Axial flux turbine MK 2

[karlb]

It has been a while since I last posted with my first turbine build here.
To cut a long story short, it only produced about 9v at 20w - not enough voltage to charge a battery.
So I geared it up 3:1 with a bike chain. You can see in the video in my sig that I get about 19v turning it by hand after the gearing up.
The problem is that the bike chain introduces too much friction and the blades only turn in a high wind and even then quite slowly. There are also problems with the chain falling off occasionally.

I have a bit more time on my hands again and am planning a rebuild.
The way I see it there were two problems: not enough blade and not enough magnet.
I plan to double the blade size and double the magnets.
I'm also going to go with direct drive this time so that less energy is lost to friction, and to keep the whole build simpler (can't be bothered with problems with the chain falling off any more). My idea is that if I get enough poles in the first place, and build my alternator properly then I won't have to deal with the whole hassle of gearing.

Last machine was dual rotor 8 magnets per rotor with 6 coils.
Planning to make this 16 magnets per rotor with 12 coils.
Magnet size is 10mmx10mmx20mm
Can anyone on here calculate the expected power from an alternator, given number of magnets and magnet size?

Heres a quick sketch, rotor on top and stator on bottom with coils shaded in.

I've gone for magnet spacing of half a magnet width here. So 5mm between each magnet on the inner edges. Could anyone let me know what spacing is optimal?
Is it best to wind the coils around a rectangular form the same shape as the magnets, or wind it around a wedge shape with one end the same width as the magnet and the other end slightly wider? With the first way the coils only touch eachother on the inner most corners, but with the second they touch along the entire length so I'm thinking this will be better?
Also can anyone recommend me a wire size? I'm trying to get a voltage of about 19v to charge 12v batteries. I know from my previous build that the voltage varies massively with different wire sizes so it would be good to get it right first time. Course I will make a test jig to test one coil and estimate from that but are there any rough calculations I can do before hand?
Thanks

[Mary B]

What kind of magnets?

[karlb]

Neodymium grade N35

[electrondady1]

i have no computer animation skills.
i use tracing paper to draw out  coil configurations .
use a thumb tack to centre it over a mag layout drawing . you can rotate it and check how the coils and mags interact.
 it can be surprising.


i recently went with wedge shaped coils with the coil legs running parallel on a radius . it seemed to give a bit  more time that the two coil legs were over the mags.
some  builders are shrinking the width of the centre hole to slightly  less than a magnet width. giving a bit more room for a few more turns.
 

[karlb]

Thanks for the advice.
Will use wedge shaped coils. My thinking was that the edges of two adjacent coils are meant to be under the same pole as the magnets turn, so it would make sense for their whole length to be touching.

In regard to the centre hole, I can see that shrinking it to slightly less than a magnet width would get you more turns, and I've even thought about doing this myself. Can anyone else confirm the theory on this?
Also what's the optimum gap between two magnets on the rotor? I've assumed half a magnet, but can anyone else confirm?

[joestue]

interesting, i found another document on optimum ratios for air gap machines
link: http://jfgieras.com/RJW_IEEE_MAG_2005.pdf

for trapezoidal magnets the optimum magnet thickness 1, while the air gap is 1.56. so for half inch thick magnets that is a 3/4th gap.
optimum magnet width is .78 to .72 pole width to arc width.
the optimum inner diameter to outer diameter ratio is .7
so this means for one inch wide magnets, you need 1.33 inch long sides of the inside diameter polygon. so for 16 poles that's 6.77 inch inside diameter.
optimum magnet length would come out to 1.5 inches, for a 9.67 outside diameter... so 2 inch long magnets aren't eating up too much...
however, the difference in area between 16 trapezoids and 16 rectangular magnets is quite large.

I find it quite convienent that the numbers in this linked document are almost identical to iron cored machines, with the exception that iron cored machines can have larger magnet coverage, 80% is a minimum, with the actual number selected to minimize cogging.

i would be willing to bet that the optimum magnet separation is much closer for rectangular magnets, for 16 poles and 1 inch wide magnets perhaps as low as .25 inch gap on the inside corners, so that would be 6.36 inside diameter.

[karlb]

Thanks Joestue,
Ordered the magnets last night, they were on offer so I decided to go for 20 pole 15 coil instead of 16 pole. So 40 magnets! Total cost £50
Should take two weeks to come so I will build other parts in the meantime.
The coil winding space on a machine with 4:3 magnet:coil ratio is 1/6 magnet width + 2/3 inside magnet gap, regardless of number of poles.
In my earlier post I assumed an inside corner magnet gap of 0.5xmagnet width was best, so with magnet width 10mm and gap 5mm I should get coil width 5mm.
If you're suggesting a 0.25xmagnet width gap then I will get a winding space of 3.33mm which doesn't seem like much.
If anyone else can confirm this then that would be good.

Not sure where your 1.56 air gap is coming from. In the paper you linked to they found that for maximum efficiency with magnet thickness 11.12mm the stator thickness was 17.5mm and clearance 2.5mm, making a total gap of 20mm, scaling this to a magnet thickness of 10mm, the total gap should be 17.8mm

[karlb]

On the parent site http://www.otherpower.com/magnetrotors.html they build their 10 footer with a gap of 1xmagnet width on the inner edge. So there seems to be a lot of different gaps being used here.
Does anyone know for certain what the optimal is?

[electrondady1]

i  have always used a spacing of one magnet width on the inside
but  the trend is to a tighter spacing .
the important ratio is the one between spacing on the rotor and the air gap between rotors. you want the space between that mag rotor surfaces to be closer than the spacing so the flux travels through the copper rather than side ways to an adjacent magnet.
so the mag spaceing dictates the thickness of your stator and the number of turns you can fit in there.

[electrondady1]

Karl if you can wait until your mag rotors are finished you can do  test coils.
that will take the guess work out of your stator.
 

[karlb]

Never realised that the ratio between the magnet spacing and the stator width was important, but now you explain it, it makes perfect sense.
I will go for a space of 1 magnet then.
This gives me an inner diameter of 12.78cm and a coil winding width of 0.83mm.
This is going to be big compared to my old machine

and yes, I will definitely wait until my rotors are finished and make test coils to get things right before setting the stator in epoxy.
While I'm waiting for the mags I can make the bearing assembly and cut the mild steel discs out for the rotors.

[karlb]

Today I spent a few hours looking for sheet metal to build my rotors. I went to local scrap yards and metal workshops. It was difficult to find what I needed and one place even said £60 for a 2m x 2m x 3mm mild steel sheet.
Eventually I found a car repair shop that gave me two brake disks for free.
I attached one to a thrust bearing and here is a video of me spinning it with some magnets on as a test:

I've calculated that I need 24 magnets per rotor to get the optimum spacing of 1 magnet width.
I worry that the size of the brake disks is overkill in relation to the size of the magnets. Although I hope that having lots of poles will compensate for the small magnet size. I'd like to hear people's comments on this.

They are quite heavy at 4.7Kg each but the thrust bearing seems to have no problem in spinning freely.
I plan to have the alternator on the ground with a spinning shaft going up about 1.5m to the blades (VAWT).
There's no way I'm mounting something this heavy at the top of a high pole.
Of course the spinning pole needs a frame with a few points on the way up having fixed bearings to keep the pole from wobbling as it spins but this is all possible.

Questions, thoughts, criticism?

[hiker]

if your worried about weight--you could double up on saw blades..
works for me...use mine on the back of my motorhome mounted to the ladder..
its survied some high winds-50plus mph- if not more..
heres a pic. on the test stand..

[sean_ork]

3mm mild steel is usually £10/ square meter, the car repair place would have had some - but it's too thin for your use

that disk could be used, but you'd have far better results using much larger magnets - having lots of poles will just reduce the dia of wire you can use, and thus the overall output - it just makes lots more work

It's not clear what bearing that's running on - consider standard wheel bearings such as linked below - these can be found at yards already attached to the disk - and as they are designed motor vehicle use the are more than strong enough

have you read the Piggot Smith recipe book ? - that will give you some pointers with regards to coil size v wire size v output

I fear that currently you are headed down a road that will lead to disappointing destination - if you are aiming for an output of a few 100 watts to charge a 12v battery then a salvaged treadmill motor would be an instant solution

http://www.ebay.co.uk/itm/Vectra-Signum-Rear-wheel-bearing-/271299222850?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3f2ab19d42

[electrondady1]

a lot of alternators have been built on this forum using brake rotors.
i have one  utilizing  a total of 144 hard drive mags split in two. for a total 288 little mags !
Crazy !
 it took a lot  patience and a long Canadian winter but i finally used them all up.

a suitable  automotive bearing like Sean has linked to would allow your rotors to really run true..
you will need jacking bolts on the top rotor to allow for fine clearance adjustments.
keep going

[karlb]

Thanks for the replies.

The bearing I'm using is from an old trolley wheel. (not the one that makes the wheel roll itself, but the one that makes the wheel pivot when you turn corners). It is not ideal but it works. Thanks for the suggestions about using a car wheel bearing, I can see how this would be much better, so tomorrow I will visit some more car repair shops and hunt one down that will fit the brake disks.

I know I would have better results with larger magnets but I already have quite a few of these magnets, and a lot more ordered and on their way, so I'm stuck with them. Building this is all fun and I would be happy with an output over 50w which should be more than achievable. This is mostly just a hobby to me and buying a treadmill motor would take the fun out of it

electrondady: your experiment with 144 poles sounds very interesting, and seems to give a lot of hope for my 24 pole machine, in the sense that having lots of poles isn't bad after all, if you managed to do well with them.
I have a lot of questions about this:
Does this mean you used 108 coils?!
What kind of wattage did your machine give out?
Wire gauge? Disk diameter? any pics?

[Mary B]

Check ebay for steel disc cutoffs. A quick check showed 8 inch ones for $13 each.

[karlb]

Thanks MaryAlana,
Found this from a quick search: http://www.ebay.co.uk/itm/1-x-5mm-Mild-Steel-Disc-180mm-Diameter-/221288974540?pt=UK_BOI_Metalworking_Milling_Welding_Metalworking_Supplies_ET&hash=item3385d9c8cc

5mm thick 180mm diameter mild steel disc. Two with delivery would be £11.50

These would be substantially lighter than the brake disks. Although I'll have to find the exact centre and drill mounting holes myself.

What do the rest of you think? Should I go for them? or stay with my brake disks?

[electrondady1]

that alt is just an attempt to make something useful out of things i had laying around .
each pole was constructed from 9 little magnets. three mags , three layers.
only 16 poles.
it's the stator that is the real exp.
32 coils,  two single phases overlapping.

finding centre, getting the two rotors to run true, are  challenges  when you build a machine with out a machine shop.

[kitestrings]

karl,

You had a few questions early on about magnet rotor gap and number of turns, output, etc.  I think this can be estimated pretty closely.  Am I correct that you are now looking at a 20-pole alternator?  I'm also not clear what the blade size and speed range your looking at...

Regarding the gap, I believe the rule-o-thumb is to have a stator somewhere near (or a tad less) than the magnet thickness.  I believe the optimum BH (highest flus density, telsa) for the Neo's is around 1.5 x the magnet thickness, but then you have to also balance the mechanical clearance and need for winding space.

Regarding e-dady's comment on alternates to CAD - I like to layout the magnets on a piece of pexi- with a pin at the center so you can see the interaction of the coils and magnets.


best, ~ks

[karlb]

thanks kitestrings and electrondady,
Looking at probably a 24 pole machine now, because of the size of the brake disks.
Blade size not sure yet, I'll try and match it to the alternator after I have built it. Since this will be a VAWT I'm aiming for about 100rpm.

Found a wheel bearing today at a car repair shop, gave it to me for free from the scrap pile. the screw holes fit the brake disk perfectly.

Unfortunately there is a lot of friction when I turn it. The brake disk does not spin for long if I spin it and let go. Whereas the trolley bearing span for around a minute.
I've doused it in WD-40 but little improvement. It turns smoothly and there are no grinding noises, just has a lot of friction. Probably it is old and thats why they threw it away, and if so then there's not much I can do. Either need to find a newer one or carry on with my trolley bearing.

[kitestrings]

Okay, I'll venture a starting point for you.  Lot's of assumptions to follow (you may be able to help fill in some).  Others may be able to come closer...

If we assume a cut-in voltage of 13.5V at 100 rpm, and 18-coil, 24-pole dual alternator.  Further assuming 10 x 10 x 20mm N-35 with an estimate Telsa of say .5T, I'm coming up with about 104 turns per coil.

And, WEG on output - if we assume #18 ga (no idea if it would fit), and say 4" per turn.   I'm going to say you'd have a forcing E at 250 rpm of nearly 19V that would give you 5-6A; or about 70 watts.  Now your total power could be close to twice this, but most of that will be in the form of losses.

~ks

[sean_ork]

Unfortunately there is a lot of friction when I turn it.

that's to be expected, the bearing you have now is just a few balls running around an open race - far too much free play for a properly balance

you might have some joy looking for a smaller version, or one intended for a trailer if you are only aiming for 50w - a couple of cheap pillow blocks may do

[electrondady1]

 the bearing is the heart of the machine your making .
it should turn freely .
i be suspicious of the one you have , they must have removed that old bearing from a car for a reason.
if you can identify the  make or model for your rotors, you may want to check the price of a new bearing at an automotive parts place. they may have an inexpensive substitute that will fit your rotors.
 compare the resistance of a new bearing  to the one you have.

you may  find a suitable car at a scrap yard with better bearings.








 

 

[karlb]

Thanks for the suggestions.
Magnets arrived yesterday and I have a trip planned to two car scrap yards this weekend. Ill take the current bearing to get an exact size match for it, and to compare the friction to any potential new ones.
Apparently they are the two biggest scrap yards around so I should be able to find something

[karlb]

It has been a few months since I posted here. This project got put away as I became more busy with other stuff. But now I have a bit more free time I've been working again.
A guy at a car scrap yard gave me two gearbox bearings for free. They turn really freely and seem quite heavy duty. I 3d printed an enclosure for both of them in ABS:


Here I have the two brake discs:


I 3d printed a mounting for both of them:

(Green plastic later changes to blue because I re-printed them due to the originals being slightly warped)
I'm not sure if ABS will be strong enough for this purpose. I've enquired with several CNC machining places to get these made in steel, but they've all said a crazy price like £100 just to make these two mounts. At the moment this is the only way I can make something to mount these brake discs to, and it seems to hold out strong enough for now.

Next I take some mild steel poles which were found outside by the bins and probably previously tent poles. I cut them up with a hack saw and weld them together to start building a frame:


Completed top and bottom frame sections:



Top section painted and has bearing attached with six screws. There is a layer of wood between the bearing housing and metal frame to act as a "washer" to stop the plastic from cracking and to keep things level:


Next, I do some more welding and painting, and here we have the beginnings of "the cage":

Each vertical pipe has a strip of copper with a hole in it welded to the top. The underside of the strip of copper has a nut welded to it. This way I can easily screw on and off the top section of the frame for maintenance.

Here I begin to superglue the magnets to the brake discs:


We have 24 poles here.
I know that I could fit more, or bigger magnets to these discs, but magnets are expensive and I'm trying to build this on a budget here. I'm using mostly scrap materials or stuff that I got for free. The magnets have been my main cost here at about £50 for all of them.

Continued on next post...

[karlb]

Here is the complete assembled unit, with a jig in place holding a test coil:

Here from another angle you can see how the top part of the frame screws onto the bottom part:


The whole thing is really strong and there is no vibration when the discs are spun. The two bearings do a good job of allowing it to spin freely. The rotor is at least 10kg with both break discs and all the magnets, but it doesn't take much effort at all to get it spinning, and it carries on going for a fair amount of time once you let go.
The shape of the frame allows me to either keep it vertical as seen in the photo, for use in a VAWT, or easily lay it on it's side if I want to connect it to a cycle generator.

Here is a close up of the test coil I have in place:


Spinning by hand at, I'd say about 100rpm, I get around 2 volts open circuit, and 0.8 amps short circuit current from one coil. Once I have all 18 coils in place this should give me a good voltage for charging a battery.

This is as far as I've got at the moment. Let me know your thoughts and or opinions for improvement

Next I need to wind all of the coils and cast the stator. I've never bought resin in large amounts before. Would this be good to order?:
http://www.ebay.co.uk/itm/170933420039?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
If not can anyone recommend where I can get some from in the UK?

[electrondady1]

excellent progress Karl.
those 3d parts your formed are slick.
polyester resin is $19.99 cdn. dollars around here .
your rotors must me running true to get such a  nice clearance on your coil
i think the air gap between your rotors is huge.
but check with others .
 i usually make my coils the same thickness as one magnet.
your showing real ingenuity.
congratulations.


 

[karlb]

Thanks for the kind comments
Yes, the rotors are running true, but you're right that the air gap is bigger than it should be. I haven't properly tightened things up yet.
I'm making my coils about the same thickness as one magnet like you suggested.
Re. polyester resin, I think I will order the one I've linked to, I calculated I will need about 250-300ml for the size of my stator mould, and there is 500ml there. There will be some left over to put on the rotors. I believe I don't need to fully encase the magnets in resin, just spread a few layers around them to keep them fixed to the disc. They already have two layers of superglue holding them on.

Re. 3d printed parts, yes they are very helpful
I was considering making the whole stator mould out of wood, but I don't trust myself to get the dimensions perfect. On the other hand, the 3d printer would get the dimensions perfect, but I don't have a big enough printer to make the entire mould. So I printed just a section:


I had a small amount of epoxy to experiment with. Just those small tubes you get from the hardware store.
I placed two coils in the mould with a layer of cling film to stop the epoxy sticking to the mould, and blu tack to stop the epoxy running outside of the desired area.
Pour in epoxy between the two coils, put another layer of cling film on top and clamp shut:



After an hour, I open it up and take off the cling film:



The two coils are epoxied together at exactly the right angle, and I have a small section of my stator made.
I can now place the coils back in the mould, rotated round by one section, and put the next coil in ready for epoxying:


I can keep building in sections and rotating round until the whole stator is built. I can cast 3 coils at a time so I'll only need 5 more casting operations to complete the stator. Each one takes only an hour to set.
Can't cast any more until I receive more epoxy, but I can wind all of the coils while I'm waiting.

[Mary B]

Unique use of scrap materials! I like it!

[electrondady1]

can you  get your program to add a tongue and groove to the ends of your stator mold segment?
maybe  they  could snap together  like K'neks .
makeing  a complete mold.

[karlb]

That's a good idea, and I could do, but I don't want to use up more printing filament if I don't have to.
I can always wind the coils for the next section or build other parts whilst one section of the stator is setting. I should have a complete stator by the end of the week

[karlb]

Just got a quote back from the CNC machiners.
About the green plastic parts I talked about in an earlier post shown here:

To machine those in mild steel would cost £480! Holy Crap.
They would be just under a kilo of steel each.
But I suppose it's the effort that goes into machining them rather than the amount of steel used. They are probably used to making thousands of the same part for each customer rather than small time guys like me who only want 2 parts.
I think I'll stick with 3d printed ABS for now!