Repairing damaged rotor bars

[arcflash99]

Does anyone have expirience with repairing rotor bars (cracked/ broken) in large motors?

I am asking out of curiosity what to do when a motor has been diagnosed with damaged bars.

Do you re-bar it?

Can you have another rotor built ahead of time (assuming you have sufficeint motor data)?

Do you just purchase another motor?

I understand the answers will vary from motor to motor, application to application. Let's just say this is a 350 HP 3600 RPM 2300 volt motor installed on a vertical pump, there is an identical spare motor/ pump for this process.

 

[oftenlost]

Sad as it might be, rebarring a motor of this size is usually not cost effective. In the case of extruded aluminum or copper alloy bars, one can sometimes patch successfully but for the majority of motors in this horsepower range which have cast rotors, the proper answer is to replace the motor. On a positive note, vertical motors are usually attached to pumps and seldom lose rotor bars unless started too often or lose a bearing.

 

[Skogsgurra]

Rotor bar damages are often diagnosed when there are other problems. Modern diagnosis tools will show sidebands indicating rotor bar problems when there are other phenomena like unstable flow (in a pump application) or other load variation. The best test is to install the spare motor and run the test again. If that one also has "rotor bar problems", then you may need to evaluate your testing methods.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[arcflash99]

My diagnosis was with an MCEmax online motor tester. The sidebands are right where where a strobe indicated the speed was. Also, there are other factors in the analysis indicating damaged rotor bar(s). The mechanical guys aren't having any vib issues with the motor so I suspect there isn't enough load (78%) to create rotor heating and produce vibrations.

When we run the sister pump there are no detected sidebands.

 

[oftenlost]

My first step would be to listen to the motor under load. Open rotor bars frequently make a growling noise. The rotors also have ring fire but most verticals cannot have their airgap witnessed during operation. I would then look at the loaded vibratory pattern on a FFT analyser such as the CSI 2120 or equivalent and look for double slip frequency sidebands and harmonics through at least the fourth. Then I would shut the motor down, energize two phases with 230 or 460volts, monitor current while the rotor is rotated manually. Deviation in excitation current usually indicate rotor bar problems.

 

[arcflash99]

The MCEmax (manufactured by PdMA) analyzes the motor's current signature analysis through an FFT. At the 5th harmonic there are several peaks just before and up to the 5th harmonic all separated by the slip frequency. This signature is a damaged rotor signature. I say this because I can have sidebands very close to the line freq that look like rotor issues and not have peaks at the 5th harmonic.


The application of 460 volts on two phases is a new one to me, but, I understand the principle behind it. That sounds old school to me.

 

[Skogsgurra]

Old school often means experience. New school often equals lack of experience.

Seen many unnecessary motor changes due to "sophisticated" measurement techniques. Testing for current variation with low voltage and turning motor slowly is a very reliable technique that I highly recommend.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[arcflash99]

This technique is equivalent to the MCEmax offline Rotor Influence Check (RIC) The tester is conected to the T-Leads and it takes a snapshot of the rotor's influence to the stator. Then the rotor is turned 5 degrees and another snapshot is taken. This continues for 90 degrees and up to 360 degrees.

Same priciple as the 460 volt application to the windings and turning the rotor.

Would you have 4 times more current when applying 480 volts to a 2300 volt motor?

One day all the whiz bang stuff I do today will be "old school" to some kid in the future.


It sounds like I got the answer I thought I was going to get. Buy a new motor.

 

[stardelta]

Dont assume you have to buy a new motor, you may be able to obtain a replacement rotor assuming its faulty and then its a case of swapping them out. Just done a similar thing with a 500hp Schorch motor but we waited 10 months! for them to deliver it so check with the manufacturers first. I would reccomend you consult a reputable repair company, this sort of job is routine.

 

[edison123]

I agree with stardelta. Unless, it is cast aluminum bars, we routinely 'rebar' rotors (copper/brass/bronze) up to 4 MW at a fraction of new rotor/motor cost.

Cast aluminum rotors are not worth repair.

The single phase test is good at detecting cracked bars.

 

[aolalde]

On a vertical pump application the normal requirement is low starting torque. This will result in low probability for fractured rotor bars.
The "New School Techniques" will give you a frequency response to uneven current in the rotor circuit. But that variation could be due to cavities on one short circuit rotor ring (not necessarily fractured bars). The rotor is not perfect but still could handle your pump for several years.
Under full load, check the shaft speed, currents, motor temperature rise and vibration. Compare to the nameplate data, standards and to the sister motor, before attempting a final action.

 

[petronila]

Hello arc flash 99, The answer for your question is depends, . If you have a stand by motor you most act quickly sending to repair the motor or replacing with a new one.
The rebaring is a good choise only if the repair cost vs replacement is atractive,but you most take account the delivery time and compare with the new one motor delivery. WHEN YOU TEST WITH THE "OLD SCHOOL" TECHNIQUES like The ROTOR TEST WITH MOTOR ASSAMBLY the current deviation should be less than 3% for a good rotor. If the current varies more than 3%, note the position where it varies and repeat the test. If it still varies at the same position, that confirms that a rotor fault is causing the variation. There is sometimes a small amount of variation, each time the rotor is turned, so the test is a little bit subjective.This works. Another "OLD SCHOOL TECHNIQUES" is to test the rotor with a growler.Take account also
For the aluminium cages rebaring , The best procedure is to use a copper alloy that gives the same resistivity as aluminum. An M42 bronze (98.5% copper, 1.5% tin) would be perfect. the closest alloy you can obtain to the 98.5% copper, the closer the motor torque/performance should be to the original aluminum rotor.
Use the same alloy for the endrings, and make them to match the cross-sectional area of the aluminum endrings. If you used pur copper for the endrings, they should be about 70% of the cross-sectional area as the original aluminum endrings..

You will also have to put internal fan blades, like the die-cast aluminum rotor endrings had. You can weld fan blades onto the endrings, or install an internal fan on both ends of the shaft. One good method is to mill radial slots in the endrings, and fit the fan blades to the slots, then braze them.

Regards

Petronila

 

[Skogsgurra]

Cu vs Al - there will be some balancing issues as well.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[arcflash99]

So, using the "OLD SCHOOL" method, how do you tell the difference between a rotor defect and bad bearings, bowed rotor, rotor not true to the stator bore?

If there is a devitation in the air gap between the rotor and the stator there will be a varying magentic influence between the rotor and the stator as the rotor is turned across the pole of the stator.

Also, can this be applied to a 480 volt motor using 120 volts?

 

[oftenlost]

It is possible that a severely bent rotor or eccentric core could affect the reading as the magnetic coupling varies as to the square of air gap but the current fluctuations would occur over a wider arc, rising more gradually and falling the same way. One thing that I beleive has not been mentioned is that a rotor with bad bars or endrings often show up in loaded operation. Sometimes, an analog clamp-on ammeter or the analog meter on the sub will bounce as the bad bar or bars pass. Bad bearings would have no effect unless the balls or rollers were extremely worn, allowing displacement of the rotor. You would feel that while turning. As to what voltage to use, it does not make a lot of difference although I prefer to get above 50% of nameplate. Most motors of this type are code F or G which means locked rotor of 5.2 to 6 X KVA or so. I would expect that 120 volts applied single phase to a 350 HP motor to give me approximately nameplate amps of 380 or so. Most 120 volt services are not rated that high. In a shop environment, this would not be a problem.

 

[electricpete]

There are three factors which suggest to me that rotor bar will have much bigger effect on current during single-phase test than airgap eccentricity:

1 - Trying to analyse the single phase test using the equivalent circuit which has two parallel branches: one through the magnetizing impedance and one through the rotor. There is locked rotor condition, so the term R2/s is very low and the majority of the current goes through the rotor branch - similar to starting condition except voltage is lower and extremely unbalancec (single-phase). Changing the airgap changes the exiting branch which is the minority of the current - don't expect a big effect. Rotor bar open circuit affects resistance through the rotor branch which is the majority of the current.
2 - The influence of displaced rotor on the exciting reluctance seen by a coil in the same phase 180 mechanical degrees opposite is as a first approximation equal/opposite and would cancel out.
3 - As was pointed out above, the influence of eccentricity affects a much wider arc than the influence of one or two adjacent rotor bars. To the extent that the arc of influence spans accross two adjacent poles, the variation within a given phase will be masked.

Winding configuration (series or parallel) will affect the above conclusions quantitatively but not qualitatively.

Of course we take this test together with all the other information available to help diagnose the problem.

I don't see a defective bearing causing dynamic eccentricity. Also a motor which exhibits dynamic eccentricity in operation probably won't exhibit dynamic eccentricity during slow rotation of the single phase test unless it has a static bow which should be evident from TIR of the shaft extension or other points if accessible.

One tell-tale sign of rotor bar problem would be higher slip than expected for a given load. Also increase in vibration over time after startup. Of course this could also be a rotor thermal bow for other reasons.

Rotor problems are among the toughest to diagnose and I would beware of any firm rules (especially anything I mentioned!). Not all the motors read the textbooks and charts about what they're "supposed" to do. The one I get a kick out of is the Technical Associates chart (often quoted and repeated by others) which shows RBPF +/- 2*LF pattern as symptom of rotor bar problem. (Of course anyone who has been monitoriong motors for any time knows this shows up on many healthy motors.) So keep an open mind, ask lots of questions like you're doing, and listen carefully to folks like oftenlost, edison, aolalde that have obviously worked with motors a lot.

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[electricpete]

Correction:
So keep an open mind, ask lots of questions like you're doing, and listen carefully to folks like oftenlost, edison, aolalde and skogsgurra that have obviously worked with motors a lot.

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