N48 Mag for 10ft Mill

[gator]

I was wondering if there would be any advantage to using N48 2x1x1/2 mags on DanB's 10ft mill.  What would have to change and approximately how much?  Coil?  Prop size and TSR?  

[Volvo farmer]

This might be a really stupid comment (and wrong too),  but I'll say it anyway :-) It seems like the biggest effect on power output is in the diameter of your mill and the windspeed. Of course, building something with ceramic magnets VS neos might make a diffference, but my gut feeling is that a N48 magnet vs a N40 magnet is not going to make much difference in the actual power you can get at a certain windspeed, with a certian diameter mill. Things like diameter of windmill and windspeed increase with equations based on the CUBE or at least the SQUARE of the power formula. Sure. it's cool to add 10% effiency to someone else's design, and there's a place for that, but personally, as a guy who wants to gather every watt as cheaply as he can, I'm looking at those types of formulas that are exponential, not fractional.

[DanB]

I've never tried it - would be intersting.  I expect you could have about 10% - 15% fewer windings and maybe bump down a gage in wire size and perhaps be a touch more efficient - or... if you could fit almost the same number of windings in there with thicker wire - a slightly larger prop running at about the same TSR.  In any case-  if you built it the same with a 10' prop I expect you could use slightly thicker wire or have a larger airgap - both things have their own added safety factor.

I should experiment with that.  My guess is the money is better spent on lower grade larger magnets but either way is moving in the same direction - basicly a slightly more powerful machine.

[Flux]

Agree with Dan. I can't find figures of Br for N48, I am not sure if the total gain is in Br or whether Hc is also increased. From N35 to N40 it is Br that goes up.

As Dan said you will have more flux for the same gap so you can manage with less turns of thicker wire and squeeze more out.

If you change nothing you will have to increase the air gap to prevent stall so there would be little gain. If you use a larger prop then the same winding would have a lower cut in speed and may match directly with the same tsr.

If you are at low volts with a long cable run, it may be better to wind with less turns of thicker wire, keep the same cut in speed and prop and save on cable run with the same performance.

Whatever you do it is not going to be that dramatic, depends on the cost of the magnets and your set up in the end whether you come off better.

Flux

[gator]

At only $1.20 more per mag from N35 to N48 that's only a $32 upgrade.  This could be fractional, I'm not sure.  I do like the idea of slightly larger gap, slightly less windings with possibly heavier wire and furl a tad bit later.  Is this worth 32 bucks?  I don't know but it will be fun messing around with it and comparing outputs.  When I start putting this thing together I will wind a coil just like danb's and then others to give a comparison; then I'll post the results.  

Cheers,  John

[oztules]

 With stronger mags you have more options. simple as that . I'm a man that likes options so i opted for n45 . This gives us the opportunity to make sloppier tolerances and still generate appreciable amounts of power. The price differential doesnt appear to be a killer, so go with it. You can always derate the gennie (bigger gap) but it's hard to go the other way if you are trying to best match the prop to your conditions.

........oztules

[gator]

Point well taken.  

[gator]

Grade   

Remanence(BrmT)

Max.Energy Product(BH)max (MGO)

Coercive Force Hcb (KOe)

Intrinsic Coercive Force Hci (KOe)   

Max Working Temp.(C / F)

N35    1170-1210    33-36    >10.9    >12    80 / 176

N40    1250-1280    38-41    >10.5    >12    80 / 176

N50    1400-1450    48-51    >10.0    >11    80 / 176

I can't get the figures for the N48's either but here is from the same place.

The greater than signs should say greater than or equal to but it gave strange characters when I put it in this comment section.

I suppose 48 would be in there somewhere.

[oztules]

http://www.cy-magnetics.com/Mag_Pros_NdFeB.htm         try this tidy looking table. ........oztules

[Flux]

That is fine, the gain is entirely in Br.

That is a very significant gain with the N50. Should give a very useful gain in output.

Flux

[willib]

If you multiply the energy product by the volume of a magnet , gives KJ ..

does anyone know what energy product is?

is it the amount of energy that a certain strength is capable of producing?

[oztules]

willib try this it a glossary of mostly mag terms

http://www.magnetfabrik.de/english/abc/service.htm

..........oztules

[nothing to lose]

Since you guys are the magnet masters

What about using smaller magnets at higher grades?

I certainly have not tried it yet, but my thought was perhaps using say a N50 I could use a 3/4" thick magnet at perhaps a lower cost and less weight and such compared to a N35 which would be 1" thick.

Would it work in that way?

Also I have those #29 arcs comming in a few days. Would have had them now but I been gone 10 days or so and the 14th I was e-mailed they did not have the full correct shipping address. I just got back online yesterday and found that e-mail

Got it fixed now and have the tracking number for them.

So once I get these and check them out, I was planning to order some thicker arcs also. Then I wondered, would it be better to have thicker ones made or a higher grade like N50 at the same 1/4" thickness?

 These are being custom made for me so I can get whatever I want to pay for.

I was going to have 4 sizes made eventaully to use in various motors.

[nothing to lose]

Since I was thinking on my arcs either thicker or a higher grade for more powerfull motor conversions I decided to e-mail the manufacturer and ask direct which is more cost effective if all else is the same. Use same thickness but higher grade, or same grade and thicker?

Should have an answer on that tommorow.

[finnsawyer]

Keep in mind that when you go to a stronger magnet you also increase the magnetic flux through the air gaps and iron parts of the alternator.  Since iron, unlike air, can be driven into magnetic saturation, it is possible that very little gain may actually occur.  One obvious bottle neck is where the magnet connects to the iron rotor.  As the magnet strength is increased without increasing its footprint area, the iron must be able to handle the increased flux through this restricted area.  At some point this becomes problematical.  It would be desirable to build two identical alternators with different strength magnets to actually determine the gain if any.  If it turns out that this truly is a problem, a possible solution would be to mount the magnets in an aluminum disk with air core stators on both sides to drop the flux to a value that doesn't saturate the iron.  Since the aluminum is rotating with the magnets eddy currents shouldn't be a problem.  Well, the currents in the coils will have an effect.  Use wood or some kind of rosin?

[jimovonz]

I have considered this myself. Rather than the 'footprint area', I believe that the restriction is the steel rotor thickness. Steel becomes saturated at somewhere around  2T which is well beyond any neo (N50 Br max ~ 1.4T), however all of the flux comming out of the pole face has to make it to the two adjacent poles through the thickness of the plate. The plate concentrates the flux from the magnets through its thickness. If you assume that half of the flux from any magnet travels directly to the adjacent magnets (one either side) then you could say the the steel plate has to be 1.4/2 x W/2 (or 1.4W/4) thick to avoid saturation - where 1.4 = the strength of the magnets divided by the saturation point of the steel and W is the width of the magnets divided by 2 (only dealing with half of the flux from the magnet in either direction). So for example if you had 2x1x1/2" N35 neos (Br 1.2T) you plate thickness should be 1.2x1/4=0.3" thick to avoid saturation. Of course you could get away with slightly  thinner plates as your magnets will never quite achieve the Br max figure in practice and the flux traveling through the plate doesn't have to travel exactly in a straight line (it can loop around the ends a bit). On the other side, some steels have a saturation density somewhat less than 2T (can be as low as 1.5T)