Flux path

[klsmurf]

Hi All,

I'm finally hopeful of having some time to work on my small axial windmill. In order to rebuild my stator better, I have a few small spools of different gauge wire for some test coils,.  I also bought some small neos that perfectly fit a small axial like mine.  I'm thinking of applying some Gubberment reasoning. Why build one when I can build two for more than twice the price?


I'd like to do some coil testing with these new magnets, but I don't have any blank rotors. I probably won't get them for a while.  The magnets on my current rotors are encased in vinyl ester and have no leakage out the back, as far as I can tell.
The figures below are my attempt to represent the two possible choices of placing new magnets on my existing rotors for testing some coils. Blank rotors I'm sure are optimal, but I'm curious.

Questions:
 1)  In the first figure, does the flux increase because of the extra outside magnets or is it disrupted because of the steel plate in the middle?
2)  In the second, since the magnets are in repulsion, are the field lines simply “compressed” in the steel plate and the flux has no increase or decrease?
         N l S      N l S                                       N l N     S l S            
         S l N      S l N                                       S l S     N l N      
         N l S      N l S                                       N l N     S l S

These questions of coarse led to more what ifs. 

3) When setting up a dual stator machine which below would be optimal?
 The first has a non-magnetic disc holding the magnets. 

Dual Stator Set Up
      
        l N     SN     S l                         l N     S l N     S l             l N     S l S     N l
        l S     NS     N l                         l S     N l S     N l             l S     N l N     S l
        l N     SN     S l                         l N     S l N     S l             l N     S l S     N l

Thanks for your help.

[SparWeb]

I may not be picturing your set-up perfectly, but I'll dive in nonetheless:

The big picture is that the backing disk of a magnet rotor is part of the flux path.  By putting magnets on the other side, you are disturbing the flux path of the magnets on the other side, and they will also disturb the flux path of the magnets you just added.  I'm sure you know this (or you wouldn't ask) but I think you're looking for a quick way to set up a few tests, just to get the coil winding right and move on...

Perhaps you could make a rough estimate that, if you did everything else perfectly, then the contribution of the backing plate to each group of magnets is 1/2 of what it should be.  That probably isn't true either, because the flux density in the rotor disk cannot increase beyond the saturation limit, and the first magnets probably already did that.  Unless the backing disk is very thick.  Perhaps it would have to be 2x as thick as the magnets themselves.

Even if there is no measurable leakage on the back-side (can a paper-clip stick?) the flux density could be high enough that your experiment could be invalid.  Depends on what you want from the results, though - if it's just to make some quick comparisons, then this is still a go.  If you intend to get detailed and mathematical then it's probably the wrong starting point.

[electrondady1]

in my own experiments i tried the first example of no.3
created a third magnet disk with no metal backing.
just set the mags up as best as i could on a grid and then cast resin around them.
it worked ok with two  stators.
  it was difficult to find center and be perfectly balanced.
if i did one again i would find center on a non magnetic disk and then create holes and insert the mags into them.
still, balance is difficult because the mass of the magnets is so much greater than the mass of the disk.