3 phase alternator design

[Igor]

Hi everyone -

It's my first time posting here so please be patient if I mess up....

I'm starting to do some experiments with wind power, and as I live in the English suburbs, only a few feet away from both my neighbours, and as I don't have a few acres to put up a tower and mill, any wind power projects must be safe, quiet and unobtrusive.

So - I've built a small model VAWT as my starting point. It's based on the Lenz2 design, but using 2 stacks of 2 blades set 90 degrees apart. It's only 20 cm diameter and 50 cm tall (8" X 20" but it sure spins at an impressive RPM (unloaded so far)

I know it's only a toy, but it would be nice if I could get a little useful 12V battery charging out of it before I go on to build something larger. So - I've built the rotor, now I need to build a small scale alternator to go with it.

I've had a look at Windstuff Ed's site and followed through the "3 phase basics" page OK - yes - I've followed it so far - but I'm wondering if there's a website that takes it a tad further ?. One of the concepts that I haven't yet got to grips with is having a different number of magnets to coils like many perm magnet alternators seem to do - and how to work out the spacings and numbers of coils and magnets. Could anyone point me in the right direction without it all getting way too technical ?

Also, is there a rule of thumb when working out what gauge of wire  / number of turns per coil when you're designing your alternator, or is it just best to look at what's gone before and base your design on that ?

I'm thinking of building a small experimental 3 phase alternator on the "brake drum" style, using magnets salvaged from computer hard discs (cut in two). As the wind speed here is less than ideal most of the time, I'd like the alternator to charge in as low a wind as possible,  even if it's only a very small trickle charge. So my thinking is to build an alternator with an effective diameter at least as large as the Lenz VAWT that it's coupled to. That will give a higher speed of the magnets past the coils, and hopefully generate more voltage, even if it's at the expense of lower current.

I'm also thinking that if I scale this up to a larger version in the future, that having a large diameter alternator will not only give a higher magnet / coil speed per rpm, but allow more room for the coils, so the advantages could be thinner coils (more magnetic flux through each coil, and less heat build up / more cooling surface area per coil) and the opportunity to use thicker wire and have less internal losses.

So please feel free to knock me down ...

Dave.

[DamonHD]

Have a look at my urban wind power stuff in London:

http://www.earth.org.uk/wind-power-pilot-autumn-2007.html

Rgds

Damon

[SparWeb]

I wrote a brief "how-to" article last year about the axial flux alternator.

AXIAL_FLUX_HowItWorks.pdf

Maybe it will help give you a basis for understanding how it works, and then it will be easier to figure out what you need to make.  Designing these things requires quite a balancing act.  There are some helpful tables in the back, too.

[Igor]

Thanks guys.

Some helpful links there.

I think there's enough info in that pdf document, along with the info on the Windstuff Now site, for me to start getting the design together. The magnet to coil ratio thing had been puzzling me. Also interesting info on magnetic flux lines.

Right - I'm off to order some copper wire !

Anyone got any tips on slicing hard disk magnets in half ? (I'm thinking of using an angle grinder with a 1mm slitting disk)

Dave.

[Igor]

Ok, more questions if you don't mind.

The hole in the centre of the coils needs to be the same width as the magnet? yes ?

The width of the "legs" of the coil should be the same width as the magnet? Would it be a mistake to make the coil legs slightly wider than the magnet, so as to make a thinner coil to decrease the gap between magnets, or is "magnet width the same as the coil leg width" regarded as the optimum ?

If you're building a relatively large diameter alternator (to increase the magnet/coil speed), what about gaps between adjacent coils ? Should the coils be as close as possible to each other or is it OK to have gaps between ?

Thanks in advance ....

Dave.

[Igor]

Ok, more questions if you don't mind. (sorry about posting twice, I'm still getting to grips with the way this board works - I meant to post at the bottom)

The hole in the centre of the coils needs to be the same width as the magnet? yes ?

The width of the "legs" of the coil should be the same width as the magnet? Would it be a mistake to make the coil legs slightly wider than the magnet, so as to make a thinner coil to decrease the gap between magnets, or is "magnet width the same as the coil leg width" regarded as the optimum ?

If you're building a relatively large diameter alternator (to increase the magnet/coil speed), what about gaps between adjacent coils ? Should the coils be as close as possible to each other or is it OK to have gaps between ?

Thanks in advance ....

Dave.

[spinningmagnets]

I'm a newbie who isn't experienced at building PMA's, but I recently learned here that getting a magnet too hot (perhaps when cutting them in half) can damage its magnetic strength, so, take your time, and keep them as cool as possible. Please post the method you use, and how the magnet cutting turned out when you're done.

[DamonHD]

I think you may be confusing me with someone who knows about alternators!  I was leaping in on the urban wind bit!

I cheated and bought my PM gen(s) so far...

Rgds

Damon

[SparWeb]

The hole in the center of the coil can be smaller than the magnet.  But, the smaller the loop, the less effective it is, so you just don't gain much.  Don't make the hole larger than the magnet, or you're winding a coil too big.

As for your second question, I think you wind a coil that is a bit thicker than your magnet - but there are some caveats to that rule and I've fallen victim to making the stator too thin, myself.  Epoxy gets soft when it's warm, and if it flexes slightly, the stator could come in contact with the magnet disk.  It's more a structural issue than a performance issue - the gap has to be adjusted according to the power the prop can put into the alternator.  

The answer to your third question is that you might as well put a few more wraps on each coil if you can, and pack them in with minimal gap between.  This isn't the end of the world.

[wooferhound]

Check out these stories about generators built from Hard Drive magnets

They all talk about how to cut/break the magnets . . .

http://www.fieldlines.com/story/2006/10/8/112046/572

http://www.fieldlines.com/story/2005/9/24/152446/359

http://www.fieldlines.com/story/2006/2/8/124610/0181

http://www.fieldlines.com/story/2006/2/9/13128/15117

http://www.fieldlines.com/story/2005/3/4/141238/1524

http://www.fieldlines.com/story/2006/9/27/112728/576

[Igor]

Thanks for those links -

It verified what I had thought - 1mm slitting disc on an angle grinder to cut the magnets in two. I use these for my classic VW restoration work so no problems there. I was also wondering how to hold them whilst cutting, but it seems they will hold themselves on a piece of steel just fine.

I occasionally get the odd scrap hard drive through work, and I have about 8 magnets so far. There's a couple of computer shops nearby so I'll have to go and pay a scrounging visit.

Here's my Mk1 Lenz "toy" turbine. it's 20cm diameter, double stacked twin blade and it's clocked a huge 960rpm freewheeling in a mild wind (est 15mph - but no anemometer yet, so can't be sure)

However, at certain speeds it seems to "resonate" and starts to visibly shake (despite being ballanced), so I'm going to re-mount it with bearings at both ends with a solid bar going all the way through the centre. If that fails, then I'll convert it to 3 blades per side.

I'm planning a large diameter alternator (The same size as the rotor itself, so the magnet/coil speed will be approx .7 - .8 of the windspeed if Ed Lenz's TSR calculation carries down to this scale.

Another question - with a well designed 3 phase alt, when the coils are disconnected, should there be any noticeable "cogging" or resistance to turning ?, because this is obviously a high rpm, low torque machine.

I'm thinking that if I can make a suitable mini 3 phase large diameter alt for this machine, (to be scaled up in future if it works well enough), if I "overbuild" the alternator (brake drum style) would it be practical to "de-tune" the alt by spacing the rotor further away from the coils to provide less drag (if needed) ?

Thanks again - Dave.