Cracked Rotor End Ring 2

[rockman7892]

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.

 

[rockman7892]

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?

 

[waross]

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

 

[edison123]

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.

 

[oftenlost]

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?

 

[edison123]

ankerwik

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

 

[ankervik]

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?

 

[edison123]

Ok. Thanks, ankervik.

 

[electricpete]

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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[electricpete]

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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[rockman7892]

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.

 

[LionelHutz]

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.

 

[oftenlost]

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.

 

[rockman7892]

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.

 

[electricpete]

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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[rockman7892]

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?

 

[edison123]

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

 

[rockman7892]

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?

 

[rockman7892]

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?

 

[oftenlost]

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.

 

[edison123]

oftenlost - lps.

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