How do I foretell Voltage?

[LEXX]

I'm at the beginning of building a PM generator and i need some help, there must be a way of deciding how long to make your coils to get the proper voltage.  For instance, I would like to make it 3 phase with 12 coils of 14 ga wire, so each phase will have 4 coils connected in delta, what should these coils ohm at?  Is ohming the proper method? and what if i used 12ga wire, would the ohms still be the same for a given voltage.  Oh yeah, my TSR is about 6, the prop is already made and being tested, and i plan to use 16 1.5" x 3/8" neo magnets per rotor on a two sided rotor design.

Thanx,

LEXX

[iFred]

Hi Lexx. The only way to know for sure what your generator will really produce is by testing. I would suggest making your rotors up and putting a single coil between the rotors and see what the coil gives you. this will give a good idea of what's going on. As for ohms, this is a rough estimate....If the coil is less then 100 turns of #14 then it will more then likely be less then 1 ohm per coil.  Delta hookup is mainly used for higher speed applications it seems according to this board (please quote me if I am wrong here!). . The 3/8 magnets are a bit on the small size, what is optimal is to get as thick a magnet as you can, this prevents magnetic shorting to the rotor plates (in essence higher is better)...As for the magnets, the hole in the center of your coil equals the size of the magnet and approx just a tad smaller (say about a quarter of an inch or so).. . What size are the rotor plates?

Hope this helps!

[LEXX]

If thicker is better then why does hugh piggot reccomend your magnet's thickness be 1/4 the width?  the rotor plates are 12 inches across, but they can be bigger or smaller, the frame for this is made for pretty much any size up to 15 inches so that i can do some experimenting, i think it should be strong enough for about a 14 foot prop as well.

LEXX

[Flux]

Lexx

3/8 thick will be ok. It should be about right for a 1/2" thick stator, to work at about the most efficient point.

Thicker magnets will allow a smaller number of turns for a given air gap. For a given size of machine you can always squeeze a bit more out with more magnet material if you are not looking to make the most cost effective use of the magnets.

Flux

[DanB]

Hi Lexx  -

Fred, and others have put fourth some good comments, and Ill add a bit.

Search this board for things like "coil shape" etc... to find lots more thoughts on the matter*.

I wonder how big your prop is?  If you know that - and you know the approx TSR that you want the prop to run at, then you should be able to determine the approx rpm that the alternator needs to cutin at.

For any given stator, and any given rpm - then the power available will be directly related to the magnetic flux through the coils.  If the magnets are too small... the prop will overpower the alternator - itll overspeed, the stator might overheat.  There are more, and less "efficient" stator designs when it comes down to the flux required to get the right result.  It's a bit tricky I think!

I usually take the approach of using a lot of magnet (because I sell them and have lots and lots available ;-)  ) and then build my stator and often find I have to open up the airgap so that the alternator doesnt overpower the prop.. causeing the TSR to fall off and the blades to stall.  Or... sometimes this allows me to run a thinner line from the batteries to the windmill I think.  There has to be a certain amount of resistance in these sytems - too little, and the prop will stall - too much, and the prop will overspeed - and, the stator might heat up too much.  The bigger the magnets you use (the stronger the flux) then the lower the resistance will be in the stator.  You can choose to either have resistance in the line, or in the stator... (there will always be some in both).  Using xtra large magnets means lower resistance in the stator - and you  may have to add some to the line.  Xtra $$ spent on magnets = less $$ spent in the line perhaps.  (am I sounding like a salesman??... these are my current thoughts anyhow).  If your running at low voltage, and the distance is long - saving a wire gage size or two in the line can add up.

That wasnt exactly your question though...

"there must be a way of deciding how long to make your coils to get the proper voltage."

Voltage is directly related to rpm, and it's directly related to the number of windings in the coil.  Since you should know exactly what rpm you need to cutin at - then you shoot to make coils that will do that.  When I start with a new design, I layout the magnet rotors Ill be working with.  Then I draw a template that is exactly the shape that I think the coils should be.  Some will say that the hole in the middle of the coil should be the size of the magnet exactly... or some will suggest slightly larger.  My thought is the hole in the middle should be exactly as high as the magnet is tall, and slightly less in width.  This is after a few fairly unscientific test I've done and I could be wrong. I think the rules may change a bit between disk, and bar magnets. Basicly though - the hole should be about the size of the magnet.  If the magnets are well spaced, Im of the opinion that the OD of the coils should be such that they pretty much touch each other in the stator.  

So once I know exactly the right shape and size of the coils, I would take some wire (usually Ill use a fairly fine gage but it doesnt matter too much) and wind one coil to exactly the right size.  Then, poke it in between the magnet rotors, and spin them at a known rpm - and measure the output of that one coil.  You'd be measuring single phase - probably RMS (depending on your meter) AC voltage.  Actual voltage in your case then, since youd have 4 coils per phase - and youd probably be wiring in Star, would be the output of the coil, X about 1.4 (this gets you about the peak voltage instead of RMS), and then multiply that X 4 (that would be voltage for 4 coils in series) and then multiply that times 1.72 (that would be about final AC voltage youd expect to see from the alternator with 4 coils per phase in series, wired in star.

and thats just 1 test coil.  Well call 'A' the number of windings you need, and 'X' the number of windings in your "test coil"

Since voltage is directly related to rpm - then you can figure how many volts your test coil would've produced at cutin rpm from whatever rpm you tested your coil at.

We'll assume your cutin voltage is 12 volts, for a 12 volt system.

So A/12 = X/whatever voltage your test coil makes at cutin rpm.

(since we know what X and whaterver voltage your test coil makes at cutin rpm then it's easy to figure out 'A')

(I think maybe Im making a very simple idea sound too complicated)

Wire gage is double the area every time you go down 3 gages in size.  (or darned close).  So... if you fit 200 windings of #20 gage in your test coil, and the results suggested you needed 50 windings, then you'd know that you could fit about 50 windings of #14 wire.

In a nutshell - I find the right size/shape for the coil,I wind a test  coil exactly that shape/size, from that I figure the right number of windings (or try...) and then I figure what thickest possible gage of wire will fit that number of windings into the given space.  Once you've picked a given size magnet and built your magnet rotors, the goal for me to to achieve the desired cutin speed with the lowest possible resistance (thickest possible wire that allows for the correct number of windings)

[Flux]

Lexx

Lots of good advice from Dan.  You say you have made the propeller, how big is it?

You say you have gone for TSR 6. That and the size should settle the cut in speed.

If those magnets are round they would go on a 12" disc, even a couple of inches bigger wouldn't hurt.

If we know your prop size then we can get some idea of the size of alternator needed.

Flux

[LEXX]

First off, thancks for all the knowledge guys, I appreciate it I didn't expect to get an entire manual from Dan but more info the better.  The prop I've built is 12 feet across with three blades, it looks pretty much like the ones on the home page labled "the triplets" other than i didn't have the patience to make that nice rounded carving at the very base of the blade, i found that just cutting with a saber saw at the proper angle got me to the same place in probably less time.  One thing tho Dan, the only two things that would let you run a smaller line to your batteries would be fewer amps or higher voltage.

LEXX

[DanB]

Hi Lexx...

"One thing tho Dan, the only two things that would let you run a smaller line to your batteries would be fewer amps or higher voltage."

I'd argue that a bit...

To some degree of course, we cannot run too thin a line.  But it's often the case, especially when we have very low resistance in the alternator, that we need to add resistance somewhere.  There has to be a certain amount of resistance in the system to allow the prop to speed up with the wind and maintain it's ideal TSR.  Consider the extreme case... if the resistance in the system (by system I mean the alternator, and the line) were 0, then it would take about an infinite amount of torque to speed the alternater up above cutin speed.  When resistance is too low in these, we might have the right cutin speed, but we might be producing way too much power above cutin speed for the prop to keep up.  To solve that...  we can add resistance to the line.

So far as the system is concerned, resistance in the line is no different than resistance in teh alternator (except Id prefer to have heat in the line than in the alternator!).  If your alternator is wound up with #14 wire...  that's just as much of a loss as if you had #14 wire in the line.  So.. if you use larger magnets, and keep the resistance in the alternator down - you can then add more to the line.

In one of the machines we made last summer, the line was only about 8' long from the bottom of the tower to the batteries.  It was a 10' machine, capable of around 50 - 60 amps in low 20mph winds.  But the alternator was producing way too much - the higher the wind above cutin the lower the TSR got and the output in higher winds was dismal.  By adding about 30' of #14 gage extension wire to the line... we got it to perform rather nicely!  A similar machine, 100' from the batteries wired up with heavier wire might have worked fine.

So I'd stick with the argument... resistance in the alternator is inversly related to the size of the magnets we use, and the lower the resistance in the alternator is.. the thinner the line we can get away with.  We've got to have a certain amount of resistance somewhere.  (of course... to much resistance in the system and the prop will overspeed  - it has to be within reason)

[LEXX]

Hi Dan,

I see what you're saying but why wouldn't your battery charger provide you with enough resistance.  This may be wrong in this situation, I just assumed that the less in-line resistance in your wire the more efficient the machine.  

LEXX

[DanB]

In the case of a wind turbine, the line, and the windmill is your battery charger...  and it has to have some flexibility to run at various speeds, and currents... dependant upon the wind.  So there has to be some resistance.  It could be 0 resistance (if the stator and the line were of superconducting material...(this is a hypothetical extreme case...) and it would be 100 % efficient at cutin speed, but above cutin speed, as the wind increased, there would not be enough power in the wind to speed the alternator up, so the rpm would remain constant - the output would remain the same, and the prop would stall and become inefficient.  By having resistance in the line, we sacrifice some efficiency in the electrical side of things in order to pick up efficiency in the prop.  This is the definitely the case with PMA's.  I suspect, if we had an electromagnetic field like most alternators (no perm. magnets) - then we could have very low resistance across the board and regulate the field so that it'd be more efficient all around, especially in high winds, but energizing the field would hurt us in the low winds.

So you're right in that that resistance is costing us...  but it's allowing the prop to run more efficiently, and if we come out ahead when it's all matched up properly.

[Flux]

Lexx

If you already have those magnets they should be ok for a 12ft prop if you furl at about 20 mph.

With them on 14" discs you should be able to get 1kW but you will probably have enough resistance in the winding without adding to the line.

I tend to agree with Dan that it is better to keep the losses ( heat) outside the machine as far as possible.

If you havent got the magnets yet I would think about something like the 2" x 1" x 1/2" blocks.

Cut in at 150rpm would be reasonable.

Flux

[DanB]

I would agree on the magnets.. they are probably a decent size for a 12' machine, if you have 16 per disk, it sounds well within reason.  But I would try to furl it early.

(just guessing)

Cutin speed... I wonder why you say 150 rpm flux, that seems quite fast for a 12' diameter machine, I'd be shooting for more like 120, perhaps even a bit less on a 12' machine.

[Flux]

Dan

I did a few quick sums on those magnets and thought they were a bit marginal on size.

If you go for a cut in at 120 it will compromise the top end somewhat.

Ultimately it depends on the wind conditions and how much you depend on very light winds.

The prop is TSR6 so I think it will run up to 8 at cut in, at about 8 mph. The TSR will fall quite rapidly above cut in, and I personally think it will give a better energy capture in higher winds as you can maintain the same TSR with less resistance.

If you live in a part of the world where there is consistant wind around 7 or 8 mph

it may be better to go for a lower speed.

Here, when we get those sort of winds they are very intermittent and the energy produced is not a lot.  I tend to think that if it is producing something at 7 or 8 and on good efficiency by 10 mph it will be ok.

Just my thoughts, you may not agree.

Flux

[DanB]

Yes.. that makes sense to me.

I would consider in this case perhaps running with a slightly smaller prop.

[Flux]

Yes I have run the figures again  and those magnets would better suit a 10 ft prop with cut in at 150, but if Lexx has made the prop and bought the magnets I would still give it a go at 12 ft.

Might be worth trying the magnets on 15" discs

Perhaps compromise for a cut in at 130 and make sure the furling comes in at 20 mph or a bit below, as it is never sharp.

Flux

[LEXX]

I am actually going to make a few props and tails to give me several different aspects on this.  This is a truly experimental machine, my plan is to build a few different rotor sets and about 6 different props just to see the different results.  Figure what the hell it only took about 6 hours to build the first prop and it was 12 feet!  Thanx for all the feed back, what size prop and TSR (I'm thinking 6 for this one too, no need to have too many variables) would you guys reccomend for a machine with 16 1 inch round .5 inch thick on each rotor?

LEXX