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Dual Rotor Toroid Core PMA build

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CraigM:
WOW! It finally sunk in, the answer was right in front of my face and I just couldn't see it. If it was a snake it would have bit me.

I knew battery voltage clamped or limited alternator output voltage but it wasn't until I thought of voltage as pressure that it made sense. A 24 volt battery system wants to push electrons out at around 26ish volts. The alternator is trying to push more electrons into the battery and has its own pressure / voltage. Not until I exceed the push back pressure of the battery with a greater pressure / voltage from the alternator will a single electron move into that battery.

So with my test I'm getting 25 volts at 190 rpm using two rows (24 turns) of 15 AWG wire. Just reaching enough voltage to get that first electron pushed into a 24 volt battery. As rpm increases I get higher voltage but at this point I can now use ohms law to say Amperes = Volts / Ohms.

I did a voltage test at 340 rpm and reached 47 volts including 1.4 volt subtracted for rectifier. I subtract 26 volts (battery pressure :) ) from 47 and get 21 volts. 21 Volts / .917 Ohms = 22.9 Amperes. 22.9 Amperes x 21 Volts = 480 Watts.

Now I have something where I can determine wattage across the entire rpm range. I get .139 volts per one rpm. I can use wind speed and TSR to determine blade rpm and plot watts out per  wind speed.

Sparweb – thanks so much for your help! I went back to your first calculations and this is what turned on the light bulb in my head... I was feeling pretty dim witted up until last night when it finally flickered. Just for fun I'm going to go back into the math so I can determine Tesla and Weber units as well.

I learned something new and it feels good. Much appreciation to the board and all who contribute.

Regards,
CM

SparWeb:
That great feeling with the "coin drops"

Don't forget to be careful with the phase-to-phase resistance and the phase-to-phase voltage as you work up from 1 phase to 3.
The 3 legs are out of phase with each other, and the resistances don't care about phase.
So voltage across 2 legs in Star is 1.73 x Vac in one leg, but resistance is 2xR in one leg.

CraigM:
I've been working on the math for my project in an effort to see what potential this PMA holds. The following slides are screen shots from my spreadsheet calculations.

I'm not one to spend much time with math but this exercise really interested me. I can now see how calculating the given information can help fine tune the design before you build.

Through a bit of research and reading of older posts in this forum I've found the DC voltage when ran through a full wave bridge rectifier still has a bit of a bumpy wave form. Peak DC voltage of that wave form is considered 1.4 times the RMS AC voltage and the average is closer to 1.35 times RMS AC voltage. I used 1.4 to determine initial cut in as the battery will see a bit of a pulsed current and 1.35 to determine anything above cut in.

I ran my calculations for a 24 volt system as well as a 12 volt system.

I guess I'm surprised at the total watts available in the 500 rpm range and the difference in total watts from a 24 volt system to a 12 volt system.

I understand there are other losses that I'm not accounting for such as heat and core losses but I don't know how to measure these at this time.

Cut in for 15 AWG at 24V seems a bit too high at nearly 8mph wind and tsr of 7. If my blades only run at 6 tsr it would make cut in rpm even higher. At 12V there is no problem with low wind cut in speed.

Cut in for 17 AWG at 24V works well in low 6mph winds but I'm concerned about it not being able to carry as much current and heating up quickly. At 12V, again, there is no problem with low wind cut in speed.

Feeling like 16 AWG would be “just right”.

Initial impression is I'm happy with the output and feel this PMA would be comfortable at 500 watts continuous but is it really capable of producing 1000 watts?

Another part of this build I'm impressed with is the low cost for magnets and magnet wire. Compare the cost of 24 neo magnets at $150 range to 16 ceramic magnets at $32 is a win. I'm also seeing less wire will be used. Believe the traditional 24 pole, 9 coil, air gap axial flux uses around 6 pounds of wire. This PMA will use around 3 pounds of wire. Of course there is the problem of finding magnet steel that can be wound as a core. Maybe I got lucky with this.

I also backed into the math to find the tesla strength in the core. Hugh Piggot talks about Proven turbines of this same design being a flux multiplier. I believe I'm seeing this after calculating 2.91 T in the core. I'm seeing nearly 10 times the tesla strength of ferrite magnets. For comparison neodymium magnets are around 1.4 T.

Believe next step is to tear down the current test coils and do one more test coil with 16 AWG wire at around 26 turns. Hope this is the sweet spot I'm looking for.

Hmmm, suppose I could do the math and get a good idea as well.

15 AWG - 24 volt


15 AWG - 12 volt


17 AWG - 24 volt


17 AWG - 12 volt


Single coil test at 190 rpm.


Tesla calculation at core.


Thanks much!
CM

SparWeb:
You may find this interesting:
https://www.fieldlines.com/index.php/topic,149585.0.html

I did carve my blades for TSR=7 but they don't operate at TSR=7.  Their TSR varies under different conditions.  Below cut-in speed, they run much higher TSR.  At cut-in, the slope of the line changes and levels off.
As the discussion in that thread explains, my blades are also under-sized for the generator, so they don't get to run at optimal TSR.

Just something to consider when you write: "...Cut in for 15 AWG at 24V seems a bit too high at nearly 8mph wind and tsr of 7. If my blades only run at 6 tsr it would make cut in rpm even higher..."

CraigM:
Thanks Sparweb!

Good real world information in the link you shared.

Doing this sometimes feels like I'm trying to put a puzzle together without having the box to see what the picture looks like... and no one else knows what the picture is as well... it's created as you fit each piece.

You try to fit a piece in here and there... some fit, some don't. But when you find a piece that fits the picture becomes clearer and you have those Ah Ha! moments.

Blade (power) matching to alternator (load) matching will take some practice.

But just like a puzzle I keep coming back, rethinking it, working it and enjoying the process.

Thanks again,
CM

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