My new project A 9 coil 12 pole axial flux alternator

[willib]

I got my magnets today, they are 1.5 inches in diameter x 1/2 inch thick .
i got 24 of them so i'm looking at a 12 pole machine.



I got the ones with the 1/4 inch holes so i will be locating them to the rotor with 1/4-28 thread bolts , and so i won't have to use any epoxy or resin.
I will be using a 10.5 inch diameter magnet rotor. and my coils will be 3 inches in diameter



The stator is ever so critical on a machine like this so i did some calculations with 13 AWG wire and i can get 87 turns , with using three turns wide of wire.
or i can get 116 turns at 4 turns deep . they will come out at 0.219" and 0.292" wide respectively or thereabouts .






I will know more about the voltage when i do a test coil.

sorry if the last pic looks big i believe its less than 60k

[willib]

This is going to be my largest Alternator to date ,with 10.5 inch diameter rotors , actually a tad less , but not much.
And a substantial bearing to handle a nice set of wooden blades.

the Rotors are off a Honda Civic front brake disks, which just happened to be the perfect size for my mags.

The bearing is from the Ag supply store, i had to open the holes in the rotors up a bit ,  it wasn't an exact match on the bolt circle diameter  
The thing about the rotors is they through the air outward when i spin it by hand, so i don't see the mags heating up that quickly , and the airflow should get sucked past the coils before exiting through the rotor.



I am making a new coil winder also , with all the parts from Lowes .




The electrical department had the two key parts ,  4" diameter electrical box covers , one to go next to the wire , and the other to support the perimeter ,with some black foam between. which i just Tapped to 3/8-16 thread.
The center of my coils is two nylon bushings , which when screwed down tight makes two fairly parallel plates , my calipers say they are just 0.004" from high to low reading around the
perimeter, 0.398" to 0.402". This is wider than i was planning on using 0.402/0.064" is six turns and change, maybe i'll just use one bushing , not sure at the moment.


Another thing about the rotors, they have 36 slots ,(between the two plates) so there is a slot every 10 degrees,  and my mags need to go every 30 degrees ,so that part worked out nice ,
 because i need to drill and tap for 1/4 inch machine screws to hold the mags in place . And its a good thing the webbing is not going to get in the way.



 I need to paint them too, before i put the mags on, don't need them rusting the coating on the mags .

The plan was to use  AWG 13 wire for this alternator ,but he didn't have any , so i got some real nice 14AWG wire .

I am expecting to get .75KW at times , out of this alternator , but i am secretly hoping for 1 KW , wind permitting of course.
I still have a LOT of welding to do , i have the bearing and the rotors on a 2by4 stand at the moment, and still need to replace the studs with threaded rod so i can turn the outer rotor towards the inner,
i did use an angle grinder to grind the hub down a bit so i could turn the inner rotor around, so the stator wasn't so far away from the stator mounts, when i get to that point.
anyway as soon as i get the threaded rod i will be able to adjust the gap between the rotors , then the test coil

[willib]

I got the coil winder together .



and wound a coil with it.



It is curing at the moment.

[willib]

The coil cured ok .


I spent all day today on it.
Getting the rotors ready to put magnets on.
Inserting the magnets
Lowering the rotor down onto the lower one
Fabricating a coil holder
doing some testing

Well i'm pretty pleased with the test coil .

It came in at 7.22 Volts at  250 RPM.

It has About 100 turns of 14 AWG wire. (  i lost count of the actual number of turns    )

I projected the output voltage on a graph.



for 9 coils total .

[jaskiainen]

It sounds to me that this generator is going for 24 volts?
I'm about to built my own with a little bit smaller magnets, but going with dual rotor 16/12 for 12 volts.
My magnet size is 1.18 x 0.4 inches.
Almost went with 12/9 but then i made some rough calculations and found that i can't use as thick wire as i wanted.
I just ordered a few feet 13 AWG magnet wire so i can do couple of tests what can i squeeze in.

I'm waiting your test drive results but your .75kW sounds a bit high for that wire size? Or is that for short period peak?
Your coil winder looks great. And really good pictures for us to see.
Nice work!

[ghurd]

"The thing about the rotors is they through the air outward when i spin it by hand... and the airflow should get sucked past the coils before exiting through the rotor."

Isn't that air flow isolated from the stator?

I often wondered about closing the factory 'vents', and drilling flow holes in from the stator side.

Or maybe tack some close fitting sheet metal on the rear of the rear dish,
drill some vents near the center of the disk,
and the air would be sucked toward the center, through the holes, and out the factory disk 'fan'.

Anyway, whatever you come up with, I am sure it will work great!
G-

[willib]

The coil thickness ranges from 0.420" to 0.439"  Oh well , i thought it was more parallel before i started.
And i must have been in a hurry because of the way it turned out, not exactly the way i would have liked it to.
also i nicked the coating on the wire taking it out of the winder.you can see it in the center.
i guess i forgot the start was inside the winder , and the hole i drilled in the plate must have scratched it off .
live and learn i guess , the next nine should be a lot better.



This next pic is when i had about  an inch and a half to go , when lowering the top rotor .
It also clearly shows the major oversight i had in the construction of this alternator.
Dan can probably tell because he must have run into this when he started constructing the volvo alternators.
The frigging hat is in the way!!
The coils won't fit all the way in.
So i gotta turn the inner rotor around and all the time i spent grinding away at the hub was for naught



The next two are of the test setup where i slide the coil in to test. And when i discovered that the hat was in the way.





My helper


G It was hard to tell from the other picture just how i was going to assemble it. When i turn the inner rotor around they both will be expelling air.

jaskiainen a 10.62 inch diameter rotor should fit your magnets very well.

If i had paid more attention when i was winding the coil then i would have a better idea of the dissipation ,but i can make an educated guess  of the length wire in one coil.
i'll see what i come up with.

thanks for looking in
william

[jaskiainen]

That hole in your coil looks a bit small? If i remember right it's so called useless wire in your coils.
Wouldn't it cause voltage cancellation when your magnet travels over both coil legs at same time?
If so, then all that extra wire in your coils only adds resistance for your windings wich lowers the output.

Someone smarter than me might want to jump in with this?


I already have cut my rotors and they are waiting to smoothing the outer side with my friends lathe.
Right now they are about 11 inches and i have ordered short piece of #13 wire to see what kind of coil i can make of it.
Hoping about 1/4 inches thick ID 30.5mm and OD 60mm.
That should make a perfect fit for my 30mm dia magnets.

[willib]

In all the scope measurements , that i have taken in the past, I have never seen any evidence of cancellation.
All the waves look like perfect sine waves.
Wether i am reading voltage or across a resistor measuring the current wave , i have never seen any evidence of cancellation with my present coil hole size in relation to magnet diameter.

Note to Wooferhound : does the post from that guy exist on the new board? Don't remember his name .

But he had three pitiful looking coils and a scope and somehow concluded the hole should be the size of the magnet  or greater. Which isn't  true

If it does exist on the new board ,i would like to debunk his results right here and now .

He has , with one thread , set coil design back to the stone age , with his erroneous conclusions.

I had a post on the old board called Percent cancellation, because i too thought that as one magnet passed over the hole that it would cancel , makes sense , right ?

But i have never , ever , seen evidence of this supposed cancellation.

and without evidence of cancellation how can it exist?

There are two  advantages to making the hole smaller than the magnet

one : as two magnets pass over the center hole they are making power for many more degrees of rotation as they pass over the hole. Where without wire there you are making none.

two : the wire used in the center of the coil adds very little resistance compared to the power it contributes .

[willib]

I've calculated the length of wire in the coil, to the best of my ability.
It comes to 48.07 feet.
At 2.525 Ohms per 1000 feet that comes to 121 milli Ohms per coil.
And for three coils , thats 364 milli Ohms per Phase.

Just for kicks i calculated that at 10A the heat dissipated , per phase would be 36.4 Watts .

And for 20 Amps the I²R losses would be 145 Watts per phase.

[wooferhound]

Note to Wooferhound : does the post from that guy exist on the new board? Don't remember his name .
But he had three pitiful looking coils and a scope and somehow concluded the hole should be the size of the magnet  or greater. Which isn't  true
If it does exist on the new board ,i would like to debunk his results right here and now .

I believe that the post that you are referring to was by finnsawyer and it never made it over to the new board. However I did find these stories that are similar...
http://fieldlines.com/board/index.php/topic,133738.0.html
http://fieldlines.com/board/index.php/topic,139275.0.html
http://fieldlines.com/board/index.php/topic,128664.0.html

[ruddycrazy]

G'day WilliB,
                I'm finally getting there with my first dual axial and when the mag plates are all finished I do intend to wind a heap of different coil config's and see which one is the best for 50mm diameter x 15mm thick N52 neo's. I'll setup my digi camera to focus on my fluke 865 meter on scope view so I can then later look at each one in detail on the puter. As I can get magnet wire dirt cheap and I do have a couple a 5kg spools of 14awg here to play with, it will be interesting to make a coil with the gap the size of mags then the exact same coil with 1/2 the mag diameter and see if they is any difference.

Cheers Bryan

[jimovonz]

In all the scope measurements , that i have taken in the past, I have never seen any evidence of cancellation.
...
But i have never , ever , seen evidence of this supposed cancellation.

willib, the cancellation introduced by making the center hole of the coil smaller than the magnet is real - whether you can identify it in your measurements or not. In fact, a certain degree of cancellation occurs when the hole is slightly larger than the magnet too (due to the field 'bulging' out in the air gap).  The cancellation is only complete (ie nett zero emf) when the magnet is dead center over the coil and equally over both legs where the potential generated is equal magnitude and opposite in direction. Of course, a coil with no cancellation has a zero emf at this point also. You will always get the maximum voltage from a coil when you wind it with no hole at all though the amount of voltage gained per turn diminishes the closer you get to the center. You can definitely make gains by having the hole smaller than the magnet - but there is a point where your increase in EMF is offset by the associated I2R loss. A lot of folk, including our hosts, advocate a hole size slightly smaller than the magnet size. As far as I can tell, not a lot has been done to determine the extent of the benefit of this practice and where the optimal point is.
I have seen a lot of 'sine like' waves from these alternators but an actual sine wave would be rare. The range of EMF profiles generated by each turn of a coil is quite varied but combined over the whole coil form a sine like waveform. I would expect a coil with no center to have an EMF profile with steeper 'sides' and a flatter top compared to a one with a magnet sized hole (and more area under the curve) but still appear 'sine like'. I have spent a lot of time trying to simulate this situation to gain some insight (as part of a larger project to simulate a whole turbine...) While I am not quite there yet, you may be interested in playing with a spreadsheet I developed that will illustrate what I am talking about. It is a MS Excel 2003 Workbook that allows you to specify a coil geometry then rotates a 3D field (obtained using Infolytica's MagNet software) past the coil to derrive the output waveform. It is currently setup for a 12" diameter magnet rotors with 24 x 2x1x1/2" N42 magnets and a 1/2" air gap. It is done mostly in VBA so you need to enable the macros for it to do its thing. Calculating the output can take a while for coils with lots of turns so be patient (approx 30 seconds for 100 turns on my machine).  You can download it here: http://mce.co.nz/temp/CoilSimOffice2k3.7z Use at your own risk!

[willib]

Note to Wooferhound : does the post from that guy exist on the new board? Don't remember his name .
But he had three pitiful looking coils and a scope and somehow concluded the hole should be the size of the magnet  or greater. Which isn't  true
If it does exist on the new board ,i would like to debunk his results right here and now .

I believe that the post that you are referring to was by finnsawyer and it never made it over to the new board. However I did find these stories that are similar...
http://fieldlines.com/board/index.php/topic,133738.0.html
http://fieldlines.com/board/index.php/topic,139275.0.html
http://fieldlines.com/board/index.php/topic,128664.0.html

Hi Woof, naaa those don't ring a bell with what i remember , i guess its gone .
Thanks for looking though

G'day WilliB,
                I'm finally getting there with my first dual axial and when the mag plates are all finished I do intend to wind a heap of different coil config's and see which one is the best for 50mm diameter x 15mm thick N52 neo's. I'll setup my digi camera to focus on my fluke 865 meter on scope view so I can then later look at each one in detail on the puter. As I can get magnet wire dirt cheap and I do have a couple a 5kg spools of 14awg here to play with, it will be interesting to make a coil with the gap the size of mags then the exact same coil with 1/2 the mag diameter and see if they is any difference.

Cheers Bryan

Hey Bryan , nice magnets .
Yes that will be interesting.
For a 9 coil 12 pole alternator are you going to use 350mm diameter rotors?
or for a 12 coil 16 pole alt. you could use 450mm diameter rotors.


I'm wondering if the magnetic field between rotors could be squeezed into the hole or otherwise moved about by the reaction of the coils, when the alternator is under load?
Does a magnetic field seek the path of least resistance i wonder. just sort of pondering in print

Hi Jim
It would be hard to notice the 'small hole effect' on an alternator , because the magnet is passing 'over the hole' at that very moment in time.
In the recent past i have made the hole equal to the distance between magnets, assuming the correct rotor size for the given magnet diameter.
I went a little smaller on this alternator , i went with a 0.750" hole  Vs. 0.784" which is the distance between magnets.
Mostly because that was what was available .
I am going to check out your spreadsheet , it will be fun. But i need a different machine , i'm on a MAC at the moment.

[oztules]

Nice to see you building a bigger unit now Willib. Looks to be coming on fine.

I did a bit on one of your posts I think, where I talked about getting the most volts per mass of copper... volts per gram as I recall.  I can't find it, but I used 50mm mags (round as always) and found that maximun volts/gram was with a round coil, and the hole size was about 80% (40-42mm). So smaller does help, but below that, the advantage starts to wander downwards.

It is worth remembering that when the magnet is above the coil hole, the emf is at zero, so any cancellation/percieved cancellation may be at the below cut in emf level anyway. In these single layer designs, both N and S operate on the coil at the same time, and EMF is generated as they approach either side/s of the coil, and going over the legs (max then)  As the coil is very much spread out under the mags, it is a creeping pulse. My coils are nearly 2 inches wide in the legs, so are covered completely when emf reaches max. As the magnet gets to the center hole, emf should drop to zero.... and both my legs see the a bit of the magnet (as they are 42mm and mag is 50 mm), and cancellation would seem obvious.... but we are not generating anything at this time anyway.

My final conclusion was that a bit smaller was helpful, and keeping the legs to no more than the magnet width (50mm here) when fully under the magnet/s when max emf was being generated.

This was for my machine... with it's own foibles, so may not work as well for everyone I guess, but  thats how  it went for me.  So I find that a smaller hole helps make more emf for the same wire mass/length/resistance, but I wouldn't go below 80%. But it is all about resistance..... if we want more emf we can use more turns, skinnier wire, but it is the resistance that is very important, and smaller holes seem to  help here.


Will watch with interest, but I have not come to terms with this new software..... I still like scoop. I only found this post by accident.


best of luck


..........oztules