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Dual Rotor Toroid Core PMA build
SparWeb:
Looking back I notice that I pointed out early on that the magnetic field reversal of 1.5T to -1.5T would give you "3 Teslas" but I shouldn't have put it that way. Sorry for the confusion. Below I went on and used 1.5T for the calculations, which is correct, so in the future just carry on that way. when I calculate the EMF I multiplied the Flux by 2. That's where the Flux goes from 0.03Wb to -0.03Wb so the total change is 0.06Wb. I did it by multiplying by 2 in that formula, and after you get comfortable with the formula you'll see why it goes there. But I started off on the wrong foot and hopefully I've cleared it up - not caused more confusion! :-\
Going on with the math, then:
The perimeter around one turn is 139mm, making 26 turns give you 3.6 meters per coil. Probably more like 4 meters because coils are never perfectly tight. There are also 8 coils per phase so upwards of 32 meters per phase of wire (and 100 meters in the whole machine). On each phase, then those 32 meters of wire will have some resistance. Your 14 gauge has 8.8 ohms per 1000 meters, or 0.88 ohms per 100 meters, thus 0.28 ohms on each phase. That's pretty good. From before, I assumed you will wire it up in Star, so the line-to-line resistance will be 2x the phase resistance, equal to 0.56 ohms.
Now it gets less exact...
Turn the rotor faster than cut-in, say 2x cut-in speed. At 480 RPM the EMF will be twice what it was at cut-in: 26.6 Volts. Deduct from that the voltage of the battery, say 14 Volts, and there's 12.6 volts to push electrons through the whole circuit. Pay the rectifier tax of 1.5 volts or so and:
Ohm's law: 11.1V / 0.56 ohm = 19 Amps. Hey wow that's okay. It won't light the world on fire but isn't a waste of time.
There you go, but it's all theory until you can go put it into practice. Have fun!
CraigM:
SparWeb,
Thanks so very very much for the time and effort you put into this, I can't thank you enough! I don't want or do I expect to be spoon fed information but your reply and the link you provided will go a long way in helping me help myself.
I want a deeper understanding of why a PMA does what it does and not just build from a recipe book. Understanding the math and physics involved will help me move to the next level.
Again, so very appreciative of your time, now it's time to study and learn from your reply.
CM
SparWeb:
That's what I'm in this for, too! :)
I don't feel like the explanation above is spoon-feeding, because it brings together a lot of different stuff into one place for a specific purpose.
Not a lot of folks studied electromechanics in college, though. It helps a lot in a hobby like this.
The most convenient table for wire gauge info is www.powerstream.com. Their table is just a copy of a copy, (like everything else on the internet) but at least it contains all the facts you need to know in the same table, not just wire gauge in one table, resistivity in another, etc. etc.
One of the big guesses in all this is the field intensity that actually does pass through the laminations. It could be 1T it could be 2T, and who will know for sure until you built it and test? May I suggest that before you wind all 24 coils, do just 1 coil, and then spin the rotors, measuring what happens. Small steps may prevent big mistakes, but by all means I hope this works!
jlt:
Wow that.s looking good. It cleared up how to attach the stator for me.
What size blades are you going to try.
The wood should work. If complete enclosed in the stator.
CraigM:
--- Quote from: jlt on April 23, 2012, 05:36:55 PM ---What size blades are you going to try.
--- End quote ---
Well, using the WAG principle, and the fact this is my very first build, and I should be stamping "experimental" all over this thing... hip shot target is 8 - 9 foot diameter blades.
The plywood stator support ring may be okay for testing but I have access to some .500 thick canvas phenolic sheet here at work and will use this for the final build. I feel it will hold up much better outside in the elements.
CM
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