Stator Thickness

[Tom in NH]

I'm admiring DanB's beautiful photos as I get ready to make my own stator for my 12 inch diameter rotors and nine coils. My coil winding tool made coils that are 1/2 inch thick. How thick do you think I should make my resin casting? Has anyone ever considered putting holes in the casting, through the centers of the coils, to perhaps aid in cooling of the coils?

This is my first stator and I must say the electromotive forces on a shorted coil between my magnets are quite astounding. Just putting the coil down inbetween the magnets produces a noticeable force in my hands, nevermind holding the coil while the rotors turn. I can see where coil vibration could become a source of fatigue over time. I assume that fiber reinforced resin is sufficient to counter it. Has anyone ever felt the need to strengthen a stator by reinforcing the casting beyond adding fiberglass cloth? --tom

[Flux]

Tom

I don't think the forces on the stator are that frightening, it is a torque in the direction where the thing is stiff. The only real danger is where the thing is mounted, if you are not careful the supports can exert a strong twisting force on the mounting ears, I don't like the 4" of allthread that many use. I like to add thick spacer tubes over the allthread and locked to it with nuts so that the supports are rigid and if you have strong steel arms then your stator is only restrained by a torque load.

At the size you are considering I have never used more than a sheet of fibreglass cloth over each side of the coils, though I do tend to chop up a bit and use it to support the fixing ears.

I am not entirely convinced that you will get any worthwhile cooling by leaving the coil centres open,you would have to drastically alter the air flow to make any difference. I have a feeling that a mixture of resin and talc conducting heat to the radial air flow will be more effective than dead pocket of air in the holes, I may be wrong.

[SmoggyTurnip]

What direction is the torque? I mean if there is a rotating force on the coil, where is the axis of rotation, is it through the center of the coil and perpindicular to the rotors or parallel to the rotors.

[Flux]

The magnets are trying to drag the stator round with them, I don't follow your question. The torque on the whole stator is about the machines bearing axis just like the prop torque.

Flux

[SmoggyTurnip]

I thought there would be a torque on the coil itself with the axis through the center of the coil.  I could be wrong though.

[Tom in NH]

Thanks, Flux. I won't bother with holes. I'll keep my thickness to that of the coils plus a layer of cloth on each side. I like your idea about reinforcing the stator supports. I could use some channel stock bolted between the stator and the supporting structure. I appreciate your expertise and your willingness to share it. --tom

[Shadow]

What size wire and how many turns on your coils?If your coils are already 1/2 inch thick, you will end up with fairly thick stator. Be nice if you could have made your coils 3/8 thick and ended up with 1/2 thick total stator.

[Tom in NH]

There definitely is torque on the coil itself when one leg of the coil is over a N-pole and the other leg is over a S-pole. When I hold a shorted coil between my two rotors with my hand and give the rotors a gentle spin, the coil wants to twist one way, then the other as the opposite poles pass by. Shake, rattle, and roll. --tom

[Tom in NH]

I'm using 14 gauge wire. I've only made two coils and I could make a new tool to make the coils 3/8 inch thick. I believe thinner wider coils would still fit with my magnet spacing. By reducing the coil thickness and spacing between poles of my mags by 25%, would there be a sufficient gain in output to make it worthwhile? What might I expect?

Thanks, --tom

[Flux]

This is not too simple, if your present coils will give you the cut in speed you want I would carry on. If you did a test coil with a gap that can take your stator thickness it will be ok. With 1/2" thick magnets a gap of 3/4" will be ok.

If you haven't filled all the space then you could have used a thicker wire but whether that is an advantage depends on the size of prop.

If you reduce thickness to 3/8 and your cut in was ok before, you will lower it and may need less turns. If you could get in the previous turns, again you could use a thicker wire.

You would have to give the full details of everything including prop to be able to give you a better answer. I tend to use coils 12mm thick for the metric 10mm magnets and the stator usually comes out about 13mm thick. For 1/2" magnets, coils 1/2" thick should be ok ands hould still run in a 3/4 gap.

I muss confess that I always take the connection from the inside via a thin copper tape and this does mean a thinner stator than if you bring a #14 wire out.

Either way as long as you can get your cut in speed with the stator thickness it will be ok. If you want to run a bigger prop you would need to use the thickest wire you can get in.

Flux

[hobot]

Flux or others,

Could you expand on the 'inside connection via thin ribbons' comment

on making a thiner coil.

Is this part of the windings or just the conductors for coil outputs?

Any other narrowness methods? A twisted wire pair seems would be thicker

than an equal guage single wire, but don't know especially wound in coils.

Hmm idea just hit me, as the upper and lower coil legs are wasted resistence,

what would happen if multi strand active coil legs were connected at ends

by single heavy strand or some ribbons of low resistence?

It might solve some corner turning and maybe allow thiner area so overlapped

coil crossings could be same thickness as the flux pumped legs?

Would connections resist enough to just nullify idea?

hobot

[Flux]

I use strips of copper about 1/4" wide and 0.010" thick soldered to the inside end and they lie flat against the coil.

Sorry I don't follow your idea.

Flux

[craig110]

Like Flux, I'm not entirely sure I understand what you are proposing, but is it to essentially cut the top and the bottom off the coil and replace them with a lower-resistance bar / wire to which all the wires from the active legs attach to?  If so, that would be paralleling all of the coil wires together and you'd end up with the electrical equivalent of just one loop of really, really, really big magnetic wire.  It would certainly have low resistance and high current, but the RPMs that would be needed to get a decent voltage out of it would be a tad bit on the high side.  :-)

Craig

[hobot]

Thanks for the insightful responses. Yes you all got my idea and summarized its

characterists I was trying to understand.

OK, its a bad idea in wind power mill rpm but

maybe a solution of compact power in engine project.

Lenght of wire in flux, not the mass of wire is what make's voltage go round.

hobot

[Propwash]

Hi Tom,

  I used West system epoxy in my stator. You can buy special fillers to thicken the epoxy. It is much stronger than polyester resin and doesn't smell as bad. West System is a marine type item and can be purchased through a marine store. All of the harware stores in this area have a complete display of the epoxy and assorted fillers that go with it. I don't know if that is the case away from the coast or not. I built the Hugh Piggot 12' mill. I used angle iron for stator mounts with short studs in the end of the angle iron flange for adjustment purposes.

                         Kevin

[Tom in NH]

Hi Kevin,

That sounds interesting. Does the epoxy pour into a mold? Do you put anything in it, like fiberglass, to reinforce it? --tom

[Propwash]

Yes Tom you can do all the things you mentioned. I placed a layer of cloth in my stator mold and wet it out with unfilled west system. I then placed my coils in the mold and poured in a batch of West System mixed with high density filler to the point where it would just pour out of the mixing container. I then put one last layer of cloth over the top and wet it out with another mixture of unfilled West System. They have a very good website that tells you all you will need to know to get the job done. It is a lot more expensive than polyester resin but really good stuff. I use it quite a bit in iceboat construction and boat construction. http://westsystem.com/

                        Kevin