Calculating magnetic resistance on moving flywheel???

[Barry1492]

Given a spinning Iron ring of a certain mass and dimensions (say about 1.5 inch by 8 inch ID and 9 inch OD, if I move magnets closer to this ring they will increasingly resist the ring from spinning....more torque will be required to keep the ring spinning at the same speed (think of magnetinc resistance in an exercise machine).

I'm trying to estimate how large and how powerful my magnets must be and how close they should be from the spinning ring. I took advanced E & M, but really have very little experinece in this area (like none!).

Can someone point me in the right direction? I'm looking for equations, weblinks, etc.

Thanks a whole lot - Barry

 

[sreid]

This is, as I'm certain you appreciate, a highly nonlinear problem involving both hysterisis and eddy current drags. It' probably best solved using magnetic FEA or;

Get a big magnet and a steel bar and measure the drag (details of a test fixture left to you). The drag will be dependent on both the magnetic field strength and the velocity. The field strength can be varied by changing the magnetic field gap.

 

[nbucska]

I think you gat more breaking force with aluminum disk

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[sreid]

nbucska,

You are correct; the eddy current drag improves with the conductivity of the disk. Copper is better yet.

 

[Barry1492]

Is this true?? I would have thought you'd want something magnetic. Can someone explain the physics that is going on here? What is causing the actual breaking resistance?

Thanks.

 

[nbucska]

Sreid: yeah, but more $$$$
Barry: Voltage is induced -- mass of wheel acts as short circuit so it acts as a shorted dynamo ( "Eddy current" )

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[itsmoked]

Barry1492: nubucksa's aluminum, would prolly be fine and cheap-er.

 

[Barry1492]

So, is it the "conductive" properties of the metal that is important and not its "magnetic" properties?? I'm basing this assumption off the fact that aluminum works better than iron yet copper works better than aluminum.

This brings up some interesting design considerations. We would obviously like to keep costs low, but we also want to keep the size of the wheel small. How would steel work? I'm not an electrical guy so forgive me if the answer is obvious. I've never heard of steel wires so I'm thinking maybe steel isn't a good choice.

Also, I need to document some of this stuff for a preliminary report. Anyone have a good link?

 

[nbucska]

The motional energy is converted to heat the wheel must be able to dissipate. You may need to cool it.


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[Warpspeed]

For this to work, you just need a moving conductor through a magnetic air gap to generate the eddy current. While an aluminium or copper disc would work better electrically, it would also add to the total magnetic air-gap (both being non magnetic).

A steel or cast iron rotor will provide a much more effective magnetic path, and more available magnetic flux, but have greater electrical resistance per unit volume.

In practice a thin aluminium disc is typically used in very, small low power light duty applications, and thicker iron/steel disc in large high power applications where considerable heat may be generated and the forces are large.

Both work, it is just a case of deciding which is most suitable for different situations.

 

[IRstuff]

Our local science center has a hands-on experiment dealing with that. You drop plates of various materials between the poles of a magnet. My recollection is that the copper plate falls the slowest.

TTFN

 

[sreid]

I can FAX you some design info on eddy current brakes if can supply a FAX number.

 

[Barry1492]

Thanks. That will be great. I'll have to get you that number tomorrow.

-Barry

 

[Barry1492]

sreid,

My work fax number is (302)831-8525. If you can fax me that info that woould be great. Send it Attn: Barry Pollock

Thanks again - Barry