Rotor with large magnets

[oregon wind]

I decided to use 1"x1"x2" magnets on my 16" diameter rotors instead of 1"x2"x1/2" magnets so that I could increase the air gap and use thicker wire in my stator, and allow more airflow for cooling my stator.

I did the old paperclip test on the back side of my rotor plates, and the paper clip sticks.  These magnets are quite powerful, and apparently I am losing some flux out the back side of the rotor.  Do you think that it warrants going to a 5/8" thick plate, or just accept a little loss and continue with 1/2"?

[Flux]

No idea of number of magnets, disc size or anything else. If it is mechanically ok at 1/2" thick then  it will be ok.

Flux

[oregon wind]

Hey Flux:

16 magnets, 16" disk for a 12' turbine.

I decided to go overkill on the magnets to be able to open the air gap substantially.

Mechanically it is fine.  Do you think the flux loss warrants a thicker disk?

[Flux]

I doubt that you would be able to measure any difference in the gap. There will be loss but I think it will be very small.

Flux

[Ungrounded Lightning Rod]

If you are "overkill on magnets" enough that you have to open the gap, who cares if some of the "lost flux" is from leakage at the back of the disk rather than opening the gap?

[oregon wind]

I would rather open the gap.  This gives me more air flow around my statorfor cooling.

Also if the bearings ever wear and possibly wobble just slightly before I notice and rectify the problem,  The added air gap would help prevent the rotors from rubbing the stator.

[oregon wind]

Thanks for the input Flux!

[Flux]

I agree with a bit wider gap for mechanical clearance but for stator heat problems I would use the extra air gap to use thicker wire with a thicker stator and generate less heat than worry about a bit of air flow and the possible but probably small cooling effect.

With the thicker stator and similar number of turns you should manage a significant increase in wire size. It will probably stall badly unless you have thin connecting wire so you will probably need to add a bit of series resistance.

If you use the same winding and just open the gap to keep the same cut in you will have the same stator loss and I doubt that your extra cooling will amount to anything worthwhile ( just a waste of magnet).

Flux

[oregon wind]

Agreed, It is mostly for mechanical clearance and larger wire size in the stator.

[Todd a]

Thicker wire yields less resistance, less voltage drop, and less heat.  I agree that if you need to cool then thicker wire is better.  The only problem is you might have problems with that powerful a generator of a 12' HAWT.  The other thing is the pull on 1" magnets is about 2x that of a 1/2" magnet so be extra careful.  These are monstrously powerful and VERY dangerous.

[oregon wind]

Hey Todd:

The generator will be quite powerful.  However, because I am opening the air gap quite substantially, it will be apropriate for a 12' turbine.

In regards to being careful with magnets, I AGREE.  When I first started playing with neo magnets, I caught both of my hands in between 2 3.5"x1.5" disc magnets and couldn't get them out.  I was alone, and every tool I would try to go near to wedge in between the magnets would violently attract.  To this day I still can't remember how I got my hands out, apparently too much of an adrenaline rush, but I was left with black and blue fingers on my right hand, and some of my flesh pinched off the palm of my left hand.

Thanks for the concern.  I am much more careful with these little buggers now!

[Todd a]

I got 2 magnet plates (8" ring of wedge magnets 2" wide and 1/4" thick) too close and it picked the plate (and bearings and collar and everything) up from about 4" away and caught 2 fingers between them. I had no feeling in the tips for days.

It seems like a waster of mighty magnets really.  Are you planning on upgrading to a larger wind turbine later (maybe with a new stator ring)?  Maybe you got a really good deal, but going with such thick magnets is adding a lot of weight too (and require thicker plates).

[oregon wind]

I'm going for large air gap, for cooling purposes.  This is a bit overkill, but I feel it is justified in the long run.

You are right, if I decide to put larger blades on it in the future, I can close the gap a bit to get more power.

[Flux]

If you want the option of a larger blade later then keeping the gap wide makes sense.

If you don't want to fit larger blades but want reliability then I would just keep a safe mechanical clearance and make the stator thicker to take much thicker wire. You said you were going to increase wire size but I don't know how much. I really think the cooling from a very big clearance gap will be small and the main cooling effect comes from the reduction in resistance of thicker wire and the lower loss generated in the first place. I see no point in having clearances greater than 1/8" between magnets and stator.

I know it will stall hard if you do this but the trick is to bring the loss back but keep it out of the stator. Just widening the gap and raising the cut in speed to avoid stall, leaving the same stator losses and hoping for better cooling I think may be much less effective.

Whichever approach you go for you will need to decide before you settle your number of turns as you will need to decide that before sorting the wire size.

With such an increase in magnet volume I suspect that you could easily have some increase in prop size and still keep the stator much cooler than the normal machines.

Flux

[oregon wind]

Let's assume that I wound 80 turns to make a 48 volt stator with 14 gauge wire.  

I can easily get this into a 1/2" thick stator.  I have a 1/16" - 1/8" clearance between rotor and stator.

By increasing magnet size by double, with the increase in gauss, I could wrap with 13 gauge wire (1/3 thicker) with maybe 60 turns.  This would still fit into a 1/2" stator. I could then have a 3/16" - 1/4" clearance, and produce the same power.

I haven't run the numbers to get the exact amount of turns needed yet.

It would take a little playing with the gap to get the power curve to be similar.

I've achieved two things . . .

first - greater space between rotors and magnets helps me sleep better at night knowing that long term my rotors will stay well away from my stator even with some bearing wear.

second - I have used thicker wire and less turns, thereby reducing heat considerably in my stator.

If I ever decided to.  I could take the same alternator, close the gap a little, which would allow cut in at a lower rpm, and install larger blades for more power.  But that would defeat the purpose of keeping the gap wide and making the stator extra beefy.

[Tritium]

Or use 2  #14's in hand (parallel) for an apparent wire size of #11

Thurmond

[oregon wind]

I made a mistake on my last post.  13 gauge is not 30% thicker than 14 gauge, I think is more like 13% thicker.

[Flux]

My view on this is that a 1/2" thick stator is reasonable for 1/2" magnets. For 1" thick magnets I would be thinking of an air gap between rotors of 1.5". That makes the gap on either side of a 1/2" thick stator very wide indeed. What I was implying was that you keep a nice safe gap of perhaps 1/8" on either side so that implies a stator of nearer an inch thick. There seems little point in running the magnets way off the BH max point.

If you must keep the stator 1/2" thick then your flux density will be way up on the 1/2" magnets and turns will drop well down. Again the reduction in turns will let you use thicker wire.

If you use this extra magnet to your advantage you will not end up with a similar power curve from the alternator.If you attempt do do that then you will be forced to drag the efficiency down to the standard unit, with similar heating problems. You just gain a bit of extra mechanical gap for peace of mind.

By going for a far more efficient alternator and reducing its performance to that of the standard one with external resistance you keep most of the heat external and not in the stator.

I would be looking for a similar number of turns of much thicker wire in a 1" thick stator, but you do it as you please, we all see things differently.

Flux

[oregon wind]

When you double the magnet size from 1/2" to 1", you don't quite double the gauss between the magnets through the stator.  I don't understand how I would be able to jump to a 1.5" gap when the gauss hasn't been more than doubled.  Maybe you could explain.

I am not stuck with the 1/2" stator so going larger is an option.  However I would like to keep mechanical clearance substantial

Thanks for your input