Doubling up magnets vs additional stators in an axial flux PMA

planetlajord

Hi all, new to register but long time reader of the forum. I need advice on magnet configuration and optimization of multiple stators in my 3 phase axial flux alternator.

 I know ring magnets are not ideal (in comparison to rectangle) but I have x 24, extremely powerful cylinder/ring magnets (1” deep, 1.25’ diameter, with 3/8” diameter hole in the center. I also have x 36+ coils (with access to many more) salvaged from street light ballasts labeled as 240 volts (2.5”x 3.25” with 1.25” by 2” hole).

None of this will be an exact science as I am using free and available materials so its what I got to work with. My struggle is determining how to best arrange these for maximum performance in a single, large turbine.

Scenario 1)

Three separate stators consisting of 9 coils each with two rotors between them with 12 magnets each (in groups of two,  so six magnet positions, nine coil positions). see diagram.

Scenario 2)

5 separate rows of stators consisting of 9 coils each, Four rotors between them with 6 magnets each. see diagram.

Should the usage influence which solution to use, I will be tying this directly to water heating elements to heat an underground (or insulated) water thermal battery. So raw wattage is my priority over clean voltage. Size and volume of thermal battery (and number of heating elements) will depend on the turbine output (hoping for 500 – 1000 gallons, perhaps too optimistic). I live on the open prairie with abundant wind (southeast Saskatchewan Canada) and turbine will be placed on a 50ft tower.

Questions:

Would the doubling up of magnets in each position (to make one longer field over a single coil) produce a stronger or wider field to accommodate/fill the coils opening? or would the gains from this be minor in comparison to having a more rotors and coils using single magnet per position?

Would the irregularities from the “figure 8” profile field (from the two combined magnets) cause inefficiencies interacting with the coil? Is a more unified field (more similar in shape to the coil) preferable?

Also considering sandwiching the rotor magnets between two metal plates for rotation precision and longevity (saw blades). Does physical connection to a metal plate effect the performance of the magnetic fields? Alternating negative, positive in a concentric circle seems to me like a plate would conduct or dull the potency of the field?

In a single rotor, single stator set up this wouldn’t matter (as only a single plate would be required and it would be on the outside pointed away from the coils) but I will have multiple rotors between multiple stators.

Is casting magnets in resin a more desirable method than steel plates ? Just concerned about the precision and longevity (machining the casting to be perfectly balanced).

 

Thanks in advance for your help!

mvas

I don't think the best design is either: Scenario #1 or Scenario #2.
Given that you have 24 magnets ...
In Scenario #2 you have two (2) poles per three (3) coils. 
I think, it is better to have four (4) poles ( N - S - N - S ) per three (3) coils
That means ... double the number of magnet positions placed in the circle on the rotor
12 poles ( magnets ) on each of the steel rotors x 2 rotors = 24 magnets.
The single thin stator ( with the coils ) is sandwiched between the two rotors.
So, two (2) steel rotors and one (1) thin epoxy stator = lowest mass

Your drawings show the opposite ...
You show each rotor sandwiched between two stators = wrong?

NO! ... Do not put a steel plate on both sides of the magnets.
The magnets get mounted onto a steel plate, with the magnets facing towards the stator

Google Search for "hugh piggott" 

Many builders consider the following criteria, when designing their PMA ...
a) the blade diameter
b) the blade speed (TSR)
c) the wind speed
d) the magnetic force
c) the # of coil turns
d) the coil thickness
e) the max watts from the PMA ( and Voc, Isc & Ohms ) 
f) RPM's at cut-in voltage

The thickness of your coils may, or may not, be optimal.
They have written whole books, that answer your questions, and reveal much more ...

Will your magnets rust?

BB.

Some links that have been recommended for DIY wind power:

Hugh Piggott - Scoraig Wind Electric site for tons of info (from mike90045)
Scoraig Wind "Recipe Book" for DYI Turbines (from Chris Olson... From his 4/11/2013 post)
www.greenpowertalk.org (added from "russ"--Like here but more wind/less solar)

-Bill

planetlajord

I will have to admit, I am a little embarrassed I didn't see the doubling of magnet positions on the rotor, just mocked it up and shaking my head that I didn't consider that before. Rookie move.

So in theory, which one is better? More coil mass per magnet position or more magnet force per coil position?

Magnets are 80 - 110lbs strength, nickle plated neodymium. There are some with superficial chips here and there so totally sealed encasement in epoxy is a must I suppose. Blade design, dimensions and diameter will be decided once I determine the amount of force required to spin the rotors.

The coils are the same gauge wire and similar turns as I have seen most DIY trapezoidal coils using similar strength (rectangular) magnets. I may sacrifice a coil to establish an exact turn count. Initial experiments revealed a spike of 1.4 volts by just waving a magnet across a single coil with my hand. Spinning two (with opposing poles) at low speed on a drill gave me spikes of 16volts which was very exciting.

Thanks for the replies and resources guys!

mvas

Your designs are still "backwards".
We put one (1) epoxy stator ( with the 9 coils ) between two (2) steel rotors ( with the 12 magnets each )
If you only have 24 magnets then you must use your upper-left "Scenario #3" layout

Your Scenario #4 ( lower-right ) with two stators & one rotor is not a "valid" design = backwards

The proper design is ...
[ Rotor with 12 magnets ] + [ Stator with 9 coils ] + [ 12 magnets on Rotor ]

Note how the two rotors have their magnets pointing "in" towards the single stator.

planetlajord

It appears I had some learning to do (but I think I get it now) thank you for your patience. I didn't realize the importance of the steel plates as backings to complete/focus/strengthen the magnetic circuit flux through the coils (hence my backward designs). So I have now narrowed it down to two new designs but it opens up two more questions (seems the jury is out on this one).

Is it better to have a single rotor of 24 magnets focused on one stater of 18 coils - consequently doubling the amount of coils recieving flux at the same time? Also in a single rotor/stator scenario, does there need to be a magnetic plate on the other side to recieve the magnetic force of the magnets through the coils?

Or

is it better to do two opposing rotors of 12 magnets, each focused on one internal stater of 9 coils - consequently focusing a stronger flux field through each coil.

Just to repeat in case it influences the decision, I do not need clean power or specific voltage levels, I am looking for the highest wattage and amperage possible to run water heating elements.

Thank you so much!

mvas

Scenario #5 is not valid = weak flux = low watts
Scenario #6 is correct = Rotor + Stator + Rotor = strong flux 
Hence the name "Dual Rotor Axial Flux PMA"