Overall density of stator laminations??

[mrpi]

I'm trying to estimate the transient heating of brushless DC motor stator. (run it for 90 seconds, how hot will it get).

I was going to assume the heat is generated in the windings (copper) and is transmitted by conduction to the laminations (steel) then into the stator housing (aluminum).

I have the stator in-hand and have weighted it. I also have the rough profile of the laminations as provided in a drawing by the motor manufacturer. I figured I'd CAD model the laminations from the drawing profile and estimate its mass without the windings. However, I suspect the laminations are not nearly the same density as if it were made from solid steel. Is there some correction-type factor I can apply to estimate the mass of the stator lamination stack? 90%ish is what I'm guessing.

Thanks.

Beat to fit, paint to match.

 

[edison123]

Laminations density is the same as of that iron, around 7.65 gm/cc.

Muthu

http://www.edison.co.in

 

[mrpi]

But there is some kind of paint or coating between the layers, thus lowering its density. I guess I was after the "bulk" density of the assembled lamination stack.


Beat to fit, paint to match.

 

[edison123]

That "paint" is an insulation varnish with is hardly 3-5 microns thick and it doesn't change the weight in any significant way.

Muthu

http://www.edison.co.in

 

[mrpi]

Huh, I thought it was thicker. Thanks!

Beat to fit, paint to match.

 

[electricpete]

Induction Machine Handbook gives a stacking factor which is supposed to account for lamination spacing and associated magnetic effects:

The stacking factor KFe (KFe = 0.9 – 0.95 for 0.35 – 0.5 mm thick laminations) takes into account the presence of nonmagnetic insulation between laminations.

(Note that end effects and vent duct effects are treated separately)

I think in some cases, the lamination insulation was not the only consideration affecting stacking factor... the burrs contributed to stacking factor < 1. However for laser cuttting, that may be very much reduced and the number higher than the range given above.


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(2B)+(2B)' ?

 

[mrpi]

So if .9~.95 is the magnetic stacking factor, I would imagine that the "density factor" would be significantly greater, seeing as the magnetic field is related to the inverse-square?


Beat to fit, paint to match.

 

[electricpete]

The desnity factor would be the same as the magnetic stacking factor defined above. The magnetic stacking factor simply accounts for the fact that a certain fraction of the volume is occupied by insulation/space and not available for iron. If you have a reliable estimate of magnetic stacking factor (which is a big if), I think it would be the correct number to use for your density calculations.

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(2B)+(2B)' ?

 

[electricpete]

By the way, you already have the weight. Density (and stacking factor) of course would not be important to determine the heat capacity of the big lump of iron representing combined core/frame.

Since you are asking, I assume you are wanting to distinguish between weight of the frame and weight of the core because the distinction is important to your thermal model? (fwiw, I would tend to think a small error in allocating among these 2 would be not much to worry about considering other likely unknowns and approximations)

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(2B)+(2B)' ?

 

[mrpi]

I have the weight of the iron laminations AND the copper windings. I was trying to differentiate between the two. But you're probably right about not worrying about it!

Honestly, I dont really know how waste heat is generated within the motor itself. Maybe that would have been a more appropriate question... hmmmm. I need to think about it some more.

Beat to fit, paint to match.

 

[electricpete]

I forgot your weight included copper. I understand better now why you were interested

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(2B)+(2B)' ?

 

[wolf39]

mrpi:

The stacking factor does take into account not only the varnish between laminations but also consider a certain uneveness of this material. Also, the laminations are not perfectly flat but slightly wavy. Stacking factors therefore also depend on the pressure forces applied to the stator core.

The dynamo steel laminations will heat up because of the iron losses generated by the magnetic flux once the motor is connected to the grid and running idle. In addition a certain loss portion is generated in the stator windings because of the magnetization current (no-load current). When the motor load is increased to rated load the magnetization current increases to rated current with the result that the short circuit losses heat up the stator winding further. Both loss fractions have to be dissipated by the cooling air flow in the vent ducts.

Wolf

http://www.hydropower-consult.com