PM Generator design

[vacuum1313]

Has anyone information on whether it is more cost effective to have a single magnet rotor with coil stators on either side or a single central stator with magnet rotors on either side such as those in Hugh Piggots Axial flux alternator designs.  The key here is cost/watt not energy density.  Also are there formulae for determining coil dimensions based on magnet size and strength?  Does magnet shape, round, square or rectangular, influence power output and if so how?

[Flux]

For a given quantity of magnets it would be crazy to use a single rotor with stators on either side. Within reason magnet shape doesn't make a lot of difference.

Flux

[vacuum1313]

Please explain why?  Both setups have 3 layers.  Piggot's is;

rotor(magnet) / stator(coil) / rotor(magnet)

The alternate is;

stator(coil) / rotor(magnet) / stator(coil)

In the 2nd both the poles of the magnet are used to do work while in the standard version only 1 pole of each magnet is used.  Even if the flux density of the magnetic field decreases across the coil would you not be able to halve the thickness of each coil, reducing the winding turns by half obviously, however you have 2 coils in the alternate for every 1 in the standard and this should achieve similar energy production?

[Flux]

I didn't really want to go into this, but if you insist, then here goes

With the dual magnet rotor you have an efficient magnetic circuit and you should have a flux density of over 600mT in the gap.

If you use the same number of magnets and fit them on both sides of a single steel disc you will have two single rotor machines. The flux density will not be defined, with a bit of luck you may manage 300mT close to the magnets but it will drop rapidly to near nothing beyond the thickness of the coils you would have used in the dual rotor version.

The net result is that the two stator windings in series will not equal the same voltage using the same coils. You have less than the previous voltage and you have doubled the resistance. Highly unsatisfactory use of magnets.

The only justification of using a single open magnet rotor is to use standard ( cheap ) magnets for a small simple basic machine rather than buy less common magnets that may work out more costly than the common ones. What you gain on simplicity may justify the means.

Hugh's small 4ft machine is a good example It uses the same common magnets as his bigger machines, it is simple for a first timer to build and the characteristics suit the 4 ft prop very well. He doesn't extend this construction to his larger machines for good reason

Flux

[vacuum1313]

Thank-you for your explanation.

[smcardle]

Hi,

I am a newby here and have a couple of small questions regarding stator/rota design for the Axial flux generator. But firstly so as not to be rude I would like to introduce myself and my interest to the community.

I live in Spain with my wife and 2 sons, my name is Steve and I am a freelance Enterprise Java architect/senior developer. This just means I am a software techy. I graduated in Electronics and computer engineering (coils and magnetic flux wasn't my thing though). I have become very interested in building my own wind turbine since I came across many of the projects you guys have posted on the net while searching for a 10Kw turbine for our home here in rural Southern Spain. Obviously we have a lot of sunlight here (around 320 days a year) and Solar is a fine option (during the day) but in the evenings and on overcast days the wind starts to pick up on a fairly regular basis. Also in Spain selling back to the grid is only just being considered so an off grid with battery backup is what I will be looking to build with an automatic switch over to the grid when and if the battery bank gets low thus allowing wind and solar to recharge my bank (however long that may take) without our domestic load.  Once recharged we will automatically switch back to renewables. This may only ever happen infrequently (or for short periods during heavy utility load) as I may be able to size my battery bank and have enough juice from the sun and wind to sustain a completely grid free system (oh pleeeese). Also I am considering a fast start standby generator with enough guts to power the home should the grid go off (oh yes, happens here a lot) and should I need to take down the solar and wind for any purpose such as damage repair. I could also use the generator to auto switch to on the occasions when the battery bank gets a bit low)

Anyway, enough about that, here are my questions....

What would be the disadvantage/disadvantage of having 2 stators and 3 rotas? i.e. in the following setup:

Rota 1 (possibly a disk brake): 12 magnets with top magnet set N

Stator 1: 18 coils (windings and thickness not important at this moment)

Rota 2 (pressed steel with magnets set midway i.e. stick out a couple of mm from each side of rota): 12 magnets with top magnet set S

Stator 2: 18 coils (same construction as stator 1)

Rota 3 (same as rota 1): 12 magnets with top magnet set N

Would this give me effectively 2 identical generators (with the advantage that this setup requires 1 rota less thus saving 12 magnets) i.e. twice the power from a single set of blades?

Would I need bigger blades for my turbine to overcome extra load resistance? and would it make my wind turbine more resistant to higher wind speeds?

Lastly, as I said earlier I am a newby to the whole build yourself a wind turbine scene and have been trying to locate a good source online that describes in detail the best way to define the magnet / coil ratio for a low RPM generator (as I said magnetic flux was not my thing).

Any and all feedback would be great, especially any ideas on improving the system I  have in mind for my home. For instance with two generator outputs I may have 2 battery banks using one as a standby. I would switch them around every couple of months so that they each get a good shot at being the primary supply.

Glad I found this site and have enjoyed reading the posts. I now hope to become a net contributer as I progress with my project.

Steve

[Tritium]

Your question comes up frequently.

The most efficient design (best use of magnets and copper) is 2 rotors and 1 stator between them.

Also magnets should be placed on the rotors N up then S up then N up then S up  etc.

The rotors are then aligned in the final machine with the N facing the S on the opposing rotors so that maximum flux will flow across the coils during rotation.

Magnet to coil ratio should be 4 to 3 for a three phase machine i.e. 12 magnets for 9 coils for example.

Thurmond

[Tritium]

That should have read 12 poles (magnets per rotor plate or 24 magnets total)

Thurmond

[spinningmagnets]

Dear smcardle, welcome. The 3-rotor/2-stator configuration has been tried twice that I know of on this forum. The idea makes some sense on paper, but in practice, the results were disappointing.

Strong magnets and copper wire are expensive, and the best performance so far (per dollar/Euro) has been the Dual magnet rotor with a single stator.

[smcardle]

Thanks for the feedback people.

I did look for posts regarding the 2 stator 3 rota design on this forum but didn't find any.

I would still like to get some scientific explanation as to why a 3 rota 2 stator design does not work (forget the magnet/copper price). As stated previously "on paper" it should work !!! Why would the magnetic flux not be strong enough across the two stators to produce the same output from both?

Surely another alternative for this type of design could be to combine both low and high wind speed usage via a switching controller, where two different stator winding configurations could be made for optimum performance at each speed.

Also, can anybody point me at any info explaining the 4-3 configuration for 3 phase stators?

Steve

[ghurd]

The multiple stator works, it just has more parts, is more complicated, and costs more to do the same thing.  

You may be missing the frequency part of the generation equasion. More magnet poles in a circle means more voltage.

Low and high wind switching is done with a "star delta switch".

"Google search the board" top right menu box.

The 4:3 is a ratio. Same ratio for most dual rotors here. 8:6, 12:9, 16:12.

It allows a single layer of coils.

G-

[spinningmagnets]

Here's a thread about 3-rotor/2-stator:

http://www.fieldlines.com/story/2008/2/1/203054/5179

IMHO it is an answer in search of a question. Its been tried (not by me) but if you still want to make one, measure the output and take pics...

[osku]

What would happen if we did not use a steel disc at all?

Only the magnets in a plastic disc and then two normal stators.

[Tritium]

Less total flux across the coils as the return path through the steel plates would not exist. (much less output vs a steel disk machine)

Thurmond