Miniature alternator design

[GreenFlux]

Hi. This is my first post in this forum. I wish to design a small hand operated generator capable of producing at least 5 watts of power rectified to charge 2.7 volt super capacitors and mobile battery. However, I don't know if it is really possible to design this with 6 cm diameter stator having, say, 4 or 6 coils. Here is the six coil design:



The magnets are rectangular in shape with dimension 10mmX5mm. 
Any useful input is welcome.

Thanks.

[Flux]

That  really is tiny and the quantity of magnet is tiny so the price you will pay is in the speed required. A few quick checks suggest that you will need a speed of 5,000rpm or more. You could keep the size quite small and probably get much more magnet in there to make life easier. A small gear driven dc motor may get you there at a far lower first cost.

Flux

[GreenFlux]

Thanks Flux for your response. Actually, I can get it to rotate at around 1000 rpm(max) using miniature planetary gear system. Let me tell you something about my earlier project. I made the generator using a good quality 2 phase stepper motor rotating at 300 rpm using a pull string mechanism, the one used to start lawn mower. I could get only a quarter of a watt at the output. Now I wish to use a machine which is actually designed to do this job, i.e. generator.
I just found this post :http://www.gotwind.org/forum/viewtopic.php?f=33&t=1717#p6022 which is almost exactly what I am looking for. With dual rotor design, is it not possible to generate useable 5W at low speeds?

Are there any small handheld motors which can generate 5W or more power?

Thanks.

[Flux]

I think you can do it if cost is not a problem. You could get a lot more magnet into your space. Very small magnets don't use space very effectively.  Don't take the magnets too near the centre, use square or slightly rectangular ones , use more of them so the gap is about magnet width at the outside ( they may virtually touch at the center), keep the coils nearer triangular to save space at the centre so that you can get the copper in with the magnets right to the outside of the discs and use thick magnets with good backing iron and make the magnets at least 12mm thick. If you can get to 1000 rpm then it may work.

If you can find details of Hugh Piggott's 2F turbine design, keep similar proportions but stick with neo and scale it down to your size then you should be on the right lines.

For large wind projects you aim for value for money in terms of magnet and copper, in your case you are looking for a working solution and in terms of cost per output it may not be so good but if the final cost is within your means then all will be well.

The Gotwind idea is on the right lines but change the proportions to get squarer magnets, more of them and don't keep the coil holes too large, you won't link all the flux with the inner turns but they will add volts and are short and contribute little resistance,

There certainly are very nice radial design motors about that will do 5 watts at 1000rpm but they will be servomotors or similar and not designed for low cost,

Another factor you may need to experiment with is the winding configuration and the voltage. Single phase is simple but a polyphase winding should do better, my instinct is that perhaps 2 phase may be best but it doesn't lend itself easily to single layer axial windings so perhaps 12 pole 9 coil 3 phase could be a good start if you base it on Hugh's 2F layout.

You will either have to accept a lot of diode loss ( even with schottky) or for rectifying to 12v or greater and using a nice efficient synchronous buck converter for the charger.

Flux

[Flux]

Not sure what magnets you can get but it looks as though making it 8 pole with magnets 12 x 10 x 10 thick might work with 6 triangular coils jammed into a stator 60mm diameter and 12 mm thick. You will need it well made with magnet rotors running true and close to the stator. I assumed a voltage of 6v, you may need to play with that depending on the capacitor or battery.

Flux

[GreenFlux]

Thanks for your valuable suggestions. After running some search on the Google for neo magnets, you are right in saying that the cost can be a real problem for me because the cost of one 12mm by 12mm by 10mm NdFeB magnet works out to be more than three dollars and I would be needing 16 of them for a dual rotor design so around $50 for magnets which is way beyond for a student like me at the moment. Nevertheless, I am not giving up. What do you think of this radial design given here?: http://www.windstuffnow.com/main/radial_air_core_alternator.htm

Is it suitable for at least 5W. It is given there in the second design that it can produce more than 20W but I don't know at what rpm. The length is not an issue. I am only constrained by diameter. It can be a long tube like machine.

Thanks.

[Flux]

If diameter is your limitation then radial is the way to go. Decent radial machines are not easy to build without special facilities, that is the main reason the axial has proved so useful.

The radial as shown in the link is basically an educational toy, it makes very poor use of materials but if you make it long enough you may get what you want if you keep the speed up.

To be effective you need closed magnetic circuits, before neo these open magnet designs were impossible and even with neo they don't use the magnet effectively. At such low power you may be able to build that thing into a steel tube and complete the magnetic circuit but in the real world the return circuit needs to be laminated to avoid an excessive eddy current loss. You could try cutting slits along the tube to reduce eddies. Another possibly better idea may be to wind soft iron wire round the outside to make a laminated tube. If you use soft iron wire that is fairly thin and cook it in a flame to anneal and oxidise it it will make a reasonable low loss core. Don't use mig wire, try to find the stuff florists use to bundle up flowers.

Try to keep the gap between your magnets and the outer core as small as possible to get a decent flux density.

Good luck, if you stick with a decent radial design 5W is possible.  It is when you try to get high power densities from small things that you run into cost issues and you have to use the best materials and use them to best advantage. Try looking up some information on servomotors and see if you can use some of the ideas.

Flux

[GreenFlux]

Thanks again for valuable insight. I think I should go with the radial design using slotted stainless steel tube 1mm thick and of diameter 60mm and length not less than 80mm. Annealed soft iron wire (is that black in color?) seems to be readily not available here. Shall I go with 4 poles and as many coils as there are in the design in the link posted earlier? Or, do I need to experiment with different design configurations? How do I decide number of poles and number of coils? I apologize if I am being too pushy.

Thanks.

[Flux]

Don't use stainless, it is non magnetic ( a few grades are but they have no virtues ).

I don't know what you can easily get or can machine, cast iron has reasonable permeability and is also fairly high resistance so not so bad for eddy currents.

The number of poles will probably be fairly low for a small radial but having a big 2 pole magnet or smaller magnets to make say 6 pole would have similar effects. If you go for single phase then one coil per magnet is easy to do. With radials the normal 3 phase arrangement will have 3 coils per magnet but you can use half that number with a different connecting arrangement, this will normally need overlapped coils. You can get down to a single layer winding by using the 4 magnet - 3 coil arrangement used in the common axials.

Flux

[Mary B]

Put out an ad wanting bad hard drives and salvage the neo magnets inside them. as long as you can find enough the same to have sets of 4 it will balance out.

Thanks for your valuable suggestions. After running some search on the Google for neo magnets, you are right in saying that the cost can be a real problem for me because the cost of one 12mm by 12mm by 10mm NdFeB magnet works out to be more than three dollars and I would be needing 16 of them for a dual rotor design so around $50 for magnets which is way beyond for a student like me at the moment. Nevertheless, I am not giving up. What do you think of this radial design given here?: http://www.windstuffnow.com/main/radial_air_core_alternator.htm

Is it suitable for at least 5W. It is given there in the second design that it can produce more than 20W but I don't know at what rpm. The length is not an issue. I am only constrained by diameter. It can be a long tube like machine.

Thanks.

[gotwind2]

This might work for you, it's a DIY radial 2 pole coreless design that comes in well below your 60mm diameter limit - It's single phase with a very 'lumpy' output, fine once rectified and smoothed with a capacitor, 5 watts at 1500 rpm.
Further details on my site here: http://www.gotwind.org/diy/12-inch-mini-turbine.htm



Have fun..