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Air gap question
By TheCasualTraveler, Section Newbies Posted on Fri Mar 14th, 2008 at 09:56:32 PM MST
magnets on one side only

I re-did my first alternator to add a second steel disc as Flux suggested so now I have an 8" disk with 12 of windstuff Ed's wedge magnets, a stator 3/8" thick and then a second steel disc (no magnets) to complete the flux circuit.
Adding the second disc gave me a more steady output at, and a little above cut in but with very good wind today it is being held back to only about 200 to 250 RPM at most. Before adding the second disc it would be spinning out of control with this wind.
The obvious thing to do (I think) is open up the air gap a little. The gap now from magnets to second disc is about 5/8". Another 1/8" should give it a little room to stretch out. Now I'm wondering...
Question - Should I just move the second disc out leaving a small space between magnets and stator and a larger space between stator and the second disc, or should I keep the stator centered between the discs? Or, does it not really matter?
I'll do it tomorrow and would like to set it up right the first time.

Air gap question | 8 comments (8 topical, 0 editorial)

Re: Air gap question (3.00 / 0) (#1)
by wpowokal on Fri Mar 14th, 2008 at 05:09:40 PM MST
(User Info)

How much is it producing at your assumed speeds?

Why do you want it to spin out of control?

To your question, it's the total air gap that maters so you choose.

allan down under
A life lived in fear is a life half lived.

Re: Air gap question (3.00 / 0) (#2)
by TheCasualTraveler (a.miklos@yahoo.com) on Fri Mar 14th, 2008 at 05:26:47 PM MST
(User Info) http://thecasualtraveler.com/wind.htm

Cut in is about 100 RPM +or- The winds are 15 to 20 plus gusts and it's only putting out 1 to 1 and a half amps. In these winds it's not even spinning enough to make that whirring sound. Before I added the second disc it would put out 3, 4, 5 amps and more in winds like this. I don't want it to spin out of control I actually like this damping effect I would just like it to be around 5 to 600 RPM or so.

But if the answer to my question is that it doesn't matter where the stator is in the gap then I will just start moving the second disc out till it raises cut in and speeds up some.
Andy
[ Parent ]

Re: Air gap question (3.00 / 0) (#3)
by wpowokal on Fri Mar 14th, 2008 at 10:46:11 PM MST
(User Info)

OK I see, apart from mechanical clearances it does not matter which side, the magnetic flux has to bridge the air gap at what point it passes through the windings of the stator is "basically" irelevent.

You need the speed for output, Ed has those magnets on special at the moment why not make it a proper duel rotor. Your origional post indicated increasing the air gap by 1/8" which is a fair bit so that should help a lot, If it was mine I would like to have both gaps even for mechanical clearances, but depending on how you have your stator mounted how easy that is to achieve.

Remember flux density is not linear with distance, for you it's a bit of a suck it and see so give it a go and gain some important data at the very least. We added 1mm to the air gap on my duel rotor for clearance reasons, and that raised the cut in from 140 to 240 rpm, maybe it would produce a tiny bit more if it was lower but I am very happy with it as is.

Oh and let us know the results.

allan down under
A life lived in fear is a life half lived.
[ Parent ]

Re: Air gap question (3.00 / 0) (#4)
by Flux on Sat Mar 15th, 2008 at 02:41:57 AM MST
(User Info)

I don't have the details of your winding but I am fairly sure you have lots of turns of thin wire. Now you have improved the magnetic circuit you have hit stall. You would do far better to raise the cut in speed by reducing the number of turns or connecting in delta or Jerry connecting it.

If you just increase the air gap you will be back close to where you were before and the thin wire with high resistance will still be a limitation. You may get some sort of compromise that is better than before but you will not gain the full potential.

Regarding the air gap on this set up with magnets on one rotor and not on the other, it is largely the total gap that matters but you will see more increase in speed with the stator nearer the blank disc compared with it near the magnet disc. You need to make coarse adjustments with the total gap but you can get fine adjustment by altering the stator position.

If this is a 12v system, before you make any changes try running it temporarily into 24v if you can find a spare battery. You will likely get far more power with the present winding.

Flux

[ Parent ]

Re: Air gap question (3.00 / 0) (#5)
by TheCasualTraveler (a.miklos@yahoo.com) on Sat Mar 15th, 2008 at 06:55:00 AM MST
(User Info) http://thecasualtraveler.com/wind.htm

This is fascinating stuff. I know I built an inefficient machine but I did it to learn from. I wanted to be able to have a low cut in for experimenting, thus the 120 turns per coil. I didn't want to have to wait for good wind days to test it and this is only that, a test alternator. Now I have a machine that hits stall and I can see firsthand what that entails.

Seems to me that if I,

" will see more increase in speed with the stator nearer the blank disc compared with it near the magnet disc"

then leaving the stator close to the magnet rotor and moving the plain disc out will make for the smallest increases in speed. So, I'm guessing as I slowly move the plain disc out increasing the air gap, cut in speed will rise as will the speed at which it stalls. That would suggest that I could increase the air gap until stall is at 5 or 6oo rpm, limiting the machine by using stall instead of furling. If thats true, it would be intresting to design a small machine that way.

Oh and Flux, we discussed jerry rigged and 24 volt charging as opposed to 12 volt in a previous post and after trying it, it appears my machine is somewhere in the middle. Too slow for 24, too fast for 12.

Thanks folks, I'll let you know how it goes.

Andy

Re: Air gap question (3.00 / 0) (#6)
by Flux on Sat Mar 15th, 2008 at 08:25:43 AM MST
(User Info)

" Oh and Flux, we discussed jerry rigged and 24 volt charging as opposed to 12 volt in a previous post and after trying it, it appears my machine is somewhere in the middle. Too slow for 24, too fast for 12."

Yes I have been looking back at that and it was true before you added the second disc. Now I think you have made it too slow for 12v.

Can't find details of the prop but 100 rpm is too slow for a 6ft wooden prop with the usual tsr.

You are learning fast. You need cut in slow enough just to get it cut in at about 7mph, there is no power in lighter winds to justify going lower. Even then you should aim to get tsr above the prop's ideal at cut in. You will then have a band where tsr is falling rapidly with wind speed but you are well up on the prop characteristic.

Now the bit about stall is not so easy to deal with. If you decide to use stall limiting as a protection you will hit the performance in modest winds very hard indeed.
You seem to be in this state at present ( but I think it is only because your cut in is too slow).

Stall is only likely to take place on the low slope of the prop's power curve between 12 and 20 mph. If you alter the air gap or add resistance to get out of stall in this region you will come up on the steep bit of the prop curve and it will never hit stall again.

I think you have to accept that you stall early and have simple control or you get out of stall mode and loose this tame nature in higher winds. You then need to rely on furling as a protection.

With Dan's big 20 ft machines there is lots of energy available at say 10 mph and if you stall at 15mph then you can live with it and have a nice tame machine with easy control.

For a 6ft machine you only have about 40W to play with in the low winds ( 10 mph) If you go for stall at 12 mph then you will probably never see above about 60W.

Your best option is not to attempt to cut in below 7mph ( 8mph may be better), never quite stall and furl when you reach the alternator limit. That will mean an alternator falling to about 50 % efficiency at somewhere over 20 mph.

This may all be wrong if you are using pvc blades but the general idea still applies although the performance may be well down.

At present with your low cut in and thin wire I suspect you are reaching 50% efficiency well below 20 mph. Increasing air gap will certainly bring you out of stall but the low efficiency will remain because of the winding resistance. You will gain considerable output with a wider gap as you get the prop up onto its power curve but a great deal of the power you gain will go into cooking the windings.

I still suggest you try at 24v with the extra disc and you may hit a far better compromise.

Flux

[ Parent ]

Re: Air gap question (3.00 / 0) (#7)
by TheCasualTraveler (a.miklos@yahoo.com) on Sun Mar 16th, 2008 at 10:34:50 AM MST
(User Info) http://thecasualtraveler.com/wind.htm

OK, the results.

As I said I added the second steel disc to complete the flux circuit and that put the machine in stall. So I then moved the second steel disc out 1/8" (the thickness of a washer) and sure enough, stall was gone but power was still improved compared to before the second disc. It was an exceptional wind day yesterday, 10 to 20 with gusts around 30. In the lowest winds it was putting out about 1 amp with 2 to 3 amps being common and I saw 1 gust take it to 7.25 amps (12 volts). I'm very happy with the results and grateful for the experience and help from everyone. Thank you all. Most of the math escapes me but I'm getting the "feel" for this stuff.

As is I still have the 64" pvc blades so I need to carve a set of 6' blades and re-test. I "feel" I will then have to bring the disc back in half of what I moved it out. Or, maybe not. We'll see.

I also have 12 more of the wedge magnets but was reluctant to epoxy them to a second disc for dual rotor thinking they might best be used on a 10" disc. I could remove the 12 I'm using on this 8" disc and make a new alt with 2 - 10" discs, 24 magnets total, and a stator of decent gauge wire instead of the #22 I'm using now, but I digress and besides right now I am studying the effects of what I have now.

I have to add some type furling after the new blades and then the final step will be putting it on the higher roof of my house which will put the top of the blades at 35' above ground. As of now they are only about 22' above ground in turbulant air.

Thanks ED for the parts and yes, I'm having fun!

Andy

Re: Air gap question (3.00 / 0) (#8)
by wpowokal on Mon Mar 17th, 2008 at 05:32:22 AM MST
(User Info)

Congratulations Andy you are gaining valuabe experience, I would go with the 10" discs.

allan down under
A life lived in fear is a life half lived.
[ Parent ]

Air gap question | 8 comments (8 topical, 0 editorial)

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