Honeywell turbine

[Flux]

Bob you are right with your example of the flywheel magneto. When looking at one coil, the flux linkage takes place in a shorter time at large diameter and as a magneto is providing a single output pulse it certainly does work.

When you look at an ac waveform with a series of linked coils, if you spread the same few magnets over a larger diameter you either end up with a miserable waveform with a high peak and low mean or you get over this by increasing the coil size and hence increase the turn length and add more resistance.

Whether you gain or not depends on the effectiveness of the geometry at small diameter, if it is too crowded at the original small diameter ( flux leakage, non optimal winding etc.) then it will be better at the best diameter so some increase will help, but beyond that things will not improve.

Changing the rotational speed of an alternator is not the same as changing the peripheral velocity by increasing diameter.

The emf depends on flux per pole frequency and number of turns. If you keep the flux per pole the same and the frequency the same you can increase emf by increasing number of turns. With more winding space due to a larger diameter you can increase wire diameter up to a point to keep the resistance down and still gain more power but there is an optimum and if you go too small for those magnets you don't have room for turns and if you go too big you can't keep the resistance down.

If you look at volts alone then within reason there is no limit to what you can get by increasing turns ( and this is much the case for the high ternsion magneto.) When you try to get power out you have to include the effect of resistance.

Having enough diameter is what you need for a given set of magnets, beyond that there is possibly a region where you gain or loose little and it becomes a cost trade between magnet cost and copper cost, but going too big in diameter will not gain anything.

Flux

[bart]

   In my simplified way of comprehending of what bob is saying:
The faster a magnet passes a coil, the higher the voltage spike.
The slower it passes results in a lower spike.
Therefore, a faster rim speed increases voltage.
I would contend that total net energy regardless of speed stays the same, but at higher speeds, thus higher voltage, gives one a better harvest of power.
    Well am I close. 

[Flux]

You are right in that the faster a magnet passes a coil the higher the voltage spike. For something like a magneto where you need a voltage spike then all is fine.

With an alternator for charging a battery things are not so simple. You have higher voltage spikes but widely spaced due to the restricted number of magnets. You get what I described before, a waveform with high spikes widely separated, the average voltage is relatively low and the period when your spikes are above battery volts is a small part of the total time of one revolution. The power you get into the battery depends on the current spike you can get during these brief intervals and this will be affected by resistance and possibly inductance.

The battery charging case is always a bit different from the run of the mill commercial alternator anyway so things can be confusing. With a conventional alternator you would have to keep the waveform somewhere near sa sine wave so you really wouldn't adopt a few widely spaced magnets on a large diameter.

Flux

[bob g]

Flux

i concur with your assessment completely, there are practical limits and compromises that must be made, and it may well be the increase in diameter may not be much before you trade off a higher instantaneous voltage for a lower mean voltage.

the only reason i got into this debate was in response to Chris' assertion that diameter has no function in generation of voltage.

my assertion is it certainly does.

i would like to go a step further.

for a given set of magnets, it is possible that just as going too large in diameter compromises the result, going too small may also present some compromises.   without understanding all the relationships it is possible that one may not be optimizing the use of his magnets with a typical machine diameter.  it might well be a common 12 inch machine using a common neo magnet might benefit from an increase in diameter to maybe 14 inches, maybe even a bit more?

now to determine if this might be true one would necessarily need to fully understand the design parameters and concerns to arrive at the optimal diameter, or ... be faced with going out in the shop and building progressively larger machines to find the spot where diminished returns arrive... the latter being less desirable in my opinion.

there are other things we might do to improve the voltage spike concerns too, things like adding pole shoes to broaden the pole and reduce the spacing between poles being one, distributed windings in the stator poles being another, certainly adding poles/phases being easy enough.

i would like to start another topic to discuss other possible benefits of going to larger diameter machine.

bob g

[Flux]

Having been brought up in an industry with conventional ac and dc machines I tended to carry on the same ideas with wind alternators. It took a while to catch on to the fact that some of the conventional ideas are not the best for battery charging wind power. I have to admit that sometimes it has been mechanical engineers that have forced to make me rethink some of these ideas.

When the requirement is for a high efficiency with a load impedance many times higher than the alternator impedance it sometimes is quite different for direct battery charging wind power where the main requirement is to screw out the maximum power at efficiencies that would be totally unacceptable in normal applications. It is here that the resistance becomes the dominating factor to such an extent that reducing winding resistance becomes a major factor.

Chris and others have shown that crowding things in does in fact reduce resistance more than the other factors work against you. It is still possible to make the diameter too small but it is even easier to make it too large.

Many of the niceties of waveform etc are of no real importance so some of the conventional things really don't help.  It is undoubtedly easier to screw high outputs out of a machine with no iron in the stator. This is far less of a problem with a mppt converter with iron cored machines where armature reaction and leakage reactance cause big problems at low efficiency.

Flux

[ChrisOlson]

for a given set of magnets, it is possible that just as going too large in diameter compromises the result, going too small may also present some compromises.   without understanding all the relationships it is possible that one may not be optimizing the use of his magnets with a typical machine diameter.  it might well be a common 12 inch machine using a common neo magnet might benefit from an increase in diameter to maybe 14 inches, maybe even a bit more?

bob - no, it doesn't.  Getting the resistance down in a wind turbine axial generator provides more benefits than making maximum use of magnet surface area.  Nobody followed my reasoning on this for a long time.  And finally Hugh tried it with ferrites and found the same thing - cramming poles in as tight as possible to keep resistance down yields better efficiency than going to great pains to utilize magnet area more efficiently.

Neo magnets really screwed up a lot of things in battery charging wind turbine design.  They're too powerful and all they basically have achieved is stalling wind turbines without MPPT.  And most of the contemporary designs have gone to great pains to maximize use of what is already too much flux in the air gap.  I put the book back on the shelf and decided there must be a better solution.

I went with very small high speed geared units.  Hugh built a direct drive (with a strange pole/coil configuration) ferrite.  Both yield pretty impressive results using the pole crowding method of construction while at the same time saving on weight, cost and size.
--
Chris

[bob g]

Chris

believe me i am all about lower stator resistance

i too fought a long an arduous battle with EE of all stripes with the lundel/clawpole alternator.

it was accepted without proof that the lundel's losses were dominated by reactance when the reality was the dominate factor was stator resistance.

in proving this concept the once popular belief that automotive alternators are only about 50% efficient at or near full load, just flew out the window.

lower resistance stators and higher generated voltages allow the same alternator (unmodified save for higher piv diodes) to operate not at 50% but at 80% efficiency and higher.

i would expect the same results with an aircore axial, keeping the stator resistance as low as possible ought to be the predominate goal in my opinion.

as you have found, reducing resistance requires increasing rpm, but you have proven that the gains far exceed the losses of the chain drive.

good on you for that one!

i will also state that just because a machine is larger in diameter does not mean it has to have a proportional increase in resistance.  splitting the resistance downward by increasing pole count "and" phases will allow for a reduction in resistance. further reduction of resistance via shortening the end connections can be done by grouping the pole groups per phase closer together, rather than diametrically opposed or distributed equally around the stator.

the chinese ST generator head does this in that there are 4poles, each pair of pole produce 120 vac, as would be expected. what is different is each pair of poles are displaced 90 degree's rather than the typical 180 degree's. the operational result is the same and the pole interconnects are proportionally shorter...
all i am saying is it proves the principle, not that they do it to reduce the resistance by an almost unmeasurable amount.
the reason the do it this way is out of simplicity, the heads were designed to be 50hz 230volt lighting generators for the far east.
when we american's wanted these heads, they changed the turn count a bit and simply split the 4 poles in half, so one half of the machine makes 120 the other half makes the other 120, together of course the 240vac.

the only downside for single phase operation in an unbalanced load situation is you have a stator that is loaded on one side instead of being balanced across the stator. this can cause some heads to whine, some to growl, while others seem just fine.

i only mention this because we could with a larger machine put in more poles, connect for many more phases to reduce resistance and group the phase poles more or less together rather than distributed around the stator. this would reduce the stator resistance a tiny bit more. 

we would not suffer the imbalance issues because all phases would be rectified and presumably the output DC would be paralleled to charge a battery bank.

as you have found out any reduction in stator resistance is generally a good thing, and if it can be easily done one might want to consider doing it.

fwiw

bob g

[jn_austin]

Interesting discussion guys. Thanks. As others have said the large diameter machine does allow you to fit in more poles and coils but that also increases drag so the RPM may be lower then for the same blade area. I think it makes sense. Now just need to work with the Daughter so she understands it. Trying to help her with a radial alternator design using a bike wheel similar to the original posts. It will be a science fair project and not really looking to make real power. Just measure what does get generated and show the performance. Thanks.