Help me spec my turbine, please.

[chux0r]

I've been lurking around this site for some time now, and I feel I've learned enough to dive in and attempt my own turbine.  I'd like to run my "plans" by the folks here, and get a feel if what I plan to do makes any sense at all, since I'm not too clear on the mathematics of the situation.

Like most people, I'll be trying to blend the conflicting values of doing this on somewhat of a budget, while at the same time getting some reasonable output.  I'd like to spend less than $200 on the turbine itself, and I'll deal with all the electronics later.  I already have a 12V bank and inverter, so that seems to have me set on 12V at this point.

It will be the classic dual rotor axial flux air core deal, since that design seems to be well-studied around these parts, so everyone should be able to give me a great deal of help.

I'll start with a front-end wheel bearing and brake rotors from a Plymouth Reliant.  We have had a dozen K-Cars at our house over the years, so these parts are 100% free.  The brake rotors are a bit small, about 9.5 inches in diameter, but I also only plan on using 1" magnets, so it should be all good.  Precisely I plan to use 12 of the 1" diameter by 1/4" disc neo (product #1) on each rotor.

I'd like to build a 12V unit, so this is where I need the real help.  All of the designs on the Otherpower site are using the giant 2" x 1/2" magnets.  That's nowhere near my price range, nor my desire for safety.  So, with 1" x 1/4" magnets, how many turns do I put in my coils to get my 12V?

Do I have to increase the number of turns to compensate for the weaker magnetic field, or is that how it works at all?

If so, I can make some assumptions... The strength of magnets seems to be roughly analogous to the raw volume of magnetic metal, so can I assume that since the 1" magnets have about 1/10 the volume of the 2" ones, that I will need 10 times the number of turns in my coil to get the same voltage?  That doesn't sound right to me at all... Since most of the 12V rotors using 2" magnets are using around 30 turns, that would give me some 300 turns.  I'm going to guess that's wrong thinking, since that much wire at the gauge required to fit in a 1/4" stator would have significant resistance at 12V.  So, I need some help here.

I'm thinking of building an 8' diameter blade, but I'm also thinking this might be too big.  Here's another area where I need some guidance.  I decided on 8' mostly because if I go and buy a board it will be 8 feet long.  I've not seen a website that says "I wish we'd made the blades smaller" so I assume that making them slightly too large is not a big issue.

A lot of variables, that's for sure.  I know the best way to get this right is to build the magnet rotors and build a test coil, so the best suggestion I could receive right now is what gauge of wire to buy.  If my number of turns will have to be large, I guess I'll be around 16 gauge.  Otherwise, I'd go with 14, and I can double/triple it up if I want fewer turns than that, since it seems the popular preference is for winding with 14 gauge wire.

Umm... yeah... I'm long winded.  So... advice, then?

[hiker]

once again-- its all their on the old otherpower board..

check it out........

[hiker]

check out the [experiments section]..........

[Flux]

Seems like quite a small  beast. Probably big enough for 6ft prop, certainly not big enough for 8ft.

Keep your magnets right at the edge of the discs.  Ideally you ought to find some wire perhaps from a scrap transformer or motor and do yourself a test coil to find out how many turns you need.

When you know this, you can calculate what size wire will fit in the space and order the right size.

The magnets are thin so keep to a stator about 3/8" thick. ( 1/2" between magnets)

Try your first test coil at about 60 turns.

For 12v dc you need 12/1.4 volts ac line volts. ( 8.6v ac )

This is  8.6/1.7 volts per phase  =5v ac.

One phase will have 3 coils, so you would expect 5/3 v for one coil.( 1.67v ac )

This is assuming that your test speed is the cut in speed. If you have to test at another speed you will have to scale the results.

Hope this gives you something to start from.

Flux

[tecker]

  Be flexable in your building practices if you have some extra rotors make one up with silicon  and use a plywood form like hiker did to test your magnet/coil configuration

and when you got it working on the bench and have some data turn your attention to the blades and hug assembly.

[nobicus]

Hug assembly?  Now you're talking my language!!!  Lady volunteers queue here!!!!

[chux0r]

Thank you, Flux.  This is very detailed and informative... just what I was looking for.  So to prove I understand correctly, I'll try to work the same calculation forward from the coil...

I should get a single coil to read ~1.67VAC with my multimeter at cut-in speed.  (This is RMS, and my meter will measure it correctly?)  I'll wire three coils in series and get 5VAC for each phase.  You didn't mention it specifically, but I assume this is wired in star, which is why the 3 phase output is 5 * 1.7 = 8.5VAC.  The rectified output will be 8.5 * 1.4 ~= 12VDC?

How do I measure the 3 phase output?  Does that mean if I measure any two legs, I should get 8.5VAC?  I know it's a simplistic question, but I haven't seen this anywhere.

Also, my understanding is that in a delta configuration, I would just leave out the factor of 1.7, and I would have 5VAC and 7VDC?

Yes, I know I'm probably asking redundant questions.  Once I understand 100%, I promise I'll help the next newbie.

Thanks.

[Flux]

Yes that's right.

The waveform will be near enough to a sine wave for you to measure it with a normal digital multimeter.

I did assume it was star connected and yes that is where the 1.7 comes from.

Personally I would avoid delta if at all possible use it as a last resort but the losses will be higher at cut in.

You should get 8.5 volts between legs, rms ac.

You can measure the dc output of the rectifier with a small resistive load across it such as a small wattage lamp.  If you try to measure it with no load on the rectifier you will get silly answers.

Flux

[chux0r]

My figuring is an 8' blade should be capable of 300 watts in a 20mph wind... the higest I'll reasonably be able to expect in my area.  You don't think these magnets are capable of that?  The 'alt from scratch' generator at WindStuffNow got reasonable power with fewer, smaller magnets.  If there's too much power in the blade at speed, I can furl it out, but I really want to get as much power from the lower windspeeds as possible.  Thanks.

[chux0r]

Thanks so much.  I've never seen it so clearly described, so I finally have a great deal of confidence that I understand this.  Your patience is commendable.

[Flux]

An 8ft prop ought to be producing nearer 700W at 20 mph.

If you are prepared to furl very early and want the best possible output in very low winds there is no reason why you couldn't use an 8ft prop.

The downside is that your alternator will be running at a lower speed and so for the same efficiency you will get less out of it.

If you want to go for 8ft I would base cut in on 200rpm, that is a bit higher than usually used but it will probably give you a better compromise.

You will have to furl at a wind speed that keeps the temperature of the coils within limits, probably at 300 to 350w.

If you have a very low wind speed the 8ft prop would make sense.  At those low winds you could probably live with the noise even if you took your prop up to tsr8. With an ironless alternator starting will be no problem.

Flux

[JB]

probably with them magnets and that size diameter rotor I would recommend nothing heavier than 17 gauge.  your coils are going to be small.That is just from my own experiments. Search the new board to find it.I posted it about 4-5 months ago? Good luck. JB

[Flux]

ChuxOr

The figures I gave you do not allow for the drop in the rectifier and also a 12v battery spends much of its time at about 13.8v so to be a bit more accurate you would be better taking the battery voltage as 15 in stead of 12.

Flux

[tecker]

 always there for last call g is the recipical of b

[devoncloud]

besides all of this, I would also put alot of emphasis on how thin of a rotor you will need considering how thin your magnets are That 1/4 inch thickness is usually with a good size magnet.  Also, trying to gain as much output from your machine as Ed did will be difficult.  He has been doing this stuff for years, and I am not familiar with the exact project of his you refered to, however most of his machines are done much differently than the dual magnet rotor design.  Most of the time he uses laminates or slotted steel laminates.  He also likes to make more coils (many smaller ones but ends up being about the same amount of wire after it is all said and done)which in my opinion drastically helps output for two reasons: (a)smaller air gap and (b)space better utilized in the stator.  Doing the coils in this way helps to produce much more power.  That's the way I see it anyway.

I think it is a great idea that you are starting small.  I don't know why but the first ones always don't turn out quite right.  I studied for almost a year before I got my feet wet, followed Hugh's plans to the T on a dual magnet rotor design, and it still did not produce as well as I hoped.  You do seem to have picked up a bunch of the measurement formulas and stuff and that is great.... those still intimidate me some, but then math and measurements have never been my strongpoint anyway.

As far as your blades, do them last.  Think about what you would like them to do though.  You should always think about your alternator and prop at the same time, but sometimes your alternator output ends up being very different than your specs on paper told you that it would produce.  When this happens you can make up the difference in blade design.

Devon

[scoraigwind]

Here is a rule of thumb for finding how many turns of wire you need in the coils.

I am going to assume that you are wiring the coils in 3-phase star (wye).  I am going to give you a number of 'turns per phase'. If you are winding 12 coils for example then you have 4 coils in each phase.  If you need 500 turns per phase then you can do this with 4 coils each of 125 turns in series.  Or you can connect 4 coils (each with 500 turns) in parallel but I do not recommend this so much.

To find the turns per phase using my formula you will need to know the following things:

System voltage V  (for example 12 or 24)

Desired cut in speed RPM  (for an eight foot blade try around 200 rpm or less)

Strength of flux in Tesla (Ferrites could give you about 0.2 or 0.3.  Using neos you can expect 0.5 or better for dual rotor machine but it does depend on the air gap and stuff like that)

total area of magnet face in square inches  (This will be the number of magnets times the face area in sq inches)

Turns per phase = 17000 x (System voltage V +1.4) / ( Desired cut in speed RPM) / (Strength of flux in Tesla) / (total area of magnet face in square inches)

[scoraigwind]

Here is a rule of thumb for finding how many turns of wire you need in the coils.

I am going to assume that you are wiring the coils in 3-phase star (wye).  I am going to give you a number of 'turns per phase'. If you are winding 12 coils for example then you have 4 coils in each phase.  If you need 500 turns per phase then you can do this with 4 coils each of 125 turns in series.  Or you can connect 4 coils (each with 500 turns) in parallel but I do not recommend this so much.

To find the turns per phase using my formula you will need to know the following things:

System voltage V  (for example 12 or 24)

Desired cut in speed RPM  (for an eight foot blade try around 200 rpm or less)

Strength of flux in Tesla (Ferrites could give you about 0.2 or 0.3.  Using neos you can expect 0.5 or better for dual rotor machine but it does depend on the air gap and stuff like that)

total area of magnet face in square inches  (This will be the number of magnets times the face area in sq inches)

Turns per phase = 17000 x (System voltage V +1.4) / ( Desired cut in speed RPM) / (Strength of flux in Tesla) / (total area of magnet face in square inches)

Example.

You plan to use 12 magnets that are each 1" diameter.  Total area of magnet faces is 12 x 3.14 / 4 = 9.4 sq inches

System voltage is 12

You want to use a fast 8 foot rotor, so cut in rpm is 200

You are using neo magnets but the magnet thickness is only 1/4 inch while the gap between magnets is 3/4 inch so we can guess flux density is 0.4

Turns per phase = 17000 x (System voltage V +1.4) / ( Desired cut in speed RPM) / (Strength of flux in Tesla) / (total area of magnet face in square inches)

= 17000 x 13.4 / 200 / 0.4 / 9.4 = 303 turns.  

If you have nine coils total and thus 3 per phase then you should have 100 turns per coil.  Connect the coils in series and then the groups in star and feed to the rectifier. Your maximum output will only be about 1-200 watts, but that's another story.

[chux0r]

Right, I figured 12 was just an example.  Thanks.

[chux0r]

That's great.  While I know the only way to be really super-accurate is to test it in the real world, I do like formulas to ballpark a starting point.  This will help me a lot.  Thanks.

[chux0r]

As far as your blades, do them last.

Umm... about that....

Well, my magnets haven't arrived yet, so I started working on the blades first.  The blades are also my personal biggest challenge.  (I'm more comfortable working with the steel and spinning things than carving wood, so I thought I'd tackle the wood stuff first.)

So much for advice.

[mtbwr]

To find the turns per phase using my formula you will need to know the following things:<p>
System voltage V  (for example 12 or 24)
Desired cut in speed RPM  (for an eight foot blade try around 200 rpm or less)
Strength of flux in Tesla (Ferrites could give you about 0.2 or 0.3.  Using neos you can expect 0.5 or better for dual rotor machine but it does depend on the air gap and stuff like that)
total area of magnet face in square inches  (This will be the number of magnets times the face area in sq inches)
<p>
Turns per phase = 17000 x (System voltage V +1.4) / ( Desired cut in speed RPM) / (Strength of flux in Tesla) / (total area of magnet face in square inches)
<p>
Example.

You plan to use 12 magnets that are each 1" diameter.  Total area of magnet faces is 12 x 3.14 / 4 = 9.4 sq inches
System voltage is 12
You want to use a fast 8 foot rotor, so cut in rpm is 200
You are using neo magnets but the magnet thickness is only 1/4 inch while the gap between magnets is 3/4 inch so we can guess flux density is 0.4<p>
Turns per phase = 17000 x (System voltage V +1.4) / ( Desired cut in speed RPM) / (Strength of flux in Tesla) / (total area of magnet face in square inches)
= 17000 x 13.4 / 200 / 0.4 / 9.4 = 303 turns.  <p>
If you have nine coils total and thus 3 per phase then you should have 100 turns per coil.  Connect the coils in series and then the groups in star and feed to the rectifier. Your maximum output will only be about 1-200 watts, but that's another story.

What is the number 17000? Is that some kind of constant?

[SparWeb]

Definitely a conversion factor.  Teslas are SI units, and RPM is imperial, so are inches.  Somehow the answer has to make all the units cancel out.