated thermal cycling, the position of one or more bars can change relative to the core... a process called thermal ratcheting.  To imagine it, remember that copper and iron have different coefficients of thermal expansion (copper moresor than iron) on top of which the copper becomes hotter than the iron.  So the copper within the core expans more than the associated core.  We can imagine that perhaps at one single location (center of the core) there is not relative motion between copper and core but on both sides the copper grows outward relative to core.  Likewise during contraction one location has not relative motion and the rest have relative motion.  If the location of no relative motion is different during expansion and contraction then at the end of the process we have movement (or at least stress).  The swaging pattern of the bars has a profound effect on this phenomenon.

The crack shown here is directly above a bar.  It looks to me as if the bar directly under the crack has migrated more toward this end over repeated cycle.  Now when the rotor heats, there is a relative push upwards by this bar and downwards by it's two neighbors.

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RE: Cracked Rotor End Ring

If you do a runout check it might shed some light on my "guess".

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RE: Cracked Rotor End Ring

I mean of course runout which indicating in the axial direciton on the face (vs rim) of the endring.  Would probably require removing that fan to get access.

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RE: Cracked Rotor End Ring

(OP)

This crack was found during a normal inspection.  There really wasn't any swinging stator current that was noticed and when I asked production if there was any unusual current increase they said it appeared to be drawing a higher current lately but its hard to say exactly because this motor is on a fan controlled by dampers and obviously damper position plays a role in the current changing.  Would there be such a noticiable current swing with a crack like this.

Electricpete it sound like what you are alluding to is the fact that this damage came about as a result of excess heating of the rotor possibly due to excess starting or starting to frequently.

 

RE: Cracked Rotor End Ring

As well as I can remember, the fan usually has a surface that lays flat against that endring to assist heat transfer from end ring to fan, different than what is shown in your photo.  Is the photo shown the as-found configuration?  Does this look normal to you guys?   

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RE: Cracked Rotor End Ring

Quote:

Electricpete it sound like what you are alluding to is the fact that this damage came about as a result of excess heating of the rotor possibly due to excess starting or starting to frequently
Yes.  

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RE: Cracked Rotor End Ring

(OP)
electricpete

yes this was the as-found configuration.

RE: Cracked Rotor End Ring

Few years ago I came across same kind of problem on 1800 KW, 11KV motor. It happened during running. Suddenly there was huge variation in current and motor became noisier. We got that repaired but manufacturer of motor suggested that whole end ring should be replaced.  

RE: Cracked Rotor End Ring

Frequent starting is a major cause of induction rotor end ring failure.
JIM
  

RE: Cracked Rotor End Ring

Were any of the rotor bars found to be loose, cracked or showing signs of local heating/arcing? Is the rotor a double cage type? it is difficult to ascertain from the photos.  

RE: Cracked Rotor End Ring

Has this rotor been repaired before?

Some repair shops have a practice of shortening the bar overhang when doing rotor repairs and this can cause this tye of problem.
Look for transients during start, i.e. open transition star delta (wye delta) starter etc.

High inertia loads require special rotors with a high thermal inertia. It is possible to have rotor damage from too high a load inertia.

Best regards,
Mark

Mark Empson
L M Photonics Ltd

RE: Cracked Rotor End Ring

This looks like a GE custom 8000 frame rotor.  you say it is on a fan.  how long does it take to get up to speed? Is there a chance that this machine is an extended start machine which could have been energized repeatedly?  If it is a GE, the endring is not cast  but milled and tig or mig welded before final machining.

RE: Cracked Rotor End Ring

(OP)

I'll try to answer some of the questions above but I'm off site and dont have all the information in front of me.  I can get more later.


I belive the motor is a single cage type.  I dont belive any of the rotor bars were damaged or loose.

From what I am told this motor was re-wound some time ago in order to increase the hp of the motor.  I'm not exactly sure how it was re-wound and am only told that it was re-wound at some point.

The few times I have seen this motor start, I did not notice any starting transients.  How would this crack cause such transients?

I'm not sure what the inertia of the fan is.  I can pull the fan data when I get back and look for this information.  If my memory serves correct I do believe it had somewhat of a high inertia.

This motor is a GE 8000 custom fram motor.

The motor does have an extended starting time.  From the few times I've seen it start I noticed that the acceleration time is around 32 seconds.  When looking in the multiline 269 relay I noticed that a custom curve was programmed for this relay to account for this long starting time.  I want to find motor datasheet to see where the rotor damage curve falls in relation to this curve.

There was an incident recently where we had a hard time getting this motor started and there were several repeated attemps to start this motor.   

RE: Cracked Rotor End Ring

Quote:

There was an incident recently where we had a hard time getting this motor started and there were several repeated attemps to start this motor.   
It sounds like a safe bet that it was related.  Either the degradation had begun and was degrading starting torque, or else the event of excess start attempts/duration contributed to the degradation.

30 second start is very long for most of the DOL equipment I work with (severe start).

Is this a DOL start or soft start or vfd?

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RE: Cracked Rotor End Ring

(OP)
This motor is a DOL start.  It is a critical process fan so once it starts it usually run continusously and is never shutdown.  It is only usually only shutdown bi-annally for shutdowns or when the motor trips on overload which rarely happens.  So this motor is not started very often but like I mentioned we had some troulbe getting it started last time it was down.

How would this crack degrade the starting torque as you mentioned and lead to starting difficulties?

RE: Cracked Rotor End Ring

Yes, broken bars and rings will make the motor start slower.  The relevant quote from ankervik's link would be:

[quote Nailen]Second, reduction in full current flow throughout the cage will reduce motor output torque as well. The motor probably won't stall under normal load, but may be unable to accelerate, or take much longer than usual to start (which can hasten further deterioration by adding to rotor heating during each start). [quote]

Here's my two cents on the subject.

As a first approximation, we consider it to resemble an increase in rotor resistance.  If you look in a textbook at varying rotor resistance for wound rotor motors, you'll see the effect of changing rotor resistance on torque speed characteristic: it slides the maximum torque toward lower speeds and decreases torque performance at all speeds above the max-torque speed and increases torque at all speeds below max-torque speed.

But broken bar is a little more than an increase in rotor resistance.  It creates an unbalance.  We can model the unbalance as the sum of forward rotating field and backward rotating field.  The backward rotating field draws current and causes voltage drop with creating useful work and in fact can create negative torque (assuming the rotor unbalance creates a stator current unbalance so we have backward rotating field on both) which decreases the net positive torque.

In the olden days it was one of the common things to look for when looking for rotor bar problems:  increased starting time and oscillation on the current meter.  Now we have more sophisticated predictive maintenance tools and we don't rely on increased starting time as a primary indication of broken bar or ring.

You mentioned 30 second DOL start and infrequent starting.  Does the thermal damage curve really allow you to go that long?

Here's my frame of reference.   We have 30 motors > 2500hp,  70 motors 300 – 1250hp, and around 1000 motors 5 – 250 hp.   None of them takes 30 seconds for DOL start so by my standards it is a long start.  Of course large motors are custom-designed, so in theory the rotor is matched to the task.   We only have two families of motors that take more than 5 seconds to start.  We have one 250 hp that starts in around 17 – 22 seconds.  It is a standard motor, operated very close to manufacturer's limit.  We have one 9000 hp that starts around 22 seconds (has to accelerate massive flywheel).  That particular rotor has a very thick copper end ring (lots of thermal inertia as well as reduced electrical resistance) reinforced by a steel end ring for strength.  

I believe for large motors this long type of start will require a very unusual rotor such as the one I mentioned.  Your copper end ring doesn't look large relative to the size of the bars.  There is no reinforcement.  

My gut feel is that your motor is certainly underdesigned for the application as others suggested.   The application includes: inertia to be accelerated, load torque during start, voltage at motor terminals during start, and frequency of starting.

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RE: Cracked Rotor End Ring

(OP)

Interisting article that ankervik posted.  I didn't realize there were all of those methods for testing rotor damage.  I also wasn't aware that such small defects such as hairline fractures in rotor bars could possibly have a signifigant effect on a motor.  I see now what others have mentioned in regard to swining stator current or current transients due to the damaged bar passing each current phase.  I would suspect that if such a transient was present you would see it alternate between phases as the damaged part of the rotor sequentially passes each phase's stator coil

Pete, I agree with you that this 30s starting time is long.  I dont have the load data (fan) in front of me but will be able to check it tomorrow.  Like I mentioned I was told that this motor was re-wound at some point in the past, so I'm not sure how easily I will be able to find the thermal damage curve curve for this motor showing the thermal limits for the rotor, and the acceleration time.  I wouldn't think that re-winding a motor would have much if any effect on the rotor thermal limits?  I would think that they would stay the same since the rotor is the same. Maybe the acceleration time would change as a result of a re-wind.  Does the motor shop typically supply a new motor datasheet after rewinding based upon test data?

I want to look at the damage curve to see if the relay curve is really protecting the rotor. I suspect it was possible that a custom curve was put into the relay because of a long acceleration time at some point in the past.  This long acc time may have been due to rotor damage and thus allowing this long acc time only heated the rotor more and worsened the problem.

While this motor is being repaired we are going to put our spare motor into service which is also a re-wound motor of the same type from what I'm told.  Usually production will just wire this motor up without thinking twice about the possbile different characteristics of this other motor.  Although the motor is supposed to be re-wound to match the existing motor I suspect there may be some slight differences that need to be looked at.  Although I'm sure the rotor type is the same and possibly the rotor damage curve the same there may be other differences such as LRC, acc time, etc.  I have a feeling it will be difficult to find the motor datasheet for this spare motor as well.  

What slight differences should you look for when replacing a motor with a "similar or identical" motor?

 

RE: Cracked Rotor End Ring

If the motor was rewound because of a burnout, it is possible that the burnout also did undetected damage to the rotor.
An inspection of the rotor may be in order.

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

RE: Cracked Rotor End Ring

Normally winding shops don't tinker with original winding design parameters. You could ask the winding shop for the original and rewind designs for comparison.

RE: Cracked Rotor End Ring

What am I missing here. If you rewound the stator for a higher horsepower and wound up with a higher locked rotor amps and kept the same rotor, why would you be that interested in the original thermal curves?  I would be surprised if the original thermal limit would have been more than 27 seconds and by pumping more current into the rotor by the upgrade, this should shorten that time, should it not?

RE: Cracked Rotor End Ring

ankerwik

In that Richard Nailen article in EA, the figures/diagrams he cites are not seen. Are you able to see them ?

RE: Cracked Rotor End Ring

Edison123
Sadly they only released the bare-bones text, diagrams & figures are not shown. If you need the complete article perhaps you could try ordering a back-issue from Barks?

RE: Cracked Rotor End Ring

Ok. Thanks, ankervik.

RE: Cracked Rotor End Ring

E.A. Magazine is of course available for free to motor professionals who request it.

I have most of the old hardcopies in my garage, but sadly only back to 2000 (that article was 1998).
 

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RE: Cracked Rotor End Ring

I should clarify - you can request a subscription for free... they send you the current issues as they come out.  Back issues are not available for free.

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RE: Cracked Rotor End Ring

(OP)

I was able to find the datasheet for the origonal motor before it was re-wound.  It actually was origonally a 1250hp motor but was re-wound to a 1500hp motor.  The spare motor we have in storage and are going to install was also a 1250hpm motor rewound to a 1500hp. I have attached this datasheet

We happen to be building a second plant On-Site here that is identical to the first and therefore have the same exact fan.  I was able to find the load data for this fan as well as the motor datasheet for the 1500hp motor that will be used on this fan.  I have attached both the 1500hp motor datasheet and fan data curve.

There was on thing in particular that jumped out at me when comparing the datasheet from the origonal 1250hp rewound motor and the new 1500hp motor that was designed for the application, and it was the Locked rotor time for each motor.  The orig 1250hp motor shows a max LR time of 25sec while the new motor shows one of 40s.  The old motor does not give an acceleration time however the new motor lists an acceleration time of 30s.  So the 32s starting time I am seeing is not that far off.

From seeing this I am theorizing this possibility.  The origonal motor at 1250hp has a lesser acceleration time and therfore the rotor was designed for a LR time of 25s.  If the motor was then re-wound therefore somehow causing the acceleration time to become longer however the rotor was kept the same it is possible that with this new acceleration time we are above the origonal 25s LR time of the rotor.  The new motor datasheet tells me that rotor on that motor was designed for the long acceleration time and therefore has a longer acceptable LR time.  Does this make sense as a possibility?

You can see from the fan loading curve that the inertia of the fan is 97746 lb-ft^2.  I'm not sure if this is classified as a high inertia or not although I guess it would be.  

RE: Cracked Rotor End Ring

I see 2 cracks. It also appears the cracks extend into the laminations.

Both data sheets show similar locked rotor, pull-up and breakdown torque percentages so I'd expect similar speed/torque curves with the re-wind just being more torque and more current. So, the re-wind would have decreased the starting time by increasing the stator current and therefore the rotor current.

It it is direct coupled that is a high inertia. I'd also expect the damage could be from attempting multiple starts. It could also just be from the starting stress.

 

RE: Cracked Rotor End Ring

i guess that i have had too many bourbons and branch in my life because I remain confused.  I missed the lock rotor time by 2 seconds and I am sorry.  We are looking at curves for a 1250 hp which has had magic HP increasing serum dispensed by some unknown someone and now everyone forgets that this is a 1250 HP in all respects except for maybe increase in wire size or chord change.  It is possible that the turns have changed but there is only so much room......
To look at the the acceleration curve of a 1250 accelerating a 1250 load and using the time of a larger machine to try to predict the life of the smaller unit does not make sense to me at all.  The LRA time is a limit, not a capability.

RE: Cracked Rotor End Ring

(OP)

The one thing that I see being a problem is that the old motor has a LR time of only 25s and the new motor has a LR time of 40s.  This new motor I would think obviously has a rotor that can withstand longer starters and longer LR times for a reason.  Dont you guys agree?

Do you guys agree that there might be an issue with the re-wound motor that the rotor is being subjected to longer starring times than it is designed for, or that the LR time of the rotor is too low and is being exceeded.

I guess the only way to verify this is the get the thermal damage curve for the rewound replacment motor, or else we are just asking for the same problem to occur.

 

RE: Cracked Rotor End Ring

I have never been involved in a motor uprate.  To my understanding there are 3 levels, in order of increasing effort/expense:

1 - Keep flux density the same.  Increase conductor loading by use of advanced insulation materials which are thinner (more copper, less thermal insulation) and higher temperature ratings.   I don't think this would result in any change in starting current or torque.

2 - Increase flux density (if there was margin in original design) by reducing number of series turns per path from line to neutral.  This could certainly increase both starting current (lower stator leakage reactance) and torque.

3 - Replace rotor.   I would think this is cost prohibitive and we might consider a brand new motor before this.

The question I don't know is which approach has been taken.   I notice the second page says "brand new motor designed for same application".  Does that mean completely new motor or uprated motor.  Also on 2nd page the rated torque is higher and torques (as percentage) are also higher.  And then it looks like motor rotor WK^2 is higher on 2nd page than on first ... was the rotor actually replaced ?
 

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RE: Cracked Rotor End Ring

(OP)

Pete

Let me clarify which motors I know the way I presented it may be confusing.

There are three motors:

The first motor I'm talking about was origonally a 1250hp motor and the datasheet for this motor was in the attachment.  Some time ago this motor was re-wound to a 1500hp motor (not sure why).  This is the motor for which we have found the cracked end rings.  I do not have a datasheet for the rewound state of this motor.

The second motor is an identical situation to the first one.  Because we keep a spare of each motor on-site this was another 1250hp motor that was re-wound to a 1500hp when the origonal motor was re-wound.  This motor was obviously re-wound to 1500hp to match the one in service.  This is now the one we are going to put in service.  I do not have any datasheet for this motor.

The third motor is a brand new 1500hp motor that was designed as a 1500hp motor.  Because we are building a brand new identical plant, we have the exact same fan application however obviosuly since this is new they ordered the appropriate size 1500hp motor.  This is the second datasheet that I attached.  So this motor is a new 1500hp motor designed for the fan application and was designed properly.  

So as you can see for some reason either the origonal fan was upgraded or it was decided that the orignal motor was not big enough for the fan and therefore had to be upgraded to a 1500hp motor.  

I am thinking that both the origonal motor that is cracked and the spare motor never had their rotors changed when they were rewound, so they have the same rotor ratings as listed on the 1250hp datasheet.  I am comparing this rotor rating to the one for the 1500hp motor designed for the same fan application in the new plant.  I am questioning weather or not if these two origonal motors (cracked one and spare) have the origonal rotors that are not thermally rated for this application.  I am thinking it would be worthwhile to try to find the thermal data for the origonal motor and find out if the same rotor was kept for the cracked motor and spare.  What do you think?

RE: Cracked Rotor End Ring

Do you see dimensional differences (in stator & rotor) between the first two originally 1250 HP motors and the third 1500 HP one ?

RE: Cracked Rotor End Ring

(OP)

Ok I went ahead and plotted the motor and relay curve for the rewound 1250hp to 1500hp motor.  I am assuming that the rewound motor kept the same LRC of 650% as listed on the datasheet and now has a FLA of 204A as shown on the new nameplate.

With that being said and assuming that the existing rotor was never changed on the rewound motors I plotted the Locked rotor time point on the graph as 25s as indicated on the datasheet.  You can see from the attached plot that this rotor damage point at 25s falls below the overload curve used for this motor.  This custom overload curve has been there since before my time so I'm not 100% sure why this curve was used.  I'm guessing it has something to do with the starting time.

So looking at this plot it would appear that if the origonal rotor was left in, it is not being protected adequately by the overload curve.  Its not to say that we bump into this point during our starts, but it is a possibility since the relay appears not to be protecting it.  

Do you agree that I am not adequately protecting the rotor of these rewound motors and that we are setting ourselves up for more problems with these rewound motors at this relay setting?

RE: Cracked Rotor End Ring

(OP)

Actually looking at this overload curve again it appears that it may not protect the brand new 1500hp motor with the rotor rated for LR time of 40s.  It appears to be right on 40s and I dont think it would adequately protect rotor from possible thermal damage.

Do you guys agree that my curve needs to be shifted down for the portion around LRC?

RE: Cracked Rotor End Ring

Rockman
This is an aluminum bar, aluminum endring custom 8000 fabricated rotor of a design still being utilized by GEVISA in Campinas, Brazil.  It appears to be about an 8411 or so frame.  Much is known about the thermal performance and endurance of these machines.  Unfortunately, much is not known by me.  A potential source for information and help might be someone like Ed Hartung who wrote an IEEE paper on the subject in 1994 or so.  before he retired from GE Motors, he was the senior engineer for that platform and I think he is still consulting in Fort Wayne.  It might pay to try to get the thermal data looked at before repair and reinstallation.  I fear that you are going to cook the second rotor too.

RE: Cracked Rotor End Ring

oftenlost - lps.

Aluminium rotors for such large capacity ? I agree that overloading that motor will cook the rotor.
 

RE: Cracked Rotor End Ring

Here is a link to an interesting article:
http://tinyurl.com/aurr5l
They talk about 4 types of rotor:  copper fab, copper cast, aluminum fab, aluminum cast.  They mention aluminum cast might be used up to around 1750HP.  We have a 500 hp aluminum fabricated rotor.

How can you tell it is an aluminum conductor rotor?  From color of bars where paint worn off, or some other feature?
 

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RE: Cracked Rotor End Ring

(OP)

Oftenlost

I agree with what you said about having the thermal data reviewed.  Since I only have the origonal motor datasheet would that be enough to give to a consultant for a review?  What other information besides load data would I need to put together to send off?  Do I need the thermal data or datasheet for the re-wound motors?

 

RE: Cracked Rotor End Ring

Rockman
This rotor is a standard design where the bars are extruded and include a sacrifice tab at the bottom of the bar that is milled off during the insertion process to insure a tight fit in the rotor slots. This prevents bar vibration which leads to cracked bars over time.  The bars are inserted and a milled end ring is installed over the bar ends and mig welded to attach the bars and fill out the end ring which is then machined to fit.  I have personally seen them in motors exceeding 5000 HP but do not know about an upper limit as most of the motors larger than 5000 hp that I have worked on were synchronous.  Motors up to 10,000 HP of so of this type are still being manufactured in the GEVISA medium horsepower factory in Brazil and just last year, had bars and endrings on the shelf to rebar a Schenectady designed motor similar to the rotor in the picture.  The right consultant can tell much from the model and serial number.

RE: Cracked Rotor End Ring

Pete
The reason that I believe it is aluminum is that 1. Aluminum is standard in this size 2. The bar tops are standard shape for aluminum bars but uncommon in copper bars which are normally rounded or square. 3. The welded end ring imperfections can be seen in the picture.  In fact, the weld appears to have holes caused by gaps in the overlay which suggest a possible previous repair.

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