Generator with ferrite magnets

[scoraigwind]

For those who followed the discussion of the dual-stator ferrite machine on this thread I would like to point to a continuation of it at the Backshed board here
http://www.thebackshed.com/forum/forum_posts.asp?TID=3504&PN=1&TPN=11

Chris claimed that a single-stator version would be 2.5 times the weight, so I did a theoretical design experiment here http://scoraigwind.co.uk/?page_id=953 that showed how using a single stator can give better efficiency at lower weight.  Chris also reports his latest 2-bladed machine within the backshed discussion.

[fabricator]

Come on Hugh, a theoretical design experiment? You and I both know there can be huge variation between theoretical and the real world, I don't quite understand the almost religious zeal in defending the single stator axial against all comers. It seems to me no design should be a dead end, there should always be constant innovation, constant experimentation.

[artv]

Hi All ,..I think until all different versions have been tested,..it's an open debate...
The more ideas the better ,better to hear the ideas, than the arguments......
Hugh , Chris , you both make great points......should join forces and make the ultimate machine.....if anybody can do it you guys could.
This is a very good thread ....need's more observation on the magnetic field.......sorry ,  now I'm thinking of a question??...artv

[oztules]

Fab,

I think the "zeal" comes from a true desire to keep the rules of the universe in perspective. It is great to see experimentation, and conclusions being drawn from them.... but sometimes what we interpret is not what we are really seeing. In this case, Hugh seems to be saying the universal laws are immutable, and no amount of CAREFUL experiment will duff them.

It is too easy to make experiments that appear to prove one thing, when in fact some other force is at play. This is particularly true of wind turbine combinations. Testing two or ten different types of blade profiles and lengths on a single turbine, tell us nothing about the profiles or lengths...... it tells us about the matching only.... only ....only.

Make no mistake, what Hugh says is true, and it does not need to be divisive. What needs to be pointed out is that the single stator recipe IS the best bang for buck for axials. No-one wants to see a point reached where people follow certain recipes for the wrong reasons.
The design has lots of possibilities, and weight need not be a problem with thoughtful design..... you don't need 2 full plates for a start......

From memory, there has been no time the multiple rotor system has worked better than an equally well funded (materially wise) single rotor design neo or ferrite.....

It is fundamentally wrong to assume that the rules of the universe can be bent to accommodate practical results that seem not to fit the  known laws..... there is always something that has skewed the result. In this case it will be hidden in the load matching and design.

That AWP that Hugh designed, can put out an easy 35amps all day into a 55v load. It is ferrite, 3.7m diam, uses .7mm wire, but has steel. I've seen it do that for years now (about 6 I think). It has no gearing, does not burn up, does overspeed sometimes, but holds together like no other I have seen..... no-one is copying it due to stator steel availability.

Some may copy Chris's, but skills are required. The same result can be obtained without these skills, with the same copper and magnets and bigger disks... thats useful too.

"It seems to me no design should be a dead end, there should always be constant innovation, constant experimentation".

And thats true. I have built the seeley radial from scratch almost, and that design works, but is not great, Chris and others have used the dual stator. Some triple F@P stators. ....all have their moments of triumph...... but it appears at this stage the easiest to build with the most promising results are the single axials, or the drum radial with steel. Flux has done well with motor cores and neo's, but in large part single axials are the norm for lots of reasons..... even though they make poor use of any magnet, compared to an iron design.... multiple ones are just worse in their use of materials.... but still work.

Whatever works for you ..... but at least know what is at work and why.



my 2C

........oztules


He he he.......I'm back...... and it's all Toms fault too

[fabricator]

Good to see you back and all your points are well taken.

[ChrisOlson]

oz makes some good points.  At no time have I claimed that multiple stator generators is the best way to do it.  It's one way to do it with ferrite magnets, in combination with the gearing helping to keep the size and weight of the generator manageable.

I had some criteria in designing this machine (I've built two more since the one shown in this thread and I've tweaked a bit).  I took a successful geared turbine with a neo generator and retrofitted it with a ferrite generator to see if it could be done.  When I built and tested the first phase of it with the turbine running WAAAY too fast it proved to me that the ferrites will work fine - just need more power.  So, after mapping a power curve for it and looking at my shaft power, it was easy to see that stacking another module on the shaft would double the power output, which was approximately what I needed to get that match.

As it turns out, it's a very powerful unit in a relatively small and lightweight package, as far as ferrite axial generators go.  And with the modular design stacked on a PTO shaft, I could add another module and make the thing 50% more powerful than it is now using the same stator mold and tooling that I used to build this one, adding only approximately 18 kg (40 lbs) of weight to it.  So the modular design of the generator holds some merit in expandability if you want to build a bigger machine.

But the criteria I started with was that this generator had to fit on my present geared head frame.  As it turns out, I had to build a longer PTO shaft for the gearbox and swap it out because the magnets were thicker and the original PTO shaft was too short.  But the thing still fit on my transmission without having to modify it other than the PTO.

Since, I have "tweaked" this design.  The latest version of it is a full 4.0 meter (13.1 feet) with a slightly longer tail boom, two blade rotor, and dual stator ferrite generator.  The stators in the 13G are both wound with 48 turns of 12 AWG wire so they're identically matched for power output.  The rear stator is skewed 90 electrical degrees with the first so I get two-phase loading characteristics, making the generator run smooth and quiet.  It is IRP configuration, requiring four wires down the tower.  The gear ratio is the same as the 12G at .4375.  I selected the two-blade rotor because I needed more rpm for the 48 turn stators.  The 48 turn stators have an internal resistance of .31 ohm so I need more power to drive it.  A two blade rotor captures ~3% less of the wind energy flowing the rotor than a three blade but it needs to run at 8 TSR to do it.  To match the power capacity of the generator I increased the swept area of the machine by 13%.  The 13G runs a tad over 400 rpm, with the generator spinning slightly over 900 rpm, at 12 m/s where it is delivering just about dead on 80 amps to a 24 volt battery bank @ 30 volts.

On 9/26/2011 the 13G averaged 1,741.8 watts output for 14 hours, 36 minutes, with peak output of 2,466.1 watts (80.07 amps) in sustained 25-30 mph wind.  This is a photo of my latest ferrite machine:



For the folks who don't like it, I extend a challenge to match it - not with theory and talk - with a real world running machine using the same ferrite magnets.  I have proven that I can play with the Big Boys with ferrite magnets.  And it's a hell of a lot more fun than the neo generators that only put turbines into stall and don't let them run.  After getting this dual stator ferrite unit tuned up and tweaked, with a rotor to match, I would suspect most folks are going to have a hard time matching it even with neos in a single stator design.  And if there's a whole bunch of theory that says you can do it, then let's see it.  Otherwise I got the only one (actually three of them now) that exists.
--
Chris

[oztules]

"For the folks who don't like it,"................and your right I don't like it...... I LOVE it. I think it is cool, well built,and a great performer......

But still the fact remains, the same theory that shows that yours works as it does, is the same theory that says the same stuff on a  single stator design, will work better. Its that simple.

If I could source those magnets as cheaply as you do, I'd consider the challenge... but sadly they are over $6.00 each in Aust.... and dearer to get to the island..... and the other problem is the large stash of neos I have...... over 100 2" disks in 1/2" and 5/8" n45 and n50...... hard to justify more magnets to the chancellor of the exchequer.

I already have 2 x 4m turbines..... and they don't get used for much other than charging the electric car/bucket of bolts.

So I have to squirm out of the challenge....... but mine to you is try to see why Hugh is correct...... I mean really try.

This from Danb says it best back in 2004  http://www.fieldlines.com/board/index.php/topic,137000.html

"But the 1st thing to consider in my opinion about this....

given the option (if diameter is not a problem) - you'll get much more power from the same magnets and wire... to stick with a single machine, rather than stacking multiple smaller machines.


Double the number of magnets on 1 disk (make it larger diameter), and you'd have twice the number of pole changes per revoltuion, so each coil makes twice as much power.  Double up the number of coils on one stator, (make it larger diameter) and you'll have twice the power.  So I think doubling the number of magnets, and coils in one alternator will yield 4 X the power, where using the same resources, and basicly building two alternators on one shaft would only yield twice the power. (I think thats right...)


so unless the diameter of the alternator is an issue, I don't see much point in having multiple rotors and stators, best to build it all into one."

Look carefully at the what and the why (forget about your situation for a minute ie rotor size). Does it make sense?

If not why not......which part of that do you take issue with given your testing of what happens in the real world...... it just works.


 and I think he is right.

The only building fellow who felt that 2 was better had a VAWT which he also felt was a winner...... given that we have not seen a worthwhile vawt to compare figures, he wold be happy to get anything I suspect.

In your situation (size constraint) your solution is right for you...... and I think it is great.



..............oztules

[ChrisOlson]

Look carefully at the what and the why (forget about your situation for a minute ie rotor size). Does it make sense?

oz, I know all this perfectly well.  I'm not a newbie  

Just that it would not fit on my turbine frame.  The dual stator modular unit does.  The thing about "wasting magnets" is a non-issue with these ferrites at $1.89 each.

But one thing I must say, that I have heard quite often, is that the single stator design would be "more efficient".  This is pure hogwash.  Power efficiency is power in minus power out.  The difference is converted to heat.  The dual stator design is just as power efficient as any single stator, and easier to achieve it because I got plenty of room for really big wire without having to wind two (or more) in hand.  When you wind parallel windings you introduce the possibility of harmonics (circulating currents) due to slight mismatches between coils, and it is harder to get good tight coils that don't vibrate.  It's also harder to keep the thickness of the stator at 12mm.  Slight changes in air gap makes a HUGE difference with ferrites and if you build the stator too thick you're going to have to add turns to make up for the increased air gap.  Building a large diameter stator 12mm thick is not a good idea.  It won't be strong enough to handle the torque from a 4 meter rotor when it's hot.  Spreading that braking torque load out over two stators reduces the load on the castings to exactly half.

There's other reasons why I think the dual stator design is not a bad idea for larger machines.  But in the interest of not wanting to sound argumentative, or pushing it as being "better", just suffice it to say that my version of a dual stator generator is successful in my eyes.  I learned from the first one as to what it takes.  This 4.0 meter machine is a flat out screamer.  When I first put it up I balanced the rotor so it had perfect static balance.  But it shook like a SOB when it yawed.  I was like, what the hell?  Dynamic balance was not good.

So I took the rotor off it, removed the balancing weights and mic'd the blades to see where the difference was.  I corrected it by sanding the blades until they were identical weight at every station down the blade.  I got it perfect without a single balance weight on it.  It runs smooth as silk now.  At lower rpm the yawing is just a big jerky.  But once it gets wound up it steers pretty nice.  I'm going to add a damper to the tail so I can use the mass of the tail to dampen the jerkiness in yaw at lower rpm.  But that's not a priority right now - I'll do that when I get time.

Otherwise it's a really fun machine to fly because it flat out HAULS A$$!  And best part is that it still cuts in at ~3.5 m/s with the bank at 25 volts.
--
Chris

Edit: This is the rotor for 13G #1 after re-balancing it and painting it the second time with DuPont Imron polyurethane enamel.  The blades are Royal Wind & Solar GOE222's with 10 degree pitch, 158mm chord, no twist, no taper.  They run at 8 TSR under load with no problem at all.

[Menelaos]

Hi to all, hello Chris,

I was kind of busy in the last weeks so my experiments with ferrits came a little short...nevertheless they arrived from poland last week...there were some problems...

In the mean time I made a good deal on bying a broken Gaussmeter and fixing it. It's a Siemens type and about 20 years old...but with calibration unit- so that is very useful :-)

As I still had some alternators flying arround, I made some measurements with interesting results:

In my latest design with dual rotors, I use magnets of 75x37x15mm N42 with an airgap of 20mm. In the gap I could measure 0,72 Tesla whereby the field decreases the more I turn to the edges of the magnets. These results did not really surprise me...

I then got my ferrits, F35 with 45x35x15mm. I chose an airgap of 15mm and was able to read 0,2-0,22 Tesla. I then wanted to find out, how much flux I would loose when I close up the magnets one and the same rotor disks as this was often issued here. so When I put them on a piece of metal with a gap of only about 1mm next to each other, the flux in teh airgap decreased to 0,19 Tesla. So this loss of flux is not really an issue as I think
So I will now change the design of my small alternator kits to ferrits soon. I still have a coupple hundret Neos left to use but I will probably run out of them early next year. With a changed coil geometry that I already mentioned earlier in this topic, I can still get hood results without having to dramatically increase the size of that alternator...

Anyway, if you do use the same dimensions on magnets but use ferrits instead of neos, you will need about 3-3.5 times more windings in your coils compared to the neo type alternator.

What I feel we have to do in order to keep the construction size to a minimum is to use wedge shaped magnets. Doing that an adjusting them to a costumized radius, I could get about 30% more coverage of magnets compares the the block shaped version which I feel is a lot!

Also I have done some more research on these ferrits after I disassembled a broken DC scooter motor. I did not quite understnad the coil geometry until I found out, that one of those magnets has 2 poles on each face....!

So basically, it would be possible to construct one big closed magnet ring for each rotor disk that is devided into a given number of alternating plus and minus poles....verry interesting as I think!

Probably, the flux density will increase by that...how much...I do not know but it would defenitively simplyfy the whole constructio a lot...but so far, this is not more than an idea for optimizing the thing using ferrits...

If anyone wants me to make some tests with various kinds of neos and ferrits on different airgaps etc, just let me know as I now have calibrated hardware for doing those things...anyway, that helps a lot for calculating the right number of turns for various airgaps.

By the way Chris, the flux density with ferrits does not decrease with bigger air gap as much as you think. After my measurements, I would keep the airgap from about 2/3 -1/1 of magnet hight wherby the magnet hight should not be more than hald of its width. making it smaller does not give a lot more flux compared the the space left for wire and making it more will indeed rapidly decrease flux, especially in the middle between the magnets, but it would rise again comming closer the the magnet plain area...

Max

[Flux]

I don't want to get too caught up in this discussion about ferrite magnets so I will just keep to a few comments.

For a given size machine you will typically have 1/3 the flux per pole so in direct comparison you will get much less output than a neo machine. the winding resistance will be much higher.

Using a speed increase as Chris has done solves  the problem in the easiest way. If you want to stick to direct drive it will have to be bigger and heavier.  Using all the magnets on larger diameter rotors with a single stator will be more logical than stacking small machines.

"So basically, it would be possible to construct one big closed magnet ring for each rotor disk that is divided into a given number of alternating plus and minus poles....very interesting as I think!"

Yes this is true and is used in the Marlec machines, they use large diameter ring ferrite loudspeaker magnets with alternate poles magnetised round the circumference. it's simple and robust but you use the magnet material better if you cut the rings into segments and reverse adjacent ones.

For axial machines sector shaped magnets are the best starting point.

Some attempts have been made to use flux concentrators to get the gap flux density up with ferrite magnets but it may not be worth the effort, you still need large pieces of magnet with large area. you may get some improvement in the reduction of turns and lower Resistance but it will be a gig and heavy machine and may be little better than just a bigger conventional machine.

Flux

[Menelaos]

"I don't want to get too caught up in this discussion about ferrite magnets ..."

Why not? ;-)

"For a given size machine you will typically have 1/3 the flux per pole so in direct comparison you will get much less output than a neo machine. the winding resistance will be much higher."

There is no doubt about that but for smaller machines up to 2.5m I think using ferrits on a single stator alternator is not that uch of a problem when the geometry of the coil is changed in some ways. Sure, this will theoretically have some disadvantages but in real life this does not really make any problems...

For a turbine of 2 m diameter, I can design the alternator to be only 33 cm in diameter and still having only about 0.6 Ohms between each 2 phases on a 12 V system if you move the theoretical cut in to 3.5 m/s of wind...

Furthermore, these small alternators can take a lot more heat then the big machines. A 500 Watt unit can easily take peaks of 1000 Watts of heat losses in short term and 500-700 watts of continious heat loss inside the stator wherby a 5 KW machine would definitively die when running on 10000 or even 5000 Watt heat loss as the wire has more surfache area on those small units...

So when it comes to eficiency- sure that one will lack...but with heat dissipation I do not quite see a problem for SMALL units. Bigger turbines can use gearing as chris does...

Regarding that, I do not quite understand why Chris will stick to a dual stator design instead of using a single stator unit of the same size and just change/doubble the transmision ratio.

I had a small unit of 30 cm diameter running on more than 3000 RPM without problems once you have balanced the magnet rotors...

Max

[ChrisOlson]

By the way Chris, the flux density with ferrits does not decrease with bigger air gap as much as you think.

Hi Max,

Glad to hear you got your ferrite magnets to experiment with.

I design my stators and gearing so the rotor is running at optimum TSR @ 6 m/s, then take what I get on either side of that - on the lower wind speed side the cut-in is what it is - on the higher wind speed side the full power is what it is @ 12 m/s.  On the 13G I designed for a 20mm air gap with the 25mm thick magnets.  During it's test run the machine was running a little bit too fast at 260 rpm @ 6 m/s and cut-in was at about 4 m/s.  I needed 230 rpm @ 6 m/s.

So I lowered the tower, disassembled the generator and took some shims out of both sections to drop the air gap to 18 mm.  Just two mm change in air gap changed the rpm @ 6 m/s from 260 to 228, and dropped the cut-in to about 3.5 m/s.

I had found this out, also, when experimenting with the 12G.  The "sweet spot" for 25mm thick magnets seems to be right around 18mm with a 3mm gap between the magnet faces and the face of the stator.  It will be interesting to see what you come up with when you experiment with that, but that was my conclusion.
--
Chris

[Menelaos]

Hi Chris,

So your 18mm gap with 25mm magnets confirms what I found out...thas pretty much 2/3 of magnet hight...
So your practical results about match with what I meassured with my Gaussmeter. two approaches with the same result :-)

What I don't quite get here: If you use gearing anyway, why do you bother with dual stator design instead of just using a higher transmission ratio?
Are you scared of the high RPM?

Max

[Flux]

I will let Chris answer your question but i will throw in a few thoughts to think about.

A single phase machine basically ony uses half the winding space. You have enough room to add another winding in the gaps. there are certain problems with this and they are worse for axial machines. There is little room for the coil end crossings, particularly on the inner radius. You invariably have to increase the copper length a lot to get a 2 phase winding in there ( same is true of 3 phase or more).

With conventional iron cored alternators the winding resistance is not really important, the load is many times the internal resistance and the reactance dominates anyway. With direct battery charging and no reactance resistance is the dominating factor so you start to loose out going for methods that use the unwound space. The big snag with single phase is mechanical vibration. by using two single phase stators positioned 90 deg electrically apart you solve the vibration problem, it then runs smooth as any polyphase winding does.

I can at least see some reasons why Chris may prefer this approach. I am sure he is not afraid of speed and chooses his windings and gear ratio to get the loading he wants with something of suitable mechanical size to fit his transmission unit.  Higher speed and smaller stators may well do the same job.

That is one reason I don't want to get too involved in this discussion. Starting with the need to use ferrite magnets I would probably approach things rather differently. The air gap approach really best suits neos. Stuck with ferrite there are advantages in sticking with iron cores, radial seems a better approach than axial and slots are likely to be an advantage but you can of course do it without iron or slots.

With resistance limited operation even radial machines with overlapped coils pose problems but they are less than with axials.

The simplest way to directly replace neo with ferrite is to use a speed increasing drive. If you want to stick to direct drive then it is certainly possible but expect it to be much bigger and heavier.

At the moment we are in a confusing situation of trying to compare direct drive neos with geared ferrite, fine as long as you take into account all the implications and don't form generalised ideas that may not be the most logical for a complete ferrite only design.

I didn't do much with ferrites as the low flux is a big issue. I could put forward a good argument for a ferrite low speed alternator to deal with the low winds and a conventional wound field slotted core machine to deal with high winds. If neo cost gets out of hand then many of the ideas i abandoned will have to be considered again and gear drives may be sensible on smaller machines than with neo.

Flux

[ChrisOlson]

What I don't quite get here: If you use gearing anyway, why do you bother with dual stator design instead of just using a higher transmission ratio?
Are you scared of the high RPM?

Hi Max,

It's a matter of torque.  When you use step-up gearing you trade torque at the generator shaft for rpm.  This is why geared drives have been used almost exclusively in large utility-scale machines.  High torque low rpm loading takes very big shafts and components.  High speed, low torque loading can use much smaller and lighter components

With a fixed pitch rotor, non-synchronous generator, the speed is constantly changing and acceleration time is important to take advantage of the extra power that can be harvested from gusts when the wind speed increases.  You may not think that's too important, but it all adds up on the KWh meter.  Weight in the rotating assemblies of a wind turbine don't really do you much good.  If you have a turbine with a very heavy rotor and generator that takes a long time to accelerate, finally gets up to speed, then you suddenly remove the wind from the picture, it almost instantly slows to a stop.  The energy contained in the rotating mass does not get transferred to the generator input because it's primarily eaten up by the blades that are now running outside their optimum performance envelope.  And conversely, the energy contained in the wind when it needs to accelerate gets eaten up being stored in accelerating rotating mass instead of being directed to generator input.

That acceleration time is dependent on how much energy from the shaft must be transferred to the rotating assembly to get it up to speed so the rotor is running at the right TSR in the gust or higher wind speed and you extract the most power possible at all times.  The gear ratio I am using is a balance between speed, torque, rotating mass, and acceleration time.  The trick to extracting more kWh from these style turbines is to keep the rotor operating at, or close to, optimum at all times.  By using gearing in my turbines I have cut the rotating mass to less than half of what you can achieve with a direct drive, while getting the power at the generator with speed instead of torque.  But you can overdo the speed and there is a point of diminishing return.  I've tried higher gear ratios but they are more suited to larger rotors.  In the 3.5-4.0 meter size range of rotors I've found that .4375 is ideal for these axial air gap generators.
--
Chris

[ChrisOlson]

The big snag with single phase is mechanical vibration. by using two single phase stators positioned 90 deg electrically apart you solve the vibration problem, it then runs smooth as any polyphase winding does.

Flux, one the things I really like about the dual stator IRP two-phase configuration is how quiet and vibration-free it runs.  The three phase is constantly groaning and moaning and you tell how much power it's making just by the sound after you learn to associate the pitch of the moaning and groaning with the ammeter.

On the three phase neo units I went to jam-nutting the stator mount a long time ago so they don't loosen up.  That's not required on the dual stator two-phase.  It runs so vibration free that there is no tendency for things to loosen up.  I don't know what causes all the harmonics in the three phase windings, but on the 12G at about 1.8-2.0 kW output the pitch of the sound reaches almost a howling noise that is very loud.
--
Chris

[Menelaos]

Hi Chris,

I cannot qiute follow your arguments...I do not quite see why the system should be able to accelerate faster if the resulting RPM of the alternator is slower because on the other hand side with your dual stator design you have more mass to get moving...but anyway, it works, so that is what counts in teh end...

I think it is time for some pictures on some experiments I have made on the ferrits.

I took some pieces of metal bar and placed the ferrit magnets on them. They are 15mm high. I simulated 2 different airgaps, 8 and 16 mm by using spacers. Because Flux leakage has alsways been a great issue on this bord, I wantet to see how much difference that really makes.

On my first test I placed the magnets next to each other with a gap between them that about equals the magnets width which is usually recommended to get the best results...but at least it should be smaller than the airgap...


On this first test with the airgap of 16mm, I get an average flux of 0,2 Tesla and 0,29 Tesla at an airgap of only 8mm.

I then placed the magnets directly attaching each other in the same constallation
The results were 0,18 Tesla on 16mm airgap and 0,27 Tesla on 8mm airgap.

From that i for myself conclude two main informations:

1. The flux density was hardly effected by the space between the magnets on one rorot disk...at least with the ferrits, with the neos I didn't try.
So for my next small axia flux alternator for a 2 m machine I will order wedge shaped magnets to form a circle where all the magnets are direcly attached to each other. For my calculation In can then get up to 55 % more magnetarea on the same rotor space but only loose less that 10 percent of Flux density- so I can gain from that!

2. Doubbling the airgap did not give me half of the flux denity or even less as one would assume. On 16mm Space, the flux denity was still about 2/3 of the one meassured on 8mm air gap. Saying that I have to add, that the flux denity meassured on the 8 and 16mm airgap was taken in the middle between the magnets and in the middle of the airgap. But on the 16mm space readings, the flux denity will increase when getting the sensor closer to the magnet surfaces, so the flux is not constant anymore, decreasing towards the middle of the airgap.

To me this means that I will design my airgap to be about 15mm as I can get quite a lot more copper into the stator without loosing too much flux density...so I can also get the resistance down using more copper and still keept the voltage up.

Third ting I will do is to change the coil geometry to what people consider to be uneffective- I don't care...as I made voltage tests that came up with similar effects on voltage hight as the space between the magnets on one disk had on the flux leakage ;-)

So I can get the whole thing a lot smaller...or keep it on nearly the same size without having too much problems with the internal resistance... :-)

Max

Ok, images dont work, I have to find a way of compressing them and will post'em then...

[ChrisOlson]

I cannot qiute follow your arguments...I do not quite see why the system should be able to accelerate faster if the resulting RPM of the alternator is slower because on the other hand side with your dual stator design you have more mass to get moving...but anyway, it works, so that is what counts in teh end...

Hi Max,

It has to do with frictional losses in the gearing.  The gearing is not "free".  At .4375 on these size machines I'm at about 97% power efficient on the transmission gearing.  I can more than make up for that in increased generator efficiency and reduced generator rotating mass and get a net gain of about 6% over what you can get with a direct drive.   But when you increase the gear ratio in an attempt to further reduce generator mass and increase efficiency there comes a point of no return where you can't increase generator efficiency enough to make up for increased losses in gearing.  It's a balance game of power efficiency of the drivetrain and generator.

Most people just assume that with gearing you get "losses" and that's "bad".  It's not when you can use it to increase the efficiency of another component of the machine that more than offsets the losses in the transmission.  I have extensive experience with this because it's what I used to do when I started out my career as a mechanical engineer for Cummins.  For most people who just slap a speed increasing drive on a wind turbine it's probably not going to work unless you do some careful matching to make sure you're getting a net gain in power output by doing it.

So what I'm saying is that just because the .4375 ratio works does NOT mean that if that's good, then more must be better.  It doesn't work that way.

On this first test with the airgap of 16mm, I get an average flux of 0,2 Tesla and 0,29 Tesla at an airgap of only 8mm.

IIRC, I measured .196 tesla at 18 mm air gap with 25mm thick C5 ferrites.
--
Chris

[scoraigwind]

I got a bit left behind by all this going on while I was not paying attention,  but Oztules did a good job of defending me thanks.  I think that I often come over as being the voice of stagnation and blind dogma which is a shame when what i am trying to do is explain the good logic behind my design preferences.  They have often evolved out of making a load of mistakes and trying stuff that did not work. 

I also love to watch others and I believe that I am learning some new tricks.  But it frustrates me to think that lots of people will go away and build stacked alternators just because Chris had good results with one due to his confined mounting space and need for a phase shift.  I still contend that he could have built a direct drive unit with the same materials but he didn't want to, and why should he?  But there is an underlying theoretical basis for all this that makes the dual stator very unattractive.

Come on Hugh, a theoretical design experiment? You and I both know there can be huge variation between theoretical and the real world, I don't quite understand the almost religious zeal in defending the single stator axial against all comers. It seems to me no design should be a dead end, there should always be constant innovation, constant experimentation.

As I wrote above, I love to see the practical experimentation, but if you stick with a single design formula as I did with the 'recipes' then it does become very predictable,  so predictable that I don't have to make test coils or to change my mind about the winding details. Under normal conditions I can scale it up and down and the alternator comes out with exactly the performance I predicted.  If it's more than 5% off then I am concerned and start scratching about for reasons (bad magnets etc).  And as you may have noticed I make them in all sizes and voltages with the same basic concept.  So I can do that design experiment in full confidence that it is real.  It's boringly easy to do, but I found the result interesting.  I didn't expect the single stator version to come out so much better (although I knew it would be a bit better).   And I am sure that the same benefits could be found with ferrites although the single phase winding is a handicap.

i wish I could model the effects of moving magnets closer together as easily as I can predict the outcome of my familiar recipe style machines.  I am motivated to use closer magnet spacings and coils with smaller holes in the middle now for fun.  I do pay attention to what other folk are doing.  I am still convinced that leakage between magnets is a serious issue, but Max makes me want to check it out again.  Thanks, guys!

[Flux]

With the early axial air gap alternators using wound poles as produced by Ferranti, Siemens and others about 150 years ago the flux leakage between poles was a serious issue, to such an extent that Mordey designed a unipolar version that eliminated this leakage and it seemed to perform better than its normal equivalent. These wound poles were very long in relation to the air gap.

With short high coercivity magnets the magnet length is tiny compared with the wound field poles and crowding the magnets is nothing like as big an issue.

The best proportions then to be when the air gap is comparable with total magnet thickness ( magnetic length to avoid confusion). it seems likely that you won't measure much difference on a flux meter until the gap between magnets comes down to something like magnet length ( thickness). Measuring gap flux with a flux meter is not the most effective way to spot increasing flux leakage, a search coil and integrator or running with a test coil may be more sensitive.

If you benefit from less wire length in the coil than you may be better off tolerating a bit more leakage flux by crowding.  if the hole in the coil is smaller than the magnet you will not link all the flux with all of the turns but if you link most of it with a turn of significantly lower resistance you may come off better.  This all depends on how you load things, with conventional alternators loaded resistively with loads many times the internal resistance this will not be a good approach. if you are charging batteries and already have significant line resistance then any reduction in coil resistance may be more use than worrying about a bit of lost gap flux due to leakage or imperfect coil loop area.

How these things interact is too complicated for me but should keep the mathematical wizards busy for a while and it may produce useful results.

[Menelaos]

So I guess I just have to find out by trying :-)

[Janne]

I can verify with my small rep that Hugh is correct with the single stator approach. It probably takes a certain level of theoretical understandment how it all works until it the reasons behind it became clear.

Anyways, I think it's easiest to understand with an example of material use. If we take a certain alternator, and make another similar one (=double stator approach) we obviously double the power it can handle. Ok simple enough. Now, if we would take the materials of the 2 alternators and build a large single stator unit, with similar magnet spacing. Let's first just imagine we kept the number of coils the same as machine #1, but just doubled the amount of magnets. In that case the magnets will cut the coils twice the speed, hence we will get 2x the voltage and also double the power handling capacity. But, because of the doubled winding space, we can also fit a second set of coils, which will further double the output power capacity.
So, in conclusion, build a stacked stator version, and you double the power handling capacity, but put all the materials in one big unit and you effectively quadruple the capacity.

So this is why it should be crystal clear why the single stator approach is the best use of materials. It's sometimes sad to see great effort put into projects that can be clearly seen as inferior to some other design from the start. Thinking outside the box is what "pushes the envelope", but it's no point in doing things that are doomed from the start.

Exceptions of course apply, like with Chris's machine (which is one gorgeous machine btw) where the generator diameter was limited.

[scoraigwind]

I hope you don't mind some commentary,
Max wrote:

"I think it is time for some pictures on some experiments I have made on the ferrits. I took some pieces of metal bar and placed the ferrit magnets on them. "

It's a pity we don't get to see the pictures but I guess the bars are on both sides like the disks in an alternator.

"They are 15mm high. I simulated 2 different airgaps, 8 and 16 mm by using spacers. Because Flux leakage has alsways been a great issue on this bord, I wantet to see how much difference that really makes."

I would expect it to make little difference to the flux in the middle of the magnet face but to reduce it dramatically at the edges.

"On my first test I placed the magnets next to each other with a gap between them that about equals the magnets width which is usually recommended to get the best results...but at least it should be smaller than the airgap..."

With very expensive magnets, I have always tried to minimise leakage and also maximise the amount of copper I could put into a stator so as to get the most power.  So for that I like to space the magnets more widely than (say) Dan.  Much more widely than Ed Lenz.  I like to build a big alternator with stator surface for power dissipation and (I thought) lower resistance, although I may have over-estimated that part.


"On this first test with the airgap of 16mm, I get an average flux of 0,2 Tesla and 0,29 Tesla at an airgap of only 8mm.

I then placed the magnets directly attaching each other in the same constallation
The results were 0,18 Tesla on 16mm airgap and 0,27 Tesla on 8mm airgap."

I can't believe you got that reading right at the edge where the magnets touch?  ...and then moving sideways it flips to the same flux in the opposite direction?  Or were the readings taken in the middle of the magnet faces?

"From that i for myself conclude two main informations:

1. The flux density was hardly effected by the space between the magnets on one rorot disk...at least with the ferrits, with the neos I didn't try."

There is no reason why the different types of magnets would behave much differently as they have similar looking demagnetisation curves..   Just rather different strengths and price tags.

"So for my next small axia flux alternator for a 2 m machine I will order wedge shaped magnets to form a circle where all the magnets are direcly attached to each other. For my calculation In can then get up to 55 % more magnetarea on the same rotor space but only loose less that 10 percent of Flux density- so I can gain from that!"

There is no doubt that you will get the most power from a given diameter disk by cramming the magnets in there.  However I would argue that when magnets are more expensive than copper you can get more bang from your magnets by putting them spaced out on a larger disk.  I come from a history when I had to save up money to buy magnets, even for a ferrite machine (1980s) but now those days are gone so you could be right.

"2. Doubbling the airgap did not give me half of the flux denity or even less as one would assume."

The relation is not linear like that.  Otherwise when the gap gets small it would go to infinity.  With a small air gap like this it's the reluctance of the magnets themselves that limits the flux.  With a larger gap its the combination of the magnet length and the air gap, with a contribution from leakage...  When you are near the sweet spot then you gain will space at about the same rate as it becomes less useful.

 "On 16mm Space, the flux denity was still about 2/3 of the one meassured on 8mm air gap. Saying that I have to add, that the flux denity meassured on the 8 and 16mm airgap was taken in the middle between the magnets and in the middle of the airgap. But on the 16mm space readings, the flux denity will increase when getting the sensor closer to the magnet surfaces, so the flux is not constant anymore, decreasing towards the middle of the airgap."

It's noticeable that you can get higher voltage if the stator is getting too close to one rotor, but of course you do have to adjust it into the centre for best reliability.  The difference is due to leakage.

"To me this means that I will design my airgap to be about 15mm as I can get quite a lot more copper into the stator without loosing too much flux density...so I can also get the resistance down using more copper and still keept the voltage up."

The voltage is balanced by the square of the resistance, so you need 22% more space to pay for 10% less voltage.

"Third ting I will do is to change the coil geometry to what people consider to be uneffective- I don't care...as I made voltage tests that came up with similar effects on voltage hight as the space between the magnets on one disk had on the flux leakage ;-)"

Keep on doing the experiments!  I think we have stuff to learn from them.

"So I can get the whole thing a lot smaller...or keep it on nearly the same size without having too much problems with the internal resistance... :-)"

To be honest the actual size has never been a thing I have had a problem with myself.  I actually like big alternators.
Thanks again.

[fabricator]

Has anyone ever cast a 36 inch stator? Will threaded rod on the periphery support the inside circle on as 36 inch stator? Will a 36 inch stator 5/8 inch thick even support itself?
It seems like such a stator would be subject to quite a bit of movement on the inside diameter under sudden yaw movements.

[Menelaos]

Hi Hugh,

Of course i do not mind comments, that is what this is about, sharing and learning :-)

"It's a pity we don't get to see the pictures but I guess the bars are on both sides like the disks in an alternator."

I can send them to you via email, my graphic converter does not get each of them smaller than 250 K so I cannot cope with the limit of 150 K...or I have to look for another tool to compress...

"I would expect it to make little difference to the flux in the middle of the magnet face but to reduce it dramatically at the edges."

Thats right but that problem appears anyway. I have tried that with magnets attaching each other and also with plenty of space between them. It is not so easy to get to see the differences here. Anyway, the flux is strongest in the middle decreasing towards the edges so the average flux will be al little lower...how much..I have no idea, so all I can do to make things comparable is to measure in the middle athough I know that this is not the best way...but it is the way I have the facilities for.

"So for that I like to space the magnets more widely than (say) Dan.  Much more widely than Ed Lenz.  I like to build a big alternator with stator surface for power dissipation and (I thought) lower resistance, although I may have over-estimated that part."

I started up this way as well but then soon changed the geometry of magnets and coils. Having much space between the magnets means that for the density of magnetic material is less then it could be...
Of course, for expensive neos this might be an aspect to prevent flux leakage but these ferrits only cost nuts...so I do not really care spending 10 Euros more on magnets and loosing maybe 10 or 15 % of flux if I can compensate that with more than 50 % of gained magnet area. It might not be the technically ideal solution but I feel this compromise would make a plus in the end.

"I can't believe you got that reading right at the edge where the magnets touch?  ...and then moving sideways it flips to the same flux in the opposite direction?  Or were the readings taken in the middle of the magnet faces?"

Readings were taken in the middle...but as I already explained I cannot really see much of a difference. If you get to the "End" of the magnet, the flux goes to nearly zero anyway...
Why...or if that really is the case...I do not know


"The relation is not linear like that.  Otherwise when the gap gets small it would go to infinity.  With a small air gap like this it's the reluctance of the magnets themselves that limits the flux.  With a larger gap its the combination of the magnet length and the air gap, with a contribution from leakage...  When you are near the sweet spot then you gain will space at about the same rate as it becomes less useful."

I know that, all I wanted to say was that realy small air gaps are not the best solutions as well as really big ones...


"It's noticeable that you can get higher voltage if the stator is getting too close to one rotor, but of course you do have to adjust it into the centre for best reliability.  The difference is due to leakage."

here I have to disagree Hugh!
With the stron neos I do not see this effect at all or only minimal. The leakage ...or whatever we want to call it, does appear with or without magnets beeing verry close. There is a certain flux on the surface of a single magnet. For my ferrrits it is about 0,2 ohm if I recall right. The further away you go from the surface, the flux will decrease fast. Higher flux can only be acchieved when u have a pair of manets and the "flux forces" add up. Now, it the airgap is too big, the flux will increase from the middle of that space towards the magnet surfaces but that has nothing to do with flux loss but with the way the magnetic field arround the magnet is formed anyway. It is limited in range and when the pair of magnets is too far from each other, the flux will continue with its original way, from one magnet pole in a circular way around the magnet to the other side...that is what we saw on that experiment...i think...

"To be honest the actual size has never been a thing I have had a problem with myself.  I actually like big alternators.
Thanks again."

I would not care as well...but the turbine does. Not size is the problem but weight. Furthermore for a bigger stator, more resin is needed, laserparts become a lot more expensive and heavy as the volume of the rotor disks rise squared to the diameter....
and last but not least...there is a point when it starts looking stupid :-D ...but thats a question of taste...

Max

[Menelaos]

Hi fabricator...your post came when i was just answearing Hught...

I have already made really big stators... the biggest one was about 1m in diamter. It will support its self but you have to be verry careful with the compilation of resin and glasfibre...

Do not make the stator thicker than 20mm, whatever you do or you will get heat problems anyway. that big stator was mounted with 30 bolts of M16 to support the structure. I did not pour the resin, I made it step by step in 4 layers. only one glasibre cloth on top an bottom will not be enough, the whole stator was stuffed with glasfibre peaces. That is what gives the strenght, not the resin..

Max

[scoraigwind]

"There is a certain flux on the surface of a single magnet. For my ferrrits it is about 0,2 ohm if I recall right. The further away you go from the surface, the flux will decrease fast. "

Ok the lines of flux don't have an end, so the only way they can become less dense is by diverging.    i call this divergence leakage although it can be called fringing.  The flux leaks away to the surrounding disks but it leaks worse if there are surrounding magnets and the air gap is large in relation to the magnet face.

" Not size is the problem but weight. Furthermore for a bigger stator, more resin is needed, laserparts become a lot more expensive and heavy as the volume of the rotor disks rise squared to the diameter.... and last but not least...there is a point when it starts looking stupid :-D ...but thats a question of taste..."

When trying to match the blades to the alternator it actually helps if the alternator is not too efficient, but we also don't want it to burn out.  So for the low tech solution a large area for heat dissipation can be the simplest way to achieve good blade efficiency and high power at once.   Compact and efficent is also interesting since you can add heaters in series if you need to, but if the large stator saves on overall cost compared to using a small super-efficient alternator that stalls the blades, then I don't mind the extra resin and the chance to look stupid.

As for problems with stator mounting I confess that I have never had a problem yet, although I would not risk putting two stators on one long stud like Chris does.

[artv]

Excelent stuff,...Hi Menelaos, in #115,...you said you could get the resitance down by adding more copper..??
does'nt adding more copper increase resistance??........thanks.......artv

[ChrisOlson]

As for problems with stator mounting I confess that I have never had a problem yet, although I would not risk putting two stators on one long stud like Chris does.

Hugh, two things on that -

• Remember that I using gearing.  Power is a function of torque and speed.  My torque at the stator is about 30% of what you have on a direct drive because of the speed.
• The dual stator unit uses five 3/8" studs and there is about 3 mm gap between the magnet faces and the stator face on both sections of the generator.  I have not had any problems with it loosening up or flexing even at extremely high outputs (over 2 kW).
  

I would not recommend that setup for a direct drive because it wouldn't hold up.  For a direct drive dual stator unit of any real capacity (1.0 kW or more) you would need a stator support on both ends of the generator with thru-studs holding the stators.  It would then become a unit standalone generator with the endplates supporting both the shaft bearings and the stators.  My unit is designed to stack on a PTO shaft turning at high speed in relation to the (turbine) rotor.  There are some basic engineering differences there which dictate the design.
--
Chris

[Menelaos]

Hi Artv,

More copper can make more or less resistance. It makes more if you use it to make extra turns or a lot more turns with thin wire. If you use it to keep the number of windings but take a wire with more diameter, this will decrease the resistance as it is dependend on the cross sectional area.

Max

[artv]

Don't thicker windings (wire dia.),....create eddy currents,..causing loses....?
Cross sectional area.....I assume that means as the magnet pole passes the coil leg......
Isn't thicker wire just to handle more amps??
Just what happens when I read these postes.... ....artv

[Menelaos]

I think that goes a little far offtopic here so only a few words more to that...

You are right about the eddy currents but too thick wire is also difficult to handle. I use it up to maximum 2mm diamter and after that use 2 thinner wires in hand.

crosssectional area only means the area of the wire that u see when you cut it...
Thicker wire can hadle more amps, thats right but the reason for this is that it has a lower resistance and thus causes less heat and thus heat which is important to our designs here...

Max

[Menelaos]

Ok...i found the right menu so here are the pictures of my meassurements:






Max