Machining laminations and testing for shorts 1

[UKpete]

We have an active magnetic bearing assembly (on a large motor) with a shaft "journal" consisting of a pack of 0.2mm laminations. These laminations are washer shaped, clamped between supporting plates and the whole pack is a shrink-fit on the shaft. It is subsequently machined to maintain concentricity and circularity and give the required airgap.

My questions are:
1. does anyone have any recommendations on how to machine the surface of the lamination pack without shorting them together.
2. having done this machining, what method could be used to test for shorted laminations?
3. when shorted laminations are found, how might they be treated?

I have done a search on Eng-Tips but suprisingly (in view of the fact that repairing rotor rubs must be common practice) the subject doesn't seem to have been covered here before - any input on any of the above questions gratefully received.

 

[aolalde]

UKpete:

I am not familiar with magnetic bearings but general electromagnetism concepts should apply.

Machining the O.D. looks quite problematic since your laminations are waved; control of embedded material looks almost impossible.
Core loop test could be at operating flux density but may be at high frequency? Excitation current through the shaft could work as in rotor’s lamination loop test.
My guess is that the lamination should be stamped to the final O.D. and for repair of shorted laminations you should disassembly the core stack and send the lamination to a site for surface coating ( check sites like;

http://www.etimotors.com/laminations.htmes

and

http://www.tempel.com/products/material/catalog/mech.html

 

[electricpete]

I was thinking core loop test as well... that's what is done for a motor stator or rotor. And observe with infrared camera.

I think in your case you are limited to one turn and the shaft would form the portion of your turn that passes thru the core.

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[mc5w]

You would be better off using splines on the shaft and on the inside of the laminations than to use a shrink fit.

My understanding of magnetic supensions and bearings for kilowatthour meters is that you use permanent magnets and that you do not need laminations. If your magnetic bearings are electromagnets they would be energized from a direct current source and the laminations yould just a be a convenient magnetic material.

 

[UKpete]

Thanks for your replies chaps. I understand about the core test but it may not suit here because what they need is a production test and because it is a large thermal mass it may be slow to heat up.

I have discovered a bit more about the machine, the motor is synchronous and rated at 28MW, 6000rpm (no typing errors!) so it is a large piece of kit and the centrifugal forces are high, so the shaft lamination section for the magnetic bearings is a heavy interference fit. The laminations are mounted on a carrier with a taper bore which in turn is a shrink fit onto the motor shaft at each end. Incidentally, this does mean that we could use multiple turns on a core test before fitting onto the shaft.

The whole set up sounds expensive I know, and aolalde I agree that it ought to be possible to avoid machining the o/d of the laminations (the tolerances on stampings being very good). However I am new to this work and there may be tolerance build-ups and other factors I don't know about; it isn't my call anyway - I am just trying to help with the production testing.

Epete, it hadn't occured to me to use an infra-red camera. I have only seen it done with thermocouples; have you ever seen a camera used for this purpose?

There is one other core test I have discovered during a web-search, the El-CID (Electromagnetic Core Imperfection Test) as used in stator cores of turbo-generators. I could only discover one manufacturer of suitable equipment though:

http://www.adwel.com/PDF/MCT-2.pdf

There are also one or two references to the test:

http://www.irispower.com/techpapers...TH HYDROGENERATOR CORE TESTING AND REPAIR.pdf

http://www.electricity-today.com/et/feb99/elcid.html

One other point, I did read in a text book that damaged laminations could be repaired by low-voltage electrolysis etching with acid - a pad soaked in 20% phosphoric acid is applied to the damaged area; but I've never heard from anyone who has tried this.

 

[Mendit]

I will get you some answers, we do work for SKF (Revolve Magnetic Bearings) and I have many good contacts who will advise. The ones we have done are always ground on the laminations.

We are currently balancing some 35 Kw high speed motors 72,000rpm these are used on high speed spindles for aircraft manufacturing machine tools. The rotor has magnetic bearing iron as well as the induction motor rotor iron. rotor weight is only 11 lbs.

When we balance these items we must never run the machine rollers on the iron. There are special tooling diameters and we run on teflon rod bearings. Balance tolerances are .35gr-mm/plane.

 

[electricpete]

I have seen core loss test several times by infrared and never with thermocouples.

The general procedure allows 30 minutes for warmup and then look for hostpots 10C rise above average core. (in addition to core loss tester criteria on overall core loss watts/pound).

I have a hard time imagining how complete coverage would be obtained with thermocouples. With the infrared camera it is easy to survey every visible square inch of surface area and identify localized hot spots, as well as saving an image to precisely identify their location and severity.

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[UKpete]

electricpete, the tests I saw were based on old practice in an old company. Coincidentally, I have just borrowed a copy of an IEEE book (published 2004) that I can highly recommend if you haven't seen it already:
"Electrical Insulation for Rotating Electrical Machines"
(Greg C. Stone, Edward A. Boulter, Ian Culbert and Hussein Dhirani).
I was interested in the insulation systems but I notice it is also quite comprehensive on lamination insulation and core testing. One other possible test that may be more suited to a production environment would be the core loss test, a simpler version of the standard core test whereby the input power to the excitation coil for a given flux (as measured by a search coil) gives an indication of the condition of the core. This is particularly useful for comparative tests e.g. before and after winding burnouts, and in our case for comparing successive core packs as they are manufactured.

Mendit, many thanks for your reply, those are exceptional balance figures. I must tell you that the company involved in the supply of the magnetic bearings are a possible rival to SKF/Revolve, I don't want to put you in an awkward position on this. In a previous job I did meet some of the guys from Revolve when they visited the UK, fine fellows. I have seen magnetic bearing journals ground on their o/d as you describe; but I'm told that it isn't desirable to grind them wet from a corrosion viewpoint, and when ground dry the surface finish isn't good enough. I don't know that from personal experience though.

 

[Tmoose]

Machining conventional laminated rotor OD with a sharp tool (to avoid smearing the material) is often specified.
We used to grind our spindle motor rotors, and it sure looked nice, but I don't know that we would have been able to tell if the performance was compomised.

 

[edison123]

UKpete,

I recently bought the book by Greg. C. Stone, Ian Culbert, et.al., an excellent book.

However, I found some errors in the equations given under core testing, which I communicated to Ian Culbert. He has accepted my corrections and assured to amend "if another edition of the book is published".

 

[electricpete]

Greg Stone wrote an EPRI document 5036V17 regarding electrical insulation in the early 1990's I think.

Does anyone know if the new book has the same info or new info?

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[Mendit]

OEM suggest don't touch the laminations, if they are damaged have them replaced. Changing the air gap would affect the sensing circuits of the magnetic bearing, and the feed back circuits which allow the bearing controler to keep the rotor in the center of the bearing. How did the laminations become damaged. Manufacturers put a lot of effort into providing protection in the event of damage by installing run down bearings. Did you have a failure, if so what happened.

 

[sreid]

As a thought on "designing out" the possibility of a short vs. "testing out" a short, would it be possible to put a chamfer on the OD of the laminations (possibly by stamping) prior to applying the oxide coating. This would leave an air gap between the laminations where the OD grind or turning is performed.

 

[UKpete]

Edison123, thanks I'll look out for this. That aside (and it isn't a mathematical-based text) I find it a very interesting and informative book. Epete, I haven't seen the EPRI document but it isn't mentioned in any of the references, which are numerous. Incidentally, the book is on Amazon:

http://www.amazon.com/gp/reader/0471445061/ref=sib_dp_pt/102-6593998-7499343#reader-link

hopefully that link works; the US Amazon lets you "search inside this book", unlike the UK Amazon site!

Mendit: I don't think they have ever damaged the laminations during a drop, the touch-down bearings have operated successfully. My original question arose because they wanted a more reliable method of production testing lamination stacks after they had been machined on the o/d.

sreid: I agree, if they were to devise a reliable and consistent method of machining the laminations, it wouldn't be necessary to test them. However, if a rogue piece of machining is allowed to slip through then the resulting costs are high. I will pass on your suggestion thanks, though I'm not sure how much material they need to remove.

Thank you to all for your responses.

 

[sreid]

I like the thermal/temperature rise ideas but would it be possible to "growel" the laminations? Two shorted laminations should cause a "Shorted Secondary" effect. I'm assuming that the ID of the laminations are shorted to the shaft?

 

[UKpete]

Yes the laminations are shorted to the shaft. We did actually try a growler but it was unsuccessful. The growler has to span the shorted laminations in order for a short circuit current to flow, and it wasn't possible to detect this with a steel rule as you would with an armature coil short (in the latter case, the "growl" from the steel rule occurs one coil pitch away from the growler when it spans a slot with a shorted coil in).

Although the growler will cause localized heating at shorted laminations, it is a long process - you have to go around slowly to give any shorted lams time to heat up. So it's quicker to do a standard core test.

Maybe an adequate assessment of the surface condition after machining can simply be made using a microscope.

 

[Mendit]

After discussions about your problem, it would appear that your packs have become delaminated (hence the wavy look) I am told that the packs are heat shrunk on to the sleeve, compressed approx 20 tons then the end restraint is fitted, some times welded. The packs are then finish ground in the bore with approx 0.0003"-0.0005" Int/inch of diameter. After shrinking on the shaft they are finish ground. Shorting of the iron is tested for using a bently probe and an oscilloscope, much the same way you would test a vibration probe sensing surface on a shaft.

 

[UKpete]

Mendit, thanks for your reply but you have lost me a bit there. I have used Bently probes for vibration measurement but do you mean a non-contact probe, and if so, is it an eddy current sensor?

 

[Mendit]

Yes, they use a Bently 7200 series probe and scan the surface while rotating the part. The probe will detect problems with the surface. It won't tell you what is wrong only that there is a change in the magnetic properties of the iron.
Support your rotor preferable on "Vee Blocks", rotate the part slowly and evenly while observing the trace on the scope screen. You are looking for consistency, variance is a sign of problem.
The probe will also show you if there are changes in the surface properties. Just like a probe sensing surface on the shafts. Burnished (smooth) surfaces give best results, or lapped (matte) with an oscillating lapping tool.

 

[UKpete]

Thanks for your help Mendit, I'll look into that.