Author Topic: How to Calculate Coils? (Read 1448 times)

Hoskald · « **on:** January 20, 2008, 12:05:15 AM »

I have perused the board and google, yet I have not come up with the answer of HOW to calculate coils to yield a close approximation for a final build (price of copper, ouch!). I understand the trade-offs (larger wire, more watts, less turns, etc.)but I have yet to find a starting point to start working... the idea is this:

5' dia rotor, tsr 8,, cutin at 300 rpms(?), 12 v, dual rotors 8", 24 N40 1"x3"x1/2" mags.

How do I take this information and determine a starting point?

Thanks!

Hos

Flux · « **Reply #1 on:** January 20, 2008, 01:43:11 AM »

I think I see a problem. Even with 2" long magnets on 8" discs you really struggle to have enough room at the centre of the stator for a hub.

I just don't see how you are going to manage with 3" magnets except as a single rotor done the way that Hugh did his little 4ft machine. I don't think you will do it single rotor with those magnets. I suppose you could fit the magnets on at an angle but I haven't drawn anything out to see if that is possible.

Flux

TheCasualTraveler · « **Reply #2 on:** January 20, 2008, 05:39:49 AM »

OK, an 8 inch disk has an outside circumference of 8 x 3.14 = 25.12 inches. Divide that into 12 spaces for magnets and you have 2.09 inches of space for each 1 ' magnet, BUT go in 3 ", the length of the magnet and the circumference of your disc at that point is 8" minus 6" ( for the length of 2 magnets) = 2" x 3.14 =6.28. Divide that by twelve and you have only about a half an inch to fit a one inch wide magnet. The solutions, use 6 magnets instead of twelve or increase the size of the disc to 12 inches or greater or get differant magnets.

Hoskald · « **Reply #3 on:** January 20, 2008, 08:36:23 AM »

See, this is what I get for not drawing it out before opening my mouth.

Thanks. I think if I go to 12" disks then the stator may be reused for other projects in the future. That's a plus. I was just worried about weight. So for the sake of argument, lets say 12" dia 1/4" thick.

Cheers,

Hos

finnsawyer · « **Reply #4 on:** January 20, 2008, 09:00:08 AM »

Unless you already have the magnets, you probably should consider using 1 by 2 inch magnets. As you mention, you should have drawn a picture. If you do, you will notice that while the magnets are rectangular the gap between them is wedge shaped. The longer the magnet the greater the tapering in of the wedge, which determines the size of the coil you can use. So, once again we see trade offs coming in. You could go to square magnets, but then you lose some nice real estate and would need a larger rotor for the same total area of magnets. Trade offs.

Flux · « **Reply #5 on:** January 20, 2008, 10:00:31 AM »

No this is silly, I have only just realised how much magnet you have. With those on 12" discs you have something suitable for 12 ft.

What do you actually have? Have you bought those magnets?

If you have them then it is going to be a real waste of material to build a 5ft machine. Yes you can do it and it will work fine but it will not do any better than much more modest magnets except in a howling gale. It will be so robust that you will never burn it out and that is a good point but no need to go crazy in over building.

Even going to 8 pole is well over the top. You could do it single rotor but again it is not a good use of magnet.

Let us know where you stand, if you haven't got anything yet then there are better options.

This is why people come unstuck by going it their own way and not following common designs, it takes a fair bit of experience to know where to start.

Flux

Hoskald · « **Reply #6 on:** January 20, 2008, 11:18:29 AM »

Flux,

thanks for the comments...no I haven't acquired the magnets yet. I am still trying to get my feet wet and this is a small scale project to start working on my skills.

TheCasualTraveler · « **Reply #7 on:** January 20, 2008, 11:24:21 AM »

If this is your first project why not just go with the ever popular 1" x 2" magnets and go single rotor? You can always rework it with a second rotor later. You'll have to keep that in mind when you make your stator though so you can make the most of the second rotor when it's added. Did you imply that you have already made 1 stator?

Hoskald · « **Reply #8 on:** January 20, 2008, 12:21:52 PM »

TCT,

haven't made anything yet except a mess!

With the cost of materials I want to make sure I understand the principals before making an ex$pen$ive mistake. And since I am a hands-on kinda person, it's been a chore.

That's why this board is such a godsend...it's much cheaper to make a fool of ones self than to screw the pooch with a lot of materials.

I would really like to make a dual-rotor so I can work out the kinks before trying to tackle a larger one.

Thanks!

Hos

Flux · « **Reply #9 on:** January 20, 2008, 12:32:38 PM »

OK 5ft does seem a reasonable size to start with.

I suspect that you would be best to stick to the common 2 x 1 x 1/2 magnets.

You may be able to do it with a single rotor using 12 magnets, but it doesn't lend itself to reasonable calculation.

8 magnets on one disc with a blank disc to return the flux would manage it quite well ( dual rotor with magnets on one disc only).

A dual rotor using 8 magnets on each disc would really be over kill but again it would be fine.

If you want to use some standard hub then I suspect you will need discs about 10" diameter to get enough room at the centre of the stator to clear the hub, but otherwise 9" discs would do.

As a rough guide, if we use the 8 magnets and spinning disc, assuming you want 12v then with a stator 1/2" thick we should be looking at about 60 turns of about #15. That would be a starting point for test coils. It would be a bit slower than your 300rpm cut in. If you do want to use the faster blades then you could cut the turns a bit but I doubt that you could manage much thicker wire.

Draw things out and check your intended hub, the only problem I can see is that it may be tight at the centre of the stator to clear the hub.

If you want to try other magnet sizes then I will have another look.

Flux

TheCasualTraveler · « **Reply #10 on:** January 20, 2008, 12:58:44 PM »

forgive me for interupting Hoskald's thread but,

Flux, you said,

8 magnets on one disc with a blank disc to return the flux would manage it quite well ( dual rotor with magnets on one disc only).

That is the first I heard of doing this, a second rotor without magnets. Does the second rotor without magnets have to have an air gap and be spinning or could it be a stationary disc behind the stator?

wooferhound · « **Reply #11 on:** January 20, 2008, 02:29:51 PM »

This story should help you out a LOT . . .

http://www.fieldlines.com/story/2008/1/6/101045/6228

Flux · « **Reply #12 on:** January 20, 2008, 02:30:26 PM »

Yes it has to have an air gap and spin at the same speed as the magnet rotor otherwise you run into eddy currents and you need a high quality laminated core and all the problems associated with iron backed single rotors.

Flux

Hoskald · « **Reply #13 on:** January 20, 2008, 07:05:20 PM »

Woofer,

That's the ticket! My Hotlist is really getting long... I think that I now understand that there must be an arbitrary starting point about which you know little. You just kick in the best you can based on what other people have done and from that point on experience will be the guide. And here I thought that it was all calculated...I am now even more impressed with the brain-trust that exists on this site!

Thanks for the enlightening kick in the seat! 8-)

Cheers,

Hos

Fieldlines.com: The Otherpower discussion board

News: