Effects of opening air gap?

[Volvo farmer]

OK, I dropped the tower again and opened my air gap some more. This is a 24 volt DanB built 10 footer. round magnets and the "HD"stator. It is wound with 2 strands of #15, 55 turns.  I measured the air gap and it is right at 3/4" now.

It's not real windy today, but I noticed the difference immediately... better speed and better output at low battery voltage. I was having stalling issues before at 26V but the thing flew great at 29V. Now the batteries are floating on solar at 26.8V and the turbine is cranking out 10-15A in what I consider pretty mild winds (sorry no anemometer).

So, having burned up my first stator (thinner wire, rectangular magnets) last fall in a wicked storm, I'm concerned about burning this one up. I know that getting the furling right is the most important thing. The machine seemed to furl well at 29V before.  I never saw the tail fully fold up, but it was not able to sustain 40 amps, the rotor seemed to push itself out of the wind every time I saw a big power spike.

So I guess what I want to know is... Is this thing going to make way more power than it did before at 28-29V and if it furled OK before, has opening up the air gap changed it's furling characteristics?  Right now, I'm afraid to let it run unattended until I can see what it does in good strong winds.

Thanks

[ChrisOlson]

So I guess what I want to know is... Is this thing going to make way more power than it did before at 28-29V and if it furled OK before, has opening up the air gap changed it's furling characteristics?  Right now, I'm afraid to let it run unattended until I can see what it does in good strong winds.

It should furl earlier than it did before because the rotor is soaking up more of the wind power and will create more thrust.  At least every turbine I've adjusted to bring out of stall and make them really fly, that's what they did.

3/4" is not a lot of air gap for those magnets either.  If you don't care about getting it to cut in at 5 mph you can safely open it up some more if you feel the blades are not running in their powerband yet.  I had my 13 footer at .975" before I wound a new stator for it with less turns of wire. That one has 1/2" thick bar mags in it and it still shut down fine with the shorting switch even with the air gap at almost an inch.  You got over 50% more magnet surface area than I got with those 2" round mags so I don't think it would be a problem to open it up to .800-.900" if you want to really let those blades fly and get more power yet.

You'll know when you've opened it too far because the blades will get noisy from running at too high of a TSR.
--
Chris

[Flux]

Can't add much to what Chris has already said. Normally increasing power out increases thrust and it furls in lower wind speed.

There is the question whether furling in lower speed will have more effect than the increase in output but it will probably be ok but I agree that you can't leave it safely until you see what it does.

You are furling and I suspect you will be ok. The failures normally come from the ones that don't furl and then pull out of stall in a high wind, some of these are so badly stalled that there is no hope of furling in the stall condition so you really have no idea where it is going when it gets out of stall.

I have always thought that most people go for too low a cut in speed and it comes at a big price in top end performance. If you are clear of stall now you are probably about right but I agree with Chris that you could try more gap. If it knocks your low wind performance badly then you have gone too far but you might still be in the improvement region.

Flux

[ChrisOlson]

I have always thought that most people go for too low a cut in speed and it comes at a big price in top end performance.

What I've learned with axial flux generators is what Windstuffnow Ed had figured out a long time ago - many people tend to use magnets that are too powerful.  If you use a magnet that has a bit less power with more surface area, like 1/4" thick wedges, it allows you to wind with a decent number of turns to get good copper density in the stator without running ridiculously wide air gaps.

You actually get better performance by doing that than by winding less turns with magnets that are too powerful, and it took me a lot of experimentation to figure out that's how Ed gets 50 amps out of a 6.5' 12 volt turbine, and yet the machine will cut in at 6.5 mph. His machines have excellent low wind performance and they don't go into stall when the wind picks up because the generator is easier to turn at high rpm's than one with lots of magnet power. It maintains a closer match to the blades' power curve from low wind to high wind speeds.

When you have one like VF has where there's good copper density in the stator PLUS very powerful magnets - that generator "tightens up" a lot quicker when you try to increase its rpm under load.  So, for only a 10 foot rotor, and going by what VF describes that he has, my guess is that the air gap is still too tight to get the performance out of it that it should deliver.
--
Chris

[Volvo farmer]

Thanks for the replies.  The "soaking up more wind power, creating more thrust" statement with regards to furling was very helpful. Now I'm not as worried.

I was actually pretty happy with the way the turbine was working yesterday. It seems like my cut in speed is still quite low as I was seeing an amp or two with hardly any wind at all. Our windy season is about over but I will continue to watch it and see how it behaves. I'm especially interested in how it will perform in 25-30 mph winds, but we might not see those for a while.

Although I'm growing a little weary of dropping the tower and jacking this thing apart, I will keep in mind that I can experiment with opening the air gap even further.  Thanks again for the help!

[ChrisOlson]

I was actually pretty happy with the way the turbine was working yesterday. It seems like my cut in speed is still quite low as I was seeing an amp or two with hardly any wind at all. Our windy season is about over but I will continue to watch it and see how it behaves. I'm especially interested in how it will perform in 25-30 mph winds, but we might not see those for a while.

Sounds like you're in the ballpark on what you expect from your turbine.  I've found that the most useful tool for tuning a wind turbine is a Doc Wattson meter.  It will record the cumulative amp-hours for you over a period of time.  Then you can make an adjustment, like opening air gap, reset the Doc Wattson and see if you get more or less over another period of time equivalent to the first test period.

If you got an anemometer that will record the average, low and peak wind speed over a period of time, that's another handy tool for tuning.

Like Flux noted, you should probably keep an eye on the machine the first time you get any real wind in case it would do something unexpected.  But with the air gap at 3/4" and the heavy duty stator you got you shouldn't have any problem shutting it down if for some reason it would get out of hand in high winds.  That's one nice thing about a 10 footer - they make decent power in decent wind, but they're not so big that you can't bring one back under control that goes wild on you in high winds.
--
Chris

[wiboater]

When you say a 3/4 inch air gap are you talking about a measurement between the 2 magnet plates or The clearance gap between each side of the stator to the magnets. In other words you have 3/8 on each side of the stator right?

[TomW]

When you say a 3/4 inch air gap are you talking about a measurement between the 2 magnet plates or The clearance gap between each side of the stator to the magnets. In other words you have 3/8 on each side of the stator right?

wiboater;

The Air Gap is the space between the magnet faces where the stator lives.

Not sure what anyone else meant just one of the conventions in this sport!

Tom

[chazsql]

Tomw - So, from how you replied, I gather if someone has a 3/4" air gap and a 1/2" stator, there is 1/8" clearance between the magnet faces and the stator.  Did I understand you, correctly? 

[bob golding]

to save some of toms brain cells.  i will answer that question. yes that is correct.

[wiboater]

When you say a 3/4 inch air gap are you talking about a measurement between the 2 magnet plates or The clearance gap between each side of the stator to the magnets. In other words you have 3/8 on each side of the stator right?

wiboater;

The Air Gap is the space between the magnet faces where the stator lives.

Not sure what anyone else meant just one of the conventions in this sport!

Tom

Thanks, That is what I thought but just wanted to make sure.

[chazsql]

Volvo Farmer - What was your air gap before you increased it to 3/4"?

[Volvo farmer]

Volvo Farmer - What was your air gap before you increased it to 3/4"?

The machine had different rotors and a different stator when I first got it. Upon installation of the new stuff, it was recommended that I add a set of washers behind the front rotor and adjust the stator. I have now added three sets of washers,  with thicknesses of about .0800, .1000 and .0850 respectively.  (Take your calipers to the hardware store, not all stamped washers are the same thickness!)

[Ungrounded Lightning Rod]

When you say a 3/4 inch air gap are you talking about a measurement between the 2 magnet plates or The clearance gap between each side of the stator to the magnets. In other words you have 3/8 on each side of the stator right?

wiboater;

The Air Gap is the space between the magnet faces where the stator lives.

Not sure what anyone else meant just one of the conventions in this sport!

Tom

Thanks, That is what I thought but just wanted to make sure.

Unless you have an iron (etc.) core in the stator (in which case it's the sum of the gaps from the N and the S magnet to their respective nearby pole pieces on the core.)  The "air gap" is the total amount of not-core-not-magnet in the closed magnetic circuit.  Copper coils with nothing but air or resin in the middle of them count as "air".

Designing iron cores into the axial-flux pancake style homebrew alternators is a pain in the tail, unnecessary, and causes problems (especially cogging) which more than negate the benefits (mainly not needing as much magnet - when the main problem with neos is they're often TOO strong).  The "just the 'air' part" is for things like motor conversions and other cored-coil radial flux designs.

[jlt]

OK, I dropped the tower again and opened my air gap some more. This is a 24 volt DanB built 10 footer. round magnets and the "HD"stator. It is wound with 2 strands of #15, 55 turns.  I measured the air gap and it is right at 3/4" now.

It's not real windy today, but I noticed the difference immediately... better speed and better output at low battery voltage. I was having stalling issues before at 26V but the thing flew great at 29V. Now the batteries are floating on solar at 26.8V and the turbine is cranking out 10-15A in what I consider pretty mild winds (sorry no anemometer).

So, having burned up my first stator (thinner wire, rectangular magnets) last fall in a wicked storm, I'm concerned about burning this one up. I know that getting the furling right is the most important thing. The machine seemed to furl well at 29V before.  I never saw the tail fully fold up, but it was not able to sustain 40 amps, the rotor seemed to push itself out of the wind every time I saw a big power spike.

So I guess what I want to know is... Is this thing going to make way more power than it did before at 28-29V and if it furled OK before, has opening up the air gap changed it's furling characteristics?  Right now, I'm afraid to let it run unattended until I can see what it does in good strong winds.

Thanks :)You say that you never saw the tail  fully fold up . I would be concerned too . have you done any mods to lighten tail or increase offset.