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magnet questions
By Stonebrain, Section Wind Posted on Mon Sep 26th, 2005 at 04:06:42 PM MST
about volume,surface and shape of magnets

Thinking about my future gen I thought a good start to look for magnets.
Before making a choice I think it would be good to review all the parameters:
1. Volume.
The more volume,the better(more flux).More flux means less windings for
the same voltage and power what means more place for thicker wire = less
losses in the gen = higher maximum power.
For smaller gennies (<7') rpm is higher,power output lower =smaller magnets.
Is this right?
What should be the minimum volume of NdFeB for a 7' genny.(9coils-12 pole)
I found out 6 - 10 cm cube is still reasonable priced ,4-10 Euro (about 4.8 - 12 $)
2 Surface.
Some say bigger surface is better.
But shouldn't the surface be smal enough to fit in the hole of the coils
so all the flux pass inside the windings? Or can it be a bit bigger?
3 Shape.
Rectangular or circular.
My intuition says circular is better because it fits better in the hole of
a coil.Is the wedge-shape really the best?Isn't there some of the flux in the corners of the magnets that doesn't pass through (all) the windings of the coils
(that are allways more or less round).
The coils are mostly oval so shouldn't be the ideal shape of the magnet
kind of oval(=elips?).
The closest to oval shape in commerce is circular.
Is a choice of circular magnets of about 7cm cube for exemple 1.5"diam 1/4"thick
big enough? Bigger? rectangular? Thicker and smaller?Thinner and larger?
What is the volume of the wedges I saw on windstuff?
And Thanks for all the great information I find on this site.
Stonebrain

magnet questions | 8 comments (8 topical, 0 editorial)

Re: magnet questions (3.00 / 0) (#1)
by Stonebrain on Mon Sep 26th, 2005 at 10:48:49 AM MST
(User Info)

Oh,I forgot to say I'm thinking about a gen with 2 rotors for the magnets
(2 x 12 magnets).

Re: magnet questions (3.00 / 0) (#2)
by Flux on Mon Sep 26th, 2005 at 11:25:08 AM MST
(User Info)

Perhaps one day someone will test every possibility or run a computer program on this but until then there is a fair bit of guesswork.

I have never used round magnets so I really don't know how they compare.

You don't give any idea of the speed you want to run your 7ft prop at.

In general I try to avoid thin magnets but with your chosen 1.5" x 1/4" thick, I think you have excessive magnet material if you intend to make a conventional machine. You could use it inefficiently and get away with it by using large air gaps.

I would be tempted to make it 8 poles 6 coil with 2 x1 x1/2" magnets but I also realise that you may have trouble with space at the centre if you use a standard car hub. If you want to keep to 12 magnet it may be worth looking at 1.25" x 1/4 thick or even 1" dia and a bit thicker.

Unless you try to produce an overlapped coil winding I don't think you will find the wedge shape to be any advantage, simple shapes are more cost effective, the most common one is likely to be the best value for money.

Minute changes to shape of holes has little effect, in general I wouldn't make the hole smaller than the magnet but in the case of round magnets, reducing the width a bit won't hurt as there is likely to be a bit of leakage at the point where the magnets are closest.

The other factor you didn't mention is the voltage and distance from batteries, if it is 12v and you have a long cable run you may be better off with too much magnet and use thinner and cheaper cables, keeping losses outside of the stator will reduce your chances of over heating it and you may be able to furl at a higher speed

[ Parent ]

Re: magnet questions (3.00 / 0) (#3)
by seanchan00 (seanchan@lycos.com) on Mon Sep 26th, 2005 at 06:06:55 PM MST
(User Info)

Hi Flux,

For my benefit please elaborate your last paragraph. I have been reading comments indirectly refering to this variable "cable run from windgen to 12 V battery" with effect to wind power generation, stalling and furling but don.'t fully comprehend how to manage this to maximise results.

SeanChan.

[ Parent ]

Re: magnet questions (3.00 / 0) (#5)
by Flux on Tue Sep 27th, 2005 at 01:48:56 AM MST
(User Info)

I have covered this before but can't find it, I will keep it short but you may find a more detailed explanation somewhere.

For an alternator to be efficient it needs a low resistance, this requires lots of magnet to keep the turns down and thick wire.

If you make it super efficient its current will rise rapidly above cut in speed and the only speed increase will be that to supply the losses as the battery volts is effectively constant.

The propeller ideally needs to run at a fixed tsr so its speed should track the wind speed, it stays on the peak of its curve and makes maximum power.

Now if you connect that very efficient alternator to your prop you run into a problem, it starts off nicely at cut in but the tsr drops rapidly as it can't track the wind speed. There is a band of tsr where the prop works reasonably well but when the tsr falls too low the prop efficiency drops right down and it goes into stall.

This is a disaster for output so you have to compromise. You keep the tsr high at cut in( above the ideal) and limit alternator efficiency so that there is a reasonable rise in speed and you reach stall at furling speed where you needing to limit power and the stall is an advantage.

This leads to the strange result that an alternator that has dropped to below 60% efficiency at 20 mph will give better overall power than one with high efficiency

Normally the cheapest option is to limit the quantity of magnet to give a winding that with the right loss ( magnets are the significant cost). If you have a lot of line loss you can use this to give the speed rise and use bigger magnets to make a winding that would have too little resistance on its own. In some cases it is cheaper to spend a bit more on magnets and save on cables, especially with long runs at 12v. There is a significant advantage in keeping the necessary loss outside the stator then you can push it higher in high winds without fear of burn out.

A couple of weeks ago there was a question about PMA efficiency from pwr I think and I covered some other options there, this may be interesting to you as I covered other ways round the problem.

[ Parent ]

Re: magnet questions (3.00 / 0) (#7)
by seanchan00 (seanchan@lycos.com) on Tue Sep 27th, 2005 at 10:21:09 AM MST
(User Info)

Thanks Flux,

I am indeed glad you wrote such a clear reply. I will be saving this for reference as there is so much info contained in it that I have not fully digested nor able to comprehend till I run into trouble when I set up my windgen. Begining to see a little light though.

SeanChan.

[ Parent ]

Re: magnet questions (3.00 / 0) (#8)
by nickelbender on Tue Oct 18th, 2005 at 11:49:42 AM MST
(User Info)

Has any one built a dual rotor machine with magnets on one side and wound salient poles on the other? This might be a solution to improve low and high wind speed performance.

[ Parent ]

Re: magnet questions (3.00 / 0) (#4)
by Stonebrain on Tue Sep 27th, 2005 at 01:05:13 AM MST
(User Info)

Thanks Flux for your usefull comment,
I realise I ve to study rpm,cut in speed and all that
funstuff.I've got to get me a windmeter to know more about
local windconditions.
Good point about the car hub.Have to get one to see diameter
of hub to see what I can fit on it.It seems to me circular magnets
fits better on a circle then rectangular ones.But the best price/volume
ratio might be the best criterium.It seems to me that overkill in
magnetic flux can't hurt.
Need some time to find a good hub.Gives me time to think about some
of the tricky questions involved.

Stonebrain.

Re: magnet questions (3.00 / 0) (#6)
by Flux on Tue Sep 27th, 2005 at 01:56:33 AM MST
(User Info)

Yes always err on the side of to much flux, you can get round that in various ways, if you don't have enough your prop will run frantic and the stator will likely get hot.

Be very careful with short magnets, a minor problem such as lumpy resin can easily force you to use a longer air gap than planned and that could easily push the cut in speed way up.

Also it is a good idea to settle the alternator first and if it goes off target you can tweak the prop diameter or tsr.

Flux

[ Parent ]

magnet questions | 8 comments (8 topical, 0 editorial)

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