Variable Air Gap

[ChrisOlson]

Non-linear springs, yes.
Headache to set up, yes.
Gyroscopic forces, yes.
Would it be worth it, not sure.

non-linear springs is a non-issue.  Almost all commonly available springs have the spring rate rated in force per unit of travel and they're progressive.

gyroscopic forces on a generator rotor are a non-issue.  Those forces are so small compared to the attraction force between the rotors that they are negligible.

After reading your post this morning, and reading Flux's post I suddenly had a wild idea and this is something that even I would try.  As Flux noted, a three-step system would be better than a two-step star/delta switch.

There is a situation where you need a tighter air gap at high speed than you need at low speed.  That situation is where you built a generator with the proper number of windings in it for delta.  This thing won't cut in until the wind is blowing at 10-12 mph so you start it up in star.  But in star the rotor runs so badly stalled right from cut-in that it really doesn't do anything until it gets switched over to delta.  This is pretty much the situation with my latest 10 foot test machine.

So what you do is widen the air gap at low speeds in star to let it spin to a higher wind speed - say in the 15 mph range or so - before switching it to delta.  But have the governor on the movable rotor start tightening the air gap at around 12 mph so when it switches to delta the transition is less drastic.  As the speed comes up after the switch, the governor would move the air gap to its tightest point, which it needs in delta to really perform.

I have a propensity for trying stupid stuff like this and this is not really all that complicated.  I'd have to work out the exact details on paper to arrive at what speed the generator needs to be operating at at the various air gap settings, and I have most of the bench test data on my test machine to do that.

Thanks for the post on your idea - now you gave me one and I won't be able to stop thinking about it until I try it.
--
Chris

[Flux]

Chris you have found much the same as I did with star/delta. The prop power curve is steep in high winds and matches delta well. With the best match in delta the ratio of 1.7:1 is too great for best star results and it stalls.

The curve is so flat in this region that ideally you would need the variable gap to operate in star mode with small gap at cut in then widening to keep it from stall. When you change to delta you need the small gap.

This wouldn't be impossible but it is far from easy, I think a compromise as you propose will certainly be a great deal better than nothing. If anyone can sort the mechanical side you probably can. I will be interested in any results.

As a mater of interest you may try running single phase in star mode by opening one input lead. This will help the stall issue and the single phase vibration is not a big issue at low power.

Flux

[joseba1]

Stalling... I'm not visualizing a way of stalling with a centrifugal governor, accelerate more slowly, decelerate more slowly, but not full stall.  If you set it up where the governor over-reacted it would kinda give you a see-saw effect, not desirable.

Opening not closeing the gap... Taking the other-way around from a mechanical point of you the governor would have to push open when rpm increased releasing the blades to accelerate faster than you would normally be used to ...

Mechanically not that much different than pushing closed but in that case a linear spring might be in order.

[ChrisOlson]

Stalling... I'm not visualizing a way of stalling with a centrifugal governor, accelerate more slowly, decelerate more slowly, but not full stall.  If you set it up where the governor over-reacted it would kinda give you a see-saw effect, not desirable.

Actually, if you do some testing with a generator you'll find that air gap adjustment is not that radical.  For instance, a 12 pole 9 coil using 2 x 1 x .5 N42 bars and 77 turns per coil (24 volt unit) will cut in at around 72 rpm in star on a 12 volt battery with the air gap at .750.  Open the air gap up to a full inch and it only raises the cut-in to 88 rpm.

Do you have a basic idea of what size machine you'd like to try this with?  Voltage, rotor diameter, and what your goals are for the variable air gap?

Perhaps you're looking at this from a different standpoint than I am.  As I pointed out earlier, the real problem is keeping the rotor out of stall at higher wind speeds and still getting a decent low wind power.  The decent cut-in speed is not to optimize power in low winds because that's a point of no return.  Anything below 10 mph is just to get things spinning and warmed up so it can go to work when the wind starts blowing.  In order to do that - to take maximum advantage of the available wind power from 10 on up - you need to see some trickle amps at around 7-8 mph.  If you don't start making some trickle amps at, say, 8 mph, by the time the wind is blowing at 10-12 and you should be seeing, say, 120 watts, you're only going to be seeing 30 watts because the generator is just coming online.

The turbine that just comes online at 10-12 mph wind speed will perform beautifully at 28-30 mph.  But you'll lose out on 70-80 watts of power, or whatever, at 10 mph where it should be working at the ideal TSR for the rotor already.  This is the value I see in using a variable air gap mechanism - optimizing the output from low wind to high wind.  But I consider low wind to be 10-12 mph.  Not "normal" cut-in speeds of 6.5-7 mph.
--
Chris

[joseba1]

Yeah, I'd like to tell you about the design, I'm actully itching to let it out.  But I will say it's a lift type turbine that can run horizontal and is designed to run in low speed, and, dare I say, turbulent air.  Tests are very positive.  ...unloaded tsr, something like 8-9, not really sure about that, I didn't take it for a drive like a lot of you guys seem to like to do.
The blades can carry a lot more surface area than your standard propeller type turbine so should be able to generate a lot more force...

Having said that then the application is to design something that can gen in low-wind conditions.  Around here our average yearly wind speed is 10mph.   We can have some pretty windy days however and it would kill me to not be able to take advantage.

[ScubaScorpion]

How about trying to rotate one rotor "out of phase" instead of trying to pull it away from the stator?
Would this have the same desired effect?
Sounds a lot easier mechanically.

The other thing I was thinking of was to use your bob weight system to change blade pitch instead, but my pea brain falls over when trying to work out what that does

Hi by the way... First post *gulp.

[ChrisOlson]

How about trying to rotate one rotor "out of phase" instead of trying to pull it away from the stator?

If you have two generator rotors on a shaft with the opposite poles attracting one another at an air gap under 1", you can lock one rotor, put a pipe wrench with a 4 foot cheater on the other one and try to twist it and you will not move it.
--
Chris

[ScubaScorpion]

How about trying to rotate one rotor "out of phase" instead of trying to pull it away from the stator?

If you have two generator rotors on a shaft with the opposite poles attracting one another at an air gap under 1", you can lock one rotor, put a pipe wrench with a 4 foot cheater on the other one and try to twist it and you will not move it.
--
Chris

That's b*****ed that idea then

[joseba1]

Re: The other thing I was thinking of was to use your bob weight system to change blade pitch instead, but my pea brain falls over when trying to work out what that does

There are some simple ways to change pitch...The one that I can think of is by using the force of the wind and/or rpm to self-pitch the blades, makes for a more complicated attachment than a fixed pitch and more to go wrong in a high wind...   Having said that, I guess you could use a centrifugal governor for that purpose it may be going along way to skin that cat.

The way that some people who have responded to my post see a variable air gap as an answer to excess voltage in high wind conditions, pitching the blades is a way to answer that question.  ...probably more simply than a variable air gap.

The way I am approaching it is to actually build the alternator to handle "extra" voltage for the extreme condition.  It makes no practical sense if your charging batteries however. 

[joestue]

The way I am approaching it is to actually build the alternator to handle "extra" voltage for the extreme condition.

what do you mean by that?

[Flux]

I think he is trying to hold the blades in stall in high wind to avoid building a furling mechanism.

I would never use this approach without some back up form of control, if it gets away for any of many reasons you have no control and only blade loss will limit the final speed. May be ok for small machines that are intrinsically strong but very frightening even then.

Flux

[joseba1]

No I'm not trying to avoid building a furling mechanism, or "stall" the blades.  My take on this is to get more electric production out of the unit.

If you'll survey the axial flux alternator marketplace there are those who are building alternators with multiple stator arrangements these extra stators/rotors  come on line in increasing high wind conditions. These units are for much larger systems than what anyone person would probably want to buy.  They are designed to handle the extra voltage.  These are available for purchase now.

My thinking is you could do the same trick for less money and complexity with a variable air gap.

Having said that, I see no reason why one couldn't build an alternator that would gen relatively constant voltage with an inverse mechanism.

[Flux]

I seem to have misunderstood your intentions. I based it on your desire to close the gap in high winds.

What are you intending to feed?  If it is a variable voltage grid tie inverter you need to do nothing except have a big enough inverter for the higher winds.

If it is battery charging then you have got the logic backwards. If your mill is tied to a fixed voltage then your alternator will inherently produce too high a voltage in high winds if you select it right in low winds. What you need is a voltage changing scheme where you reduce the turns in high winds. Once you have reduced turns then in theory you can increase the wire size and lower the resistance.

As I have never found a scheme that can do this we need to approximate. You can certainly achieve part of this by incerasing the air gap with speed and this will keep the voltage rise within limits and prevent stall. It does nothing to deal with the resistance problem so you have a lot of unnecessary wire at high speed with more resistance than you want and this limits your maximum current and drops efficiency.

Winding change schemes and multiple stators can give you chances to reconnect for the correct voltage at certain speeds and get you a near approximation to the ideal variable turn/variable wire size alternator.

Your proposal would probably more accurately suit direct heating schemes where if you choose the load at cut in, the volts will not rise fast enough to track the cube law in high winds. Conventional controllers add more load to increase the power at the lower voltage but your variable gap scheme would let you work with a single fixed load. The wide gap at start up would limit the power into the fixed load resistor to keep you on track near start up where conventional alternators go into stall. If your gap reduction brought you into the correct voltage in high winds to still maintain full load in high winds then it may work very well but you would to shift the gap width very considerable just beyond start up where the prop curve is very flat.

In the end the practicality or otherwise depends on the conditions you are working under but for battery charging it is only a partial solution and you need to shift the gap the other way to see any benefit.

The same is true of the pitch control schemes discussed here. To some extent pitch control can gain you some matching in the working region and increase production from a non ideal alternator but normally pitch control is used to control things once you exceed the maximum alternator rating. It is a very sharp and effective way to limit power and if speed based it has the very real advantage that it protects the blades if you loose load ( furling won't do this).

Flux

[joseba1]

Flux,

I know most on this board, you included, are very practical in nature, I totally respect and admire that.  I feel the DIY wind energy is far more practical than probably most things you can buy.   And, following in the footsteps of successful implementations can give the newbie a feeling of accomplishment.

However, my interest in wind energy is more experimental.  Without getting into some long bullwinder about why I'm interested, I'll just say I feel there is a lot of untapped potential and a lot of unsolved problems. 

I'll let others debate the practicality of WHY something is done.  I'm more interested in the HOW.

j

[jimovonz]

Somehow my fixed gap alternators always seem to manage to produce a fixed rpm/volt... 

I've never tested one yet that does.  If it has 10 rpm/volt @ 120 rpm, it'll be like 9 @ 250 and 8 and 400, depending on the winding configuration and magnets used.

Chris

Oh well now you have: http://www.youtube.com/watch?feature=player_profilepage&v=EtUY8_Pt3PU

By your own measurements you have an increasing rpm per volt:
65.5v @ 483rpm = 7.37 rpm/v
104v @ 770rpm = 7.43 rpm/v
172.6v @ 1284rpm = 7.44rpm/v

[fabricator]

If it runs horizontal its fighting the wind on half the rotation no matter where the wind comes from.

[joseba1]

"If it runs horizontal its fighting the wind on half the rotation no matter where the wind comes from."

Did you mean Vertical?