steel on magnets. Needed?

[acedovick]

I have heard a couple of different things about this and I'm not sure what's right.

I tried searching but magnets, steel and wood come up in way to many posts.

so my question is do magnets on a generator need to be stuck onto steel?  Does this make it better and if so how much?

I have seen a generator made out of wood using no steel but I do not know if the same design might yield more energy output if it were mounted on steel.

Mag-Neato

[Flux]

To use magnets effectively they need to be used in a closed magnetic circuit that includes steel at every point except where the coils go ( air gap). The smaller the air gap the more you out.

With a typical axial machine with magnets 1/2" thick and mounted on steel the air gap for a dual rotor set up is about 3/4". If you mount the magnets on a wooden disc then the air gap is not well defined but basically you add the distance between magnets twice so the total gap goes up to something like 3" and you have to add leakage flux on this because of the extra gap.

Considering that the output depends roughly on the square of the total flux then you can see that with no steel in the construction you are building a toy. Yes it will work but very badly and you are wasting most of the money you spent on expensive magnets.

The wooden alternators are fun but toys, perhaps they should come with a serious warning to this effect to deter those who think that wood will work as well as steel.

Flux

[acedovick]

thanks flux,

I was also curious about using a halbach array to generate electricity.  I know this makes the out facing poles much stronger.  IE pushes all the flux to one side of the array

| up | |left| |down| |right|

this shows the alignment of the north poles in the array

http://www.matchrockets.com/ether/halbach.html

do you think it would be a waste to do this.  I dont mind paying for lots of magnets to build an excellent generator

however I dont know if would be more effective to just use all the magnets in a north south configuration.  also the added weight might affect it.

know nething about this?

If it is a waste could you link me to a good tutorial on making a generator.  broad question I know.  I just need the basics for one less than 2ft diameter.

thx

[wooferhound]

Here is a very good .PDF  file with plenty of good information about the basics of making a wind generator.

http://www.sparweb.ca/Forum/AXIAL_FLUX_HowItWorks.pdf

[Jeff]

I always imagined: if I could get a company to mold carbon-fiber rotors, 12-14" dia., and have a machine shop build "pockets" from a good iron-core material. You would have the light weight, strength, and durability of some great rotors. BUT, the magnets needs that magnetic steel around them to realize the full potential of the flux path. A nice little rectangular "box", about 1/4" to 3/8" thickness all around, but with one end open to put the magnet in. Barely 0.001" to 0.002" shallower than the magnets. You could have these made to fit both the magnet, and the cast carbon-fiber rotor, so no location problems or worries about "escaping" magnets! Less mass to get moving, the strength is there, and it could be machined to run extremely true (within 0.001"), AND the magnets are nested in a nice little open-ended "box" that will give you maximum flux without all the extra dead weight!

[acedovick]

my roomate works at a shop that deals a lot with generators and he has proposed to me that the steel is not needed.  He said that using a halbach array you could complete the magnetic circut without the steel and it would be much more powerful.

comments?

[ghurd]

"it would be much more powerful" than what?

A standard configuration of the same cost?  Or the same magnet volume?

I expect he means diameter.  Diameter is not a problem that needs fixed.

Fixing magnets to steel has enough problems.  Wood is going to have a lot more.  

I have my doubts wood could support the pull of a decent dual rotor, and the magnets would pull off some wood chips as they pulled away.  It is a lot of pulling force from disk to disk.

G-

[jimovonz]

If there is an absolute requirement for no steel and you were looking to obtain the highest flux density from the smallest volume/mass then perhaps a halbach array could be appropriate. In anything else the use of a halbach array represents a fairly inefficient use of magnets (translates as "expensive"). A lot of people on the board here considering this fail to realize that the typical arrangement of 5 magnets in a halbach array produces not one but two magnetic poles (N,S) on the 'active' side. You are basically using 3 magnets to direct the flux of two.

To achieve an even coil spacing the logical arrangement would a ring of magnets around the perimeter of your rotor (assuming something similar to the current axial flux alt popular on the board) in a continuous halbach array where only every second magnet is an active pole. The opposing rotor would be configured in a similar fashion with the opposing poles opposite. You are using twice as many magnets and still require something substantial to mount them on.

Quite some time ago I ran some simulations in Vizimag to compare the field strength in the gap of an alt configured in this way to compare to the more traditional setup..

Halbach, no steel, max flux in gap 0.43T:

Standard magnet configuration, no steel, max flux in gap 0.28T:

Standard magnet configuration, steel back, max flux in gap 0.82T:

As you can see in this example, using half as many magnets and choosing steel as your backing gives you approx twice the flux in the gap. Considering that the addition of Nd and B to your Fe costs you an order of magnitude more than the Fe alone, I wouldn't even consider making an alt using a halbach array...

[pvale]

jimovonz;

That is pretty respectable flux across an air gap. What did you use as the air gap measurement. I design large power transformers, and use 1.7-1.8T in a stacked, 3 leg, round cross section core at 60Hz. Now, when I design preventive autotransformers for LTCs, they have gaps, and I limit the flux density to about 1.2T. So, for static magnets, I suspect that .82T is a very good flux. Would stacking 2 magnets at each position help any? Those graphics show pretty plainly that steel backing to direct the back flus is essential. I am starting to see the efficiency of the axial dynamo.

Finally, what are you using to plot the fields?

Perry

[jimovonz]

Hi Perry, These are simulated and not actual experimental results. I recovered these images from an old hard drive and I don't have the actual models I used. The flux densities I gave were actually part of the image file names before I cropped them for posting. I used Vizimag (www.vizimag.com/vizimagtr316.zip for a 30 day trial) which is very easy to use and allows spot measurements of the flux at any point as well as plotting along specified lines. With out looking at the specific models I used I can't say exactly what magnet strength, steel permeability and dimension I specified. I usually used values that mirrored what we would typically see here on the board. I certainly would have kept these values consistant between each case so regardless this is still a relevant comparison. Using Vizimag has certainly enabled me to better understand the influence of the magnetic circuit in out alternators. Yes, the steel is a critical performance factor and to consider building an alt without it and expect similar results is just nonsense. The likes of Halbach arrays are certainly interesting to investigate but unless your doing it for novelty value and have deep pockets, I wouldn't attempt to make an axial flux air gap alternator like we see here utilising them. Stacking your magnets definately gives improved results - download Vizimag and see exactly how much for yourself (be sure to report back!) Though generally folks would advise that if you are after the best 'bang for your buck' magnet wise then you are better off using more individual magnets on a larger diameter alt rather than doubling up (within limits - it depends on the relative size of the air gap you intend)