12V Wind Turbine - Voltage question

[owen_a]

Hi,

I have this wind turbine (http://m.ebay.co.uk/itm/251320008073?nav=WATCHING_ACTIVE&skus=Output:12V&varId=550270984212) in the UK, wall mounted about 18ft, its not in the best position, but it still turns pretty fast, enough to hear the blades spinning from inside, and for it to generate 17VDC (Even though its only rated for 12v).

The issue is, when its spinning quite fast, its only generating about 5-7 volts, 7volts plus is when you can start to hear the blades making a noise. I'm just wondering why it has to spin that fast to generate anything? It seems like it needs Arctic winds constantly just to get any use out of it.

Is there anything I can do? It generates three-phase AC. If I had a 24v turbine, I'm presuming it wouldn't need to spin as fast to generate about 12-14v?

Thanks,
Owen.

 

[sophiecentaur]

Perhaps it is faulty in some way. If the alternator has some open circuited turns, you would expect it to under perform. Or else, the bearings may be worn or the blades distorted so it makes a lot of noise even at low speed (I have had that). IS there a published spec of output volts and current for different wind speeds? My 12V, 2A turbine needs a lot of wind to give any more than the 2A claimed - but that is to spec.
I read the (your) advert and it is a presented in a rather 'quaint' way. Have you any reason to expect it to work as well as they claim? Not everything you can buy on eBay is what it claims to be. OTOH, my Rutland 503 turbine is very low power but does exactly what the makers claim - it cost more than yours when I bought it a few years ago. Perhaps you get what you pay for?

 

[Mike_In_Plano]

Since I was a lad, I dreampt of building one of these for my grandfather. But, my studies have shown that you really do need quite a wind speed to produce the advertised power, and lesser speeds just produce dribbles of energy.
Another frustration, and this is on the part of folks such as yourself, is that these machines produce a varying amount of optimal voltage versus speed. That presents a challenge regarding power conversion. I know any number of people who would start scratching out a design for a switching power converter, but having something on the napkin is a long way from having something in your home that won't start a fire...
If you substituted a 24 volt alternator, you'd simply have scaled the problem to a different range. I suspect someone has dealt with this and produced a power converter to change the varying source voltage to a fixed voltage for use. If not, perhaps it's a good time to use the phone and ask why not.

 

[owen_a]

Hi,

Thanks for your replies. I have seen the turbine reach 8A, not sure at what voltage, but most likely 17v, which gives us about 130watts (from the top of my head). That was only for about a second before the massive gust of wind died down and dropped it to 6A and back down to 0. That was by putting an analogue Amp metre in series with the output of the charge controller I have.

It seems like its working fine, but until I see some sort of graph showing the voltage or amps versus wind speed, I don't know what to think.

Could you tell me what sort of problems i might have if I went for a 24volt instead?

Thanks.

 

[Mike_In_Plano]

A 24V, 300 W machine gives the same power as a 12V, 300W machine when each is running at rated speed, voltage, and current. Tying a 24V alternator into a 12V battery will allow it to charge at 1/2 of rated speed, but it will only deliver 1/2 of its rated power.
The 24V machine has thinner wire to accommodate more windings. Thus, it's made to deliver less current in return for delivering a higher voltage. With a strong wind, you may damage the alternator or blow fuses / breakers.

 

[owen_a]

Ah I see. When you mean damage the alternator, is that because its spinning too fast? It has electro magnetic breaking built in (unless that's down to you for breaking it manually).

 

[sophiecentaur]

Hi,

Could you tell me what sort of problems i might have if I went for a 24volt instead?
Thanks.

There is no point in trying to beat the system in this way. The windings will have been designed with (hopefully) the right number of turns and the right gauge of wire for 12V operation. That will be the optimal design for a 12V battery bank. Working a 24V alternator into a 12V battery bank could roast the wire or, most likely, bring in the Current Limiting regulator (Electromagnetic brake). That, of course, would mean that you get less peak power to your battery.
It may be possible to choose a better site for the turbine than the one you are using (?). A small, hand held anemometer (as used for sailing etc) could be used to test all points in your property. An in-depth survey like that could give you a good return on your effort, in the same way that you would (more obviously) choose the best site for a PV array to avoid shading.
Height can make a big difference.
P.S. Is it too late to send the unit back? I don't like to be too negative but, after the experience of having run my wind turbine for several years*, I would not buy another one. PV has become much cheaper since I made my original choice and, in moderate Latitudes, and not in the "Arctic" winds, the Wind solution for low powers may well not be the best. But the price you paid is surely not too much to spoil the fun of your experiment. It can supplement a PV array if you want a reliable supply.
*They are very noisy in the confines of a small boat!

 

[owen_a]

Hi,

Thanks for your reply. I was wondering sending it back and try to find another turbine that generates around the same but at a lower RPM, although I don't think I will without the price getting higher and higher.

As for the turbines current position, its not in a good place like I said before, its close to the house and only has proper access from three directions. Its not in the middle between the two houses. I originally wanted it wall mounted at the side of the house so the turbine is above the roof, but I'm not sure how much the planning permission will cost on a normal UK house in a culdesack.

 

[sophiecentaur]

another turbine that generates around the same but at a lower RPM

I think it's down to swept area. If you can go twice the diameter (will look massive, I expect) then you can probably use less than half the wind speed. If you look at Websites of 'professional' installers you can get an idea of what's needed for a seriously workable system. It's no wonder that everyone is not doing it. They are very costly.
Is there another wind turbine in your locality? There is only one near me (high on a hill in Brighton) and that is on a School campus.
If you look upon this as a bit of entertainment then you will get good value out of your money but any truly useful system would create howls of objection from neighbours. It could be well worth while doing a bit of a survey, as I suggested. It's seriously hard work to move a support mast to another part of the house, chasing the prevailing wind.
Good luck with it.
(Look at the local Government site to find planning applications for one.)

 

[jim hardy]

I have this wind turbine (http://m.ebay.co.uk/itm/251320008073?nav=WATCHING_ACTIVE&skus=Output:12V&varId=550270984212) in the UK, wall mounted about 18ft, its not in the best position, but it still turns pretty fast, enough to hear the blades spinning from inside, and for it to generate 17VDC (Even though its only rated for 12v).

The issue is, when its spinning quite fast, its only generating about 5-7 volts, 7 volts plus is when you can start to hear the blades making a noise. I'm just wondering why it has to spin that fast to generate anything? It seems like it needs Arctic winds constantly just to get any use out of it.

Well, your statements about speed and volts are confusing.
Are you getting ~6 or ~17 volts?

I note "rated " speed is 1850 which will look mighty fast for a near four-foot propeller.

They claim in paragraph "Introduction"

The turbine begins to supply power in wind speeds as low as 15m/s.

15 m/sec is 33moh. edit> make that mph
But the user manual says

Electricity per month 30KWH（when the wind speed in 5.8m/s）

And specifications say

• Rated current: 25A/12.5A
• Rated speed: 1850r/m
• Max power: 400W
• Number of blade: 3 pcs
• Starting wind speed: 3.5m/s
• Cut-in wind speed: 3.5m/s
• Cut-out wind speed: 18m/s
• Security wind speed: 15m/s
• Rated wind speed: 12.5m/s

Inconsistent at best. 12.5m/s is about 29.8 mph.
Let's take the instruction manual's number of 30 KWH per month with wind at 5.8m/sec
that's ~ 1 kwh/day = 41²/₃ watts, which at 12 volts is ~3½ amps.
at wind speed of 5.8m/sec = 13 mph.

The quick test for a permanent magnet generator is to plot unloaded volts versus RPM and compare to design.
If you're not getting design volts
You could have a defective generator or a defective rectifier.

If you are getting correct volts for a known RPM,
then we need to put a number on your perception of "arctic wind".

old jim

 

[sophiecentaur]

Yes Jim. The devil is in the detail and that spec is not one that should be used in deciding whether or not to invest a 'lot of' money. At that price, we are dealing with a 'fun project' and nothing more. I am sure it is written in the hope that not-too-technical people will buy it as a serious addition to their home electricity supply. In my experience, you really do need pretty high wind speeds for a small wind generator to do its stuff and that involves noise.
eBay would help in any argument about the seller's responsibility to replace the unit if it is not doing what it claims. (As long as the unit is returned soon after the transaction.) If a replacement does the same thing then you should get your money back. But you will need to be able to measure wind speed reliably.

 

[jim hardy]

Decades ago Mother Earth News printed designs for home-made turbines using car alternator and bicycle sprockets.
In those days a 30 amp Chrysler alternator cost only $19 new.
The simple ones without internal regulator are still cheap, probably around $39..

 

[sophiecentaur]

Decades ago Mother Earth News printed designs for home-made turbines using car alternator and bicycle sprockets.
In those days a 30 amp Chrysler alternator cost only $19 new.
The simple ones without internal regulator are still cheap, probably around $39..

Then you need to design and build a suitable turbine which will not shake itself to death and also deliver the required torque at reasonable wind speeds. Getting the balance right would be beyond many home constructors.

 

[owen_a]

I actually have two vehicle alternators, but can't get past the issue of it causing a high amount of torque when it's trying to excite is magnetic field. I thought they'd only be useful for hydro, since after it's got past that 'high torque', it carries on rotating easily while generating electricity. I can get access to alternators at my college, so if there's a better one for wind, tell me which one and I'll see if there's one on the shelf.

As for the turbine, I only wanted something to charge batteries, not something to 'reduce bills'. I have currently got my turbine connected to this rectifier (although proven to be inefficient to it's 2v+ drop on the diodes), and then connected to a 'boost buck' which 'steps up' the voltage to 14.4v which I variably set. I then had the output connected to my solar charge controller so I can see if it's outputting any power when it's spinning, and it reached 14.4v @ 1A - though there was barely any wind at that moment, so it didn't rotate as fast as it did before.

Obviously I dislike this method, as it's like having a power station capable of generating MW of power, and you're unable to fuel it, so you're only able to generate a kW (poor example).

Thanks for the replies though.

PS: jim hardy, remember my previous thread on alternators for wind turbines?

Also, is this the article you meant when on about Mother Earth News?

I found this article as well on this guys alternator-based wind turbine. How does this even work?

 

[jim hardy]

PS: jim hardy, remember my previous thread on alternators for wind turbines?

I recall that there was such a thread but don't remember it very well... "Sometimer's" syndrome i guess, can remember details of conversations from 40 years ago but not 40 minutes...

The Mother Earth News article that i remember was from around 1970
it described a far more primitive machine using mostly bicycle parts
i think the blades were plain flat boards more like traditional windmills in western movies.

I found this article as well on this guys alternator-based wind turbine. How does this even work?

That article is short on data. I believe he needs a LOT of wind to make electricity.
He appears to use a "one wire alternator" which we didn't have in 1970... they must be revved up to a thousand or two RPM to start generating, as explained here:

http://bob_skelly.home.comcast.net/~bob_skelly/alternator_conversion/wiring_alternator1.html

For the 1-wire, self-exciting design, the field windings are not energized via the ignition switch; instead, a special circuit is built into the internal voltage regulator that senses the rotation of the alternator’s rotor. The rotor must turn at sufficient speed to trip the circuit which excites the field windings and starts the charging process. This “cut-in” speed is affected by several things and is typically higher with certain high amperage alternators. Prior to reaching the cut-in speed, the charging system is not activated and the battery will be discharging. However, once the cut-in circuit is tripped, the alternator will charge at all speeds, even very low ones, until the alternator’s rotor comes to a complete stop. At that point, the circuit will shut off and wait for the process to be repeated.

Typically, after starting the engine, the engine must be revved above 1200-2000 RPM to turn-on the 1-wire alternator.

I have not built such a gizmo myself so can't give you an informed opinion.
His pictures look to me like a high wind location - observe how the trees in background lean...
Should you undertake to build one of these, plan on some testing. Friends of mine mounted their wind turbine on a flatbed trailer and took it out on lightly traveled roads to test its performance.

old jim

 

[jim hardy]

I actually have two vehicle alternators, but can't get past the issue of it causing a high amount of torque when it's trying to excite is magnetic field. I thought they'd only be useful for hydro, since after it's got past that 'high torque', it carries on rotating easily while generating electricity.

errrr,,,ummm... not sure i understand the concern over torque.
dont forget energy is conserved
if you expect electrical energy to come out of an alternator , you must put mechanical energy into it as torque X rpm .

decent layman's explanation of excitation here



that's why the first article included a centrifugal switch for the field.

 

[sophiecentaur]

A vehicle alternator is designed to rotate at 3000+RPM at least so using it for wind generation will always require some gearing / chain / belt arrangement to multiply up the speed. That will make the starting friction effects worse and be a serious problem unless some sort of centrifugal clutch (or equivalent) is used to allow the turbine to get going, in order to get over the initial torque needed. It seems a very unsuitable candidate, to me - despite being cheap and available.

 

[owen_a]

errrr,,,ummm... not sure i understand the concern over torque.

If you get an alternator and begin to rotate its shaft by using a drill for example - making sure that there is a bulb in series with the input where the dashlight connection would go, then when it gets up to speed, you'll feel a big torque in which the bulb will begin to go out, since its trying to excite the rotors magnetic field. And of course, it will be inducing a current in the stator, so that will have a magnetic field.

That's why I said I'd only really use an alternator with some sort of water wheel, so I can get a wheel from an old bicycle let's say, and attach that to the water wheel, and you have your ratio. This is because once you've got past that 'excitation torque' it'll freely spin again like it does when you spin it by hand, but it will be generating this time. I only know from experience and experimenting around with them.

 

[sophiecentaur]

But what about the rotation speed requirement? A car alternator doesn't produce much power at ticker - at which speed the alternator will be doing not far short of 3k rpm. Surely what's needed is an alternator with many more poles.

 

[owen_a]

You're absolutely right, I'd still be stuck with a high speed requirement, in this case, a lot higher. The easiest would be to just get a PMA or build one. I actually have an alternator disassembled and just needs the claws being opened to fit a donut magnet in there.

 

[sophiecentaur]

a donut magnet

I donut (haha) think you are likely to get a suitable replacement permanent magnet - even making a suitable yoke could be a problem. (All yoking apart)
There is some sense in there somewhere!

 

[owen_a]

I couldn't think of the correct name, so I thought of something which sort of resembles it, haha.

I see what you did there also *sigh* haha.

There are some sites which sell magnets which could potentially fit. There's videos on YouTube of people doing it, and text based websites of people explaining to you how to do it.

 

[sophiecentaur]

It will all be down to how competent you are at machining stuff to the sort of tolerances that you need for very small gaps and well balanced rotating bits. I can use tools but wouldn't be able to do that sort of thing well enough - I take my hat off to people who can.

 

[jim hardy]

that's why I'm skeptical whether that alternator driven by radiator fan actually works.

I haven't tried your turning an alternator by hand while applying field. That it cogs as the claws pass slots is plausible though, and it you've felt it then it's so.

Here's an interesting article on "souping up " automobile alternators, I've linked to it before in old threads.
At higher RPM they'll make higher voltage, so one can extract more watts at same current.,
http://www.rle.mit.edu/per/ConferencePapers/cpConvergence00p583.pdf

Surely what's needed is an alternator with many more poles.

indeed, at low rpm you need more flux reversals per second to make voltage.
e=ndΦ/dt and with fixed amount of iron you can push only so much flux, so you must increase frequency by adding poles to give more reversals per revolution...
That's why they went to alternators in the first place, so they could add poles thereby improving output at low RPM.
I think 12 is a typical number of poles for a car alternator, compared to two or four poles in the old generators.

Here's an interesting paper by a guy who rewound a Toyota alternator with more turns of smaller wire so he could operate it at low speed.
It was a demonstration so he just wound it single phase..
I only skimmed his article . His intent was to rewind for 200 RPM but he was unable to physically fit in enough turns. He did achieve a design RPM of 251though, but it it's unclear whether he actually tested it.
http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1145&context=eesp

 

[Merlin3189]

A car alternator doesn't produce much power at ticker

Can I respectfully doubt this. I think car alternators produce useful output at tickover (800rpm). Engine speed is multiplied 2 - 3x by pulleys to rotate alternator at ≅ 2000rpm . Some alternators are produce 50% of rated output at that speed. (Ref.[/PLAIN] Delco-Remy)

 

[sophiecentaur]

I think car alternators produce useful output at tickover

I would like to believe that but it goes against my experience with a marine diesel (1973 vintage) with a fairly elderly but recently referred alternator. With the engine on tickover the 'lamp' will often stay on and my clamp meter never shows any significant charge current until I have much more than 1k rpm. The regulator was replaced not too long ago and it does the same thing. I was even contemplating finding a smaller alternator pulley to beef things up a bit. I guess it's probably a matter of age (alternator's not mine).

 

[jim hardy]

there's a pretty common failure that'll give one that symptom

The lamp alone does not pass enough current to "flash" the field.
So there's a resistor in parallel with the lamp, to deliver more current than the lamp alone would allow.
On old Ford trucks it's behind the dash on instrument cluster and virtually inaccessible.

When that resistor burns open the lamp still illuminates
and the alternator will commence charging if you rev it fast enough to bootstrap itself up with that feeble field current, on friend's old Ford truck a little more than 2000 engine RPM,.
But with a good resistor it ought to bootstrap itself at idle.

On a friend's truck we installed a "Field Flash" pushbutton.
A headlamp would probably have done same job.

old jim

 

[OmCheeto]

Can I respectfully doubt this. I think car alternators produce useful output at tickover (800rpm). Engine speed is multiplied 2 - 3x by pulleys to rotate alternator at ≅ 2000rpm . Some alternators are produce 50% of rated output at that speed. (Ref.[/PLAIN] Delco-Remy)

From my experiments and shop manual data, I concur.
The shop manual for my 1984 Isuzu impulse lists the following output vs rpm for the alternator (I added the watts):

 rpm  amps   volts  watts
 800     0    13.5      0
 900    10    13.5    135
1000    20    13.5    270
1200    30    13.5    405
1500    40    13.5    540
2000    50    13.5    675
2600    60    13.5    810
4500    70    13.5    945

Rpm is for rotor speed. Speed ratio is listed at 1.96. Engine idle is 900 rpm. [ref = shop manual]

So the alternator has a minimum speed of 1760 rpm.
Eyeballing that against the chart, it would be putting out ≈45 amps.
From my recollection, the alternator light never came on at night, with everything on, sitting at a stop light.

I don't know what automobiles normally consume, but when my alternator tension pulley fell off my last vehicle, I hooked up 3 x 50 watt solar panels, and drove home, which is about 12 miles. So with no auxiliaries, the car could not have been pulling much more than 10 amps, as that is about what the panels were capable of providing.

that's why I'm skeptical whether that alternator driven by radiator fan actually works.

Me too.
Matching the maximum output of my alternator to the output from the Wind Power app @ The Engineering Toolbox, I come up with a windmill diameter of 3.2 meters, to get 965 watts, @ 10 m/s (22 mph).

3.2 meters is about 10 feet.
hmmm... That's pretty big.

 

[jim hardy]

From my recollection, the alternator light never came on at night, with everything on, sitting at a stop light.

The light sees the difference between battery and alternator voltages.
That's why i prefer an ammeter, it'll show you when alternator can no longer keep up with demand, usually because of worn brushes.
A voltmeter can tell that also but requires closer attention,

Nice data, OM

 

[OmCheeto]

The light sees the difference between battery and alternator voltages.
That's why i prefer an ammeter, it'll show you when alternator can no longer keep up with demand, usually because of worn brushes.
A voltmeter can tell that also but requires closer attention,

Nice data, OM

It's been quite awhile since I've owned a vintage vehicle. I think the alternator generator light on my '61 beetle was always on, regardless of how steep the hill was I was driving down.

But I probably should thank @owen_a for starting this thread. I've been considering turning that old alternator into a windmill project for years, but never got around to doing the math. I'm now shying away from a HAWT, and will probably build a VAWT.

VAWT: Vertical axis wind turbine
HAWT: Horizontal axis wind turbine

I saw a VAWT design a few years ago, that used sailboat inspired physics. I can't find it now, but I thought it was pretty cool.
(Mainly because I'd thought of the idea myself years before, but discarded it, as mere Om style, wishful thinking silliness)

[edit] Just found it!

Warning, if you don't speak French, turn off the sound.

​

 

[OmCheeto]

Oh dear. That video is from 2006.
Vaporware!
Glad I never tried to build one.

It did strike me as a bit of an engineering nightmare.
But beautiful to watch, in a physicsy kind of way.

 

[Averagesupernova]

I have rebuilt a few older delco alternators and what I find is this: There is a 6-diode bridge off of the stator that is the main rectifier. This feeds the voltage regulator and the main battery post. The output of the rectifier assembly is the point that the regulator looks at to determine whether or not to increase the field current in order to raise the output voltage based on a comparison to an internal voltage reference. Then there is a diode trio that connects from the stator to another terminal on the regulator. This I consider a power terminal. If this terminal has no voltage on it the regulator will not allow field current in the rotor. This terminal ALSO is fed through the dash board indicator when the ignition is turned on. So what happens is when the ignition key is turned on the regulator is powered up through the light bulb. This is enough to tell the regulator to turn the field on based on the battery voltage. When the alternator comes up to speed the diode trio rectifies the output from the stator and the regulator is now being powered by the diode trio. The dash indicator will no longer light since it has about 12 volts on each side of it. At this point the bulb can be removed with no change in operation of the alternator since the regulator is being powered through the diode trio.

 

[jim hardy]

If this terminal has no voltage on it the regulator will not allow field current in the rotor. This terminal ALSO is fed through the dash board indicator when the ignition is turned on. So what happens is when the ignition key is turned on the regulator is powered up through the light bulb.

Good description.

I'm accustomed to a resistor in parallel with the lamp. I both gives you alternator should lamp burn out and it let's "bootstrapping" start at lower RPM than with lamp alone.

 

[owen_a]

@OmCheeto thanks!

I was actually going to build a VAWT using one of these alternators (I think they're delco, I'll check later). I used a bicycle wheel and a 200cm fan belt to get my ratio. I think it was 1:10. The only thing that stopped me, is the fact that it wasn't easy turning that wheel. You needed a little bit of force which I doubt the wind could do unless if it was really windy. This is when I thought that a HAWT might be a lot better since the force against the turbine is stronger. However, the blades would need to be quite big - you did the math above.

What if you had a resistor in series with the field coil, so it reduced the amount of current in order to self excite? Would it still self excite but reduce that initial torque? There was a site which I was reading a few months ago, and they used an alternator as a generator for a water wheel, and used a resistor to do something like this.

 

[OmCheeto]

@OmCheeto thanks!

I was actually going to build a VAWT using one of these alternators (I think they're delco, I'll check later). I used a bicycle wheel and a 200cm fan belt to get my ratio. I think it was 1:10. The only thing that stopped me, is the fact that it wasn't easy turning that wheel. You needed a little bit of force which I doubt the wind could do unless if it was really windy. This is when I thought that a HAWT might be a lot better since the force against the turbine is stronger. However, the blades would need to be quite big - you did the math above.

Actually, I didn't do any math. I just plugged numbers into the "computer" until I got my answer.
The equations are on another page:

Power generated from the wind
The theoretically available power in the wind can be expressed as
P = 1/2 ξ ρ A v³
where
P = power (W)
ξ = efficiency of the windmill (in general less than 0.4, or 40%)
ρ = density of air (kg/m³)
A = area wind passing through perpendicular to the wind (m²)
v = wind velocity (m/s)

If you look at the variables in the equation, there's really only one that you have control over, and that's the "area" of the wind turbine.

 

[Merlin3189]

"If you look at the variables in the equation, there's really only one that you have control over, and that's the "area" of the wind turbine." I think this a very important point. There is a danger of thinking, 'if only we used a different alternator, or geared it up, or down, or whatever', then we could make it work. But the essential point is, there is only a certain amount of energy/power available from a moving mass of air (and apparently*, we can only extract about 60% of that.) Nothing you can do with your alternator can alter this.

All you can do is to optimise the extraction / conversion process, so I think you need to look at the steps in the process and see which ones can be improved. Two items related to the alternator which I think could be changed to help you are both electronic.

What might help you most with your main problem (getting output at low wind speed) is synchronous rectification. Instead of using diodes, as shown in the circuit of #33 and used universally in automotive systems, you can use mosfet transistors (and associated control circuitry.) Diodes have a roughly constant voltage drop of 0.7V, so that two in series in the standard circuit drop 1.4V. To get 12V output you therefore need to generate at 13.4V and waste 10% of your power in the diodes. Mosfets do not have a constant voltage drop, but a very low resistance of only about 0.01 V per amp. So at low currents, say up to 10 A, the voltage drop is less than 0.2 V, wasting only 1 - 2% of your power. At high currents you eventually lose out to diodes, but then you are not so worried about losing a bit.

Another thing you can do with controlled transistor rectification, is simply to switch it off at low speeds. This could solve your starting torque problem: if no power is being extracted, there is no electromagnetic drag and your alternator should rotate freely, limited only by friction.

I think the reason this does not seem to feature in many amateur wind turbines, is that automotive alternators usually have diodes built in and possibly regulation as well. But I guess it would not be too difficult to remove or bypass them.

If you get away from automotive alternators and have them made specifically for your purpose (possibly, increasing the cost 10x of course!) then you can also do things such as SophieC's suggestion of increasing the poles to make it function at lower speed (but higher torque.) Doing so does not give any more power, but by avoiding gears / pulleys will reduce frictional loss.* "Betz Law
In 1919, a German physicist Albert Betz, based on conservation of momentum and energy, he proved that the maximum possible energy that can be derived from a wind turbine cannot be more than 59.3 percent, or 16/27 of the potential energy in the wind.
In practice, no wind turbine has ever achieved the Betz limit."

 

[owen_a]

What might help you most with your main problem (getting output at low wind speed) is synchronous rectification. Instead of using diodes, as shown in the circuit of #33 and used universally in automotive systems, you can use mosfet transistors (and associated control circuitry.)

Using MOSFETS? hmmm, I've never used anything other than diodes for rectification. Something like this maybe? (second picture)

 

[jim hardy]

Something like this maybe? (second picture)

Clever, that self biasing... No timing circuity required to synchronize them.

 

[Rive]

I have currently got my turbine connected to this rectifier (although proven to be inefficient to it's 2v+ drop on the diodes), and then connected to a 'boost buck' which 'steps up' the voltage to 14.4v which I variably set. I then had the output connected to my solar charge controller so I can see if it's outputting any power when it's spinning, and it reached 14.4v @ 1A - though there was barely any wind at that moment, so it didn't rotate as fast as it did before.

If you change the rectifier bridge to some schottky type you can save some on the voltage loss (without spending too much on special rectifiers), but I think the root of the problem lies elsewhere. You already nailed it with using a charge controller, but on the other side you also lost it with using the step up converter.
A charge controller (at least a good one) will try to optimize the power pulled based on the input voltage: higher voltage comes in -> it'll draw higher current (it 'thinks' that when there is more light/wind/whatever there will be higher voltage, so it'll pull more juice from the source).
But with the step-up you already fixed the voltage -> it'll always work with the current which belongs to the 14.4V -> the system will deliver only the fraction of it's maximal potential.

Also, as this setup will draw only a ~ fixed power, it cannot effectively slow down the rotor in high wind: it'll always overspeed (this is what you are experiencing, right?).

I think what you need is simply to match the output voltage range of the turbine and the charge controller - and forget the step-up.

 

[jim hardy]

What if one collected his power at 6 volts instead of 12?
Seems to me he'd halve his speed requirement
and a slow turning propeller has better efficiency , as the Wright Brothers figured out.
http://wrightstories.com/propeller-design-demonstrates-the-genius-of-the-wright-brothers/

 

[OmCheeto]

If you get away from automotive alternators and have them made specifically for your purpose (possibly, increasing the cost 10x of course!)

As a perpetual pauper, that's never been a viable option for me.

then you can also do things such as SophieC's suggestion of increasing the poles to make it function at lower speed (but higher torque.) Doing so does not give any more power, but by avoiding gears / pulleys will reduce frictional loss.

Hence, why I hobble together old junk, and do the sciency stuff:

The last experiment was to determine how much power was being lost by the fan belt.
It turned out to be around 100 watts.

I decided that either direct drive, or a more flexible serpentine belt would be more efficient.

[ref]

 

[jim hardy]

Hence, why I hobble together old junk, and do the sciency stuff:

that's how real progress happens.

I kept this poster over my desk for years.

picture courtesy of http://www.greenwichworkshop.com/details/default.asp?p=2106&t=0

 

[Howard]

Sounds like you need to gear up the drive between propellor & alternator. See the alternator performance curves to get the correct speed. I designed & built one of these a while ago that still works well. I'll dig through my piling cabinet; keep your fingers crossed.
Regards, Howard.

 

[Merlin3189]

Owen & Jim point to a mosfet circuit which I had not realized could be done and seems a good simple idea as an immediate replacement for a diode bridge.
I had been think of a more complex circuit like this or an even more complex one (which I can't find a link for at the moment) which included input from a regulator. I must say though that I have not built nor used any of these circuits, I am simply aware of the technique and thought it might be another step improvement to the system.

I notice a comment, " it cannot effectively slow down the rotor in high wind: it'll always overspeed (this is what you are experiencing, right?)." and wonder what the issue is here? It seems to me that, if you draw current to slow the turbine by loading it, you risk overheating the windings. Isn't that the problem of high speeds? Would it not be better to limit the output current at high speeds and let it run as fast as it wants? If mechanical damage due to centrifugal effects is the worry, would it be better to have some sort of mechanical governor to feather the blades say? (Maybe mount the blades on a spring-loaded screw, so that as centrifugal forces pull them outwards - physicists please excuse my language - they twist slightly.)

Oh, & BTW OmCheeto, "As a perpetual pauper, that's never been a viable option for me." i heartily concur ! I wasn't so much suggesting you did this, as pointing out that difficulty if you wanted to have a tailor made alternator. All respect and encouragement to those who are attempting to do this themselves.

 

[jim hardy]

If one accepts the complications of a boost/buck regulator he can collect several times the alternator's rated power by letting it run at higher speed,
Higher voltage at rated amps = more watts;

and he can generate in low wind at lower RPM. where he can only make less than 12 volts .

Owen's self biasing mosfet rectifier circuit should enhance efficiency at low voltage

that MIT link in post 24 should really enhance high wind performance.I predict some young folks will be making smart rectifiers with Arduinos soon enough.

A 150 amp car alternator at 40 volts would absorb 6kw divided by its efficiency,
surely that'd keep most homebuilts down to safe speed short of a hurricane?

 

[sophiecentaur]

Imo, it would be the turbine design that would impose the final limit on the overall power obtained. That's a far more difficult problem to solve than making a good rectifying circuit.

 

[OmCheeto]

Oh, & BTW OmCheeto, "As a perpetual pauper, that's never been a viable option for me." i heartily concur ! I wasn't so much suggesting you did this, as pointing out that difficulty if you wanted to have a tailor made alternator. All respect and encouragement to those who are attempting to do this themselves.

A windmill is now #743, aka "at the bottom", on my list of random projects to build.

Imo, it would be the turbine design that would impose the final limit on the overall power obtained. That's a far more difficult problem to solve than making a good rectifying circuit.

I ran across a 59 page paper yesterday on wind turbine tower design. Pages 23 through 29 list just the variables involved.
6 pages of variables.
I can't even solve a problem with 6 variables, period.

But I do like Merlin3189's idea of blade feathering.

 

[Rive]

I notice a comment, " it cannot effectively slow down the rotor in high wind: it'll always overspeed (this is what you are experiencing, right?)." and wonder what the issue is here?

In high wind you not just 'can' draw the rated power of the turbine, but you also 'have to' draw that power to keep the RPM within the specified limits.
Your actual setup cannot draw that power, so the RPM goes out of the specification.

 

[Merlin3189]

Imo, it would be the turbine design that would impose the final limit on the overall power obtained. That's a far more difficult problem to solve than making a good rectifying circuit.

Absolutely agree. Pretty much everything about the electrical conversion process is understood. How we apply it depends mainly on costs. But the wind! I have read that Einstein hoped that when he died, God would explain turbulence to him. When I read about something like mosfet rectification I can see at once how it could help, but if I see a picture of a wind turbine, I often have no idea whether it's better or worse than another, or even whether it would work at all! But being so complex does make it a plausible area for experimentation.

you also 'have to' draw that power to keep the RPM within the specified limits.

I think maybe I missed something in the posts along the way, but I'm still not sure what this RPM limit is about. I'm not saying there isn't good reason to limit the rotary speed, just wondering what it is.
If you do draw power to keep the RPM down, you are still putting stress on the mast / mounting (which may or may not be your critical factor.) If you let the rotor freewheel, then you do reduce this stress, though your centrigfugal stress rises. Depends what your main concern is.

 

[sophiecentaur]

I'm not saying there isn't good reason to limit the rotary speed, just wondering what it is.

Is it not to avoid it tearing itself off its mounts and chopping up everything it meets with? I must say, feathering is a lot safer feeling than electric braking but it's a much more complicated mechanism. I guess a cheaper half way house could be to have a method of steering it off the wind with the vane when the speed gets excessive - an electromagnetic clutch could do the job but you'd need to have a pretty stable control system or it could end up spinning round and round on its vertical axis.