Bolting laminates together and its effect on eddy currents. 1

[Intermesher]

The intention is to have numerous bolt-holes located at a constant radius around the ring of the laminates in a stator. Then bolts will be inserted in these holes to hold the laminates together and to attach the stator to the supporting structure.

My concerns are;
~ Will steel bolts detrimentally affect the laminates ability to minimize eddy current?
~ Alternatively, will inserting the steel bolts in a sleeve and then inserting this assembly into the holes meaningfully minimize the eddy currents?
~ Alternatively, will the use of titanium bolts meaningfully minimize the eddy currents?
~ Is there a better suggestion?

Thank you

 

[electricpete]

Certainly if you put the bolts in a sleeve and also use insulating washers at the ends, you have eliminated all concerns.

When I came from transformer world into motor world, I was surprised to find out that the motor laminations generally are shorted together by various supports around their outer periphery. It is not a problem because there is no shorted-loop path which flux can flow through (would only be a problem if you added shorts near the bore of the stator). The flux-intercepting area of the possible loop path is generally minimized if your bolts are close to the outer periphery and the length of the stator is short.

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(2B)+(2B)' ?

 

[electricpete]

you have eliminated all concerns.

I meant to say all concerns assocaited with circulating currents. Of course drilling a hole in the stator laminations may have some tiny effect on magnetic performance.

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(2B)+(2B)' ?

 

[electricpete]

Actually "drilling a hole" would create big potential for burrs which is a problem in itself. Probably you are going to have to holes stamped during the lamination manufacture processes?

Maybe you can tell the big picture of the size of the motor and whether it is a new motor or addressing problem on an existing motor?

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(2B)+(2B)' ?

 

[electricpete]

To circle back to original questions: here's my input fwiw:

~ Will steel bolts detrimentally affect the laminates ability to minimize eddy current?
Yes, it can.
~ Alternatively, will inserting the steel bolts in a sleeve and then inserting this assembly into the holes meaningfully minimize the eddy currents?
Yes. I think you also need some form of insulation (washers) at the ends.
~ Alternatively, will the use of titanium bolts meaningfully minimize the eddy currents?
Haven't heard of this.
~ Is there a better suggestion?
If you put the bolts around the outside of the stator, they do not cause circulating current problems.

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(2B)+(2B)' ?

 

[wolf39]

Intermesher:

Pete answered your question pretty well.

Wolf

http://www.hydropower-consult.com

 

[Intermesher]

Pete, thanks for the time and answers that you have provided.


The following is an answer to your question and an elaboration on the query of this thread.

Yes, the intention will be to remove all burrs.

The following is a picture of a purchased stator for an outrunner PM motor.

http://www.unicopter.com/1910_FP_1.jpg

It has bolt-holes that are filled with plastic. The motor under development is for a much different application and its stator is to be extremely modified so as to extract significantly more power.

Re insulating washers:
The final motor will be air-cooled. The bolts are to attach a portion of the aluminum housing directly to the outer stators at each side of the stator stack. The hope is that these two [aluminum/laminate] cemented joints will extract heat from the laminates, but the bolts will only transmit a minimal amount of flux between these two outer laminates.

Dave

 

[waross]

If the diameter of the bolts is significant compared to the magnetic path you may have eddy currents induced in the bolts.
I would use stainless bolts or cadmium bronze bolts. Still with an insulating sleeve and washer.
A sketch of the magnetic portion of the stator with the hole locations would help.


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

 

[jpts]

You should leave those "holes" open on the outer periphery side (basically U-shape slots where you place the bars) and have the bars non-magnetic (stainless steel). In that case you have no flux linking the bars-->no eddy current problems in bars (and that should not affect in the laminations either).

If you make real holes to stator sheets you will have some eddy currents in bars even with insulating sleeve and non-magnetic bars. Insulating the bar from stator stack helps only that on stator end, the current from one bar cannot go the next and all around the full periphery. In that case the consequencies would be quite catasthropic.

My recommendation would be stainless steel bars in U-shape slots on the outer periphery. In that case no insulating sleeves are needed. This is quite common way to do. I know that on the other hand, insulated bars are also quite often used with closed holes on stator sheet (especially with large generators). I guess this is more a question of mechanics which way to do.

But especially if you have high frequency, I would use U-shape slots for bars.

 

[Intermesher]

waross,

This is the drawing that you requested. The lower left quadrant is of the proposed new lamanate. The other three quadrants are the laminates of the existing motor.

http://www.unicopter.com/Temporary/1926_All.jpg

The inward facing 'teeth' are to be located in a hollow core. There will be only one or two of these teeth per laminate. They will be staggered at various azimuths to allow for forced air cooling.


warross & jpts

Your advice has only been scanned, so far. But it will be studied in more depth tomorrow. Thanks

Dave