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Dual Rotor Toroid Core PMA build

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CraigM:
Hello All,

I have a project I started and could use a little help and guidance.

First thing I'd like to mention is this build is more of a learning tool for myself on PMA theory and less of an exercise in getting this up in the air and making power. Flying it will come in time but I'm relocating to Florida soon and don't see the maiden flight happening for another year.

Specifications:
Dual rotor 11.250” OD
Toroid core 11” OD x 7” ID x .600 thick
Three phase, 24 coils, 8 per phase
Magnet poles, 8 per rotor, N to N & S to S facing each other.

A while back I came across some rather large sections of silicon steel at the local scrap yard. I started a radial PMA with the intent of using the silicon steel to make my own un-slotted core but it was slow going and a rather daunting task. So with a pile of extra material and a guillotine shear at my disposal at work I started cutting .600” wide strips to be wound as a toroidal core.

Much of the information gathered was gleaned from this posting. http://www.fieldlines.com/index.php/topic,145994.0.html

The winding of the core was rather uneventful as I was lucky to have 36” lengths of strip to work with. The material (.015”) also had the factory insulation so no extra insulation was added. I began the winding by using thinned epoxy between layers but this was really messy and hard to control so after only a few winds I switched to cyanoacrylate adhesive. I found I could wrap multiple layers and then apply thin CA glue to the edges and it would wick into the core and with a small puff of chemical vapor set almost instantly.



The next step was adding the coil spacers which were cut from .125” thick G-10 (also called Epoxyglass or Garolite). It's what I had on hand and an educated guess (at best) of how thick the coils will be. If needed I can always add to the spacer thickness by adding a thin layer of mmm… let's say balsa wood. The spacers were glued to the core using JB Weld and were held in place by magnets while the epoxy set up. It was slow going and a giant pain in the A$$ gluing 48 spacers to the core but it came out really strong. My first attempt was with 5 minute epoxy and wanting to know how strong it was I purposely tried to make if fail and it did. With the JB Weld I can hold on to the spacers and shake it back and forth with no failure in the adhesive. The photo shows a plywood ring (cut in two) that supports the assembly. Not sure how comfortable I am with the strength of the plywood so I may change this out with some Phenolic sheet. Each spacer will then be drilled and mechanically fastened to the support ring. The support ring of the stator will also have mounting holes to allow adjustment between the rotor plates.







So the bones of this PMA are nearly complete with a little cleanup work to do on the stator. What I need to decide (figure out) at this point is what gauge wire to use and what magnets to use. I'm leaning towards using Ferrite magnets (especially for the salt air of the Florida coast) but I'm unsure of overall size. In the posting mentioned earlier “Rewinding a Proven Stator” Flux mentioned keeping the flux density in the core below 1.5T and ideally lower if you want low core loss in low wind. How do you determine flux density in the core? If I take the tesla strength of ferrite at .38T and using for example a 1” x 2” x 2” magnet (feeding from both rotors) and condense that to the cross section of the core .6” x 2” I get 4 sq. in. magnet area divided by 1.2' core cross section = 3.333 x .38T = 1.266T in the core. Is this correct? Or can I push the magnet square inch size up until I hit the max 1.5T core density? I have my eye on 1” x 2” x 6” ferrite blocks that I'd like to cut into two wedge shaped blocks.


Black outline represents 1" x 2" x 2" Ferrite magnet.


Black outline represents 1" thick Ferrite magnet with 4.9 sq. inch of area.

The number of winds per coil will require a single test coil to be made first. The width between the coil spacers is .85” +/- .015” (best I could do eye-balling spacer placement). Depending on wire gauge (13 or 14) I should be able to get 12 to 13 winds in one layer. Ideally I'd like to get full layers (1, 2, maybe 3?) for each coil and not end up with something odd like one and a half layers to reach the desired cut in voltage. Have any ideas how to accomplish this?

I have to admit the electrical side of things is my weak point and what I need most to study and learn about. Mechanically I think I'll do okay but any and all input is welcome and appreciated.

Quite a bit to start a new post with, I'm sure I missed something.
Thanks, Craig

SparWeb:
Personally I would go overboard with the math, but that's me and my weakness.  As for giving advice, I think you've worked through it well enough to go ahead and do it.  The rough-size estimation of the magnets looks good enough to show that there will be an output of useful proportions.  Let the geeks make it better later.

One calc that I'd like to see first, since you haven't worked out how many turns to use, is a rough idea of the EMF per turn of wire.  With that you can get a voltage per # of turns, and that will start us working out wire gauge and turns for a given battery voltage.  I feel like opening a couple of the calculator webpages and see what drops out.  You've given a bit of geometry to start...

Have you worked out just HOW you will wind the thing's coils, yet?

SparWeb:
Back of envelope calcs say that 14 gauge may work out okay, but you will need two layers in the coil, with 13 turns in each layer across between the spacers.  I'm not sure how easy it will be to work with 14 gauge.  You can always wind "two-in-hand" with 17 or 18 gauge and get nearly the same result.
I'm coming to 250 RPM cut-in, for 12V battery charging, 3-phases in Star.
Phase resistance about 0.3 ohms maybe.

CraigM:
Thanks for the reply SparWeb.

In a previous post Hugh P. mentioned having wound a Proven stator and did so by transferring wire to a bobbin and then passing the wire through the center of the torrid core and winding the coils by hand. Having never worked with 14 gauge wire I'm not sure what difficulties I may run into. The coils are wound prior to attachment of the outer supporting ring.

I have no idea how you arrived at the approximate EMF per turn of wire using only the geometry given. Where can learn more about how to do this?

Thanks again,
CM

SparWeb:
My math was done hastily, there may be errors.
A good reference for physics/science math is Hyperphysics:    http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

I drew your stator to scale (not as hard as it sounds for a former CAD jock like me) and got 7/8" space between your wedges.  Wire wound between the wedges will lay relatively flat on the first layer and the second will settle on the grooves made by the first.  So a fair guess of the number of turns per layer is just the space across divided by the width of a wire.  0.85 / 0.064 = 13 turns, or thereabouts for 14 gauge, per layer.

Each turn is a loop, each loop encloses the laminations.  Taking your estimage of field intensity at face value (I can do no better right now) at 1.5 T and let's also say that it's evenly distributed through all the laminations (again I can't argue against it right now).
So that means that through every loop of wire 1.5T of mag field passes, and when the next magnet has come and gone the field will be -1.5T, meaning it points the other way, now.  The total field reversal is 3 Teslas.

Back to geometry, your laminate cross-section is 2 inches by 0.6 inches, or 1.2 square inches.  Now we must turn to the metric system because flux calculations in imperial is insane.  The area is 774 square millimeters.  Also worth calculating is the length of each turn:  perimeter of the rectangle is 139mm.  For right now all we need is the area enclosed in each loop, and get back to wire perimeter later.

The definition of FLUX is the total field that passes through a loop at right-angles.  Your loops fit this description, so through each turn, you can say that 1.5Teslas pass, and there are 26 turns if you put on 2 layers of 14 gauge.

1.5T * 774 mm*mm * 26 = 30186 microWebers.
This should be done in "meters" not "millimeters" so there are 1 million square mm in a square m:

30186 / 1,000,000 = 0.03 Webers.

A Weber is the standard unit of FLUX.   It's at this point that you can't mix up "field" and "flux".  Kinda like mixing up "Watts" and "Watt-hours"; they mean different things.  The flux is a unit of the field within a certain area, pointed in a certain direction.  Once the flux goes from N to S it will reverse the direction.  This is the effect that causes electrical potential to be created by electrical machines of all types.  The voltage you measure is scientifically called "electromotive force" or EMF which is produced every time the total Flux changes.  If the flux is constant there is no EMF.  Change the flux rapidly or change a large flux more slowly, either way it will produce an EMF which makes electrons move.

240 RPM is a nice round number that suits the speed of WT rotor blades at the size of machine you are building.   240 revolutions per minute = 4 revolutions per second.  Your rotor and stator have 8 "poles".  The flux will reverse completely 4 times per revolution.  Together your stator will experience 4*4= 16 flux reversals per second.

We're getting close to the end.  The formula for the EMF is to multiply the total change in flux by the rate at which it changes.

EMF = 2 * Flux * frequency = 2 * (0.03 Weber) * (16 per second) = 0.96 Volts

Each phase has 8 coils in it they will all be in series so their voltage will add:     0.96 * 8 = 7.68 Volts

If you connect the phases of your stator in Star, then the voltages on the line will increase by the square-root of 3.

7.68 Volts * 1.73 = 13.3 volts.

That is roughly good enough as a cut-in voltage for battery charging a 12 volts system.
The variables under your control are the wire gauge and size of magnets.  Increasing the magnets will add more flux (but at diminishing return) and reducing the wire size will increase the number of turns.  Doing either or both of these things will increase the voltage, and it's quite feasible to re-do this to size it up for a 24 volt battery system.

On the other hand, we haven't worked out the amount of current that will flow.  I'd better hit the "Post" button because I've typed enough already!

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