Low power output, 24 pole 3 phase alternator
By rfirth, Section Wind Posted on Wed Aug 20th, 2008 at 06:28:00 PM MST
What did I do wrong? Inappropriate wire sizing, or less than ideal magnet backing metal? Or both?

I mounted a wood frame on a bicycle wheel, and placed a 1/4 inch thick, 18 inch diameter (13.5 inch inner diameter) ring of steel on the rotating wood frame. The plate is made of a stack of thin sheets of steel. On the ring, I've placed twenty-four 2x1x0.5 inch NIB magnets. I also made a test coil from some copper wire I stripped from a dishwasher motor I found down the road the other day. I don't plan on using that wire in a final version - I've probably damaged the enamel coating on the wire while removing it, and the wire is probably too thin anyway. But I have lots of it for testing. My first question is - my NIB magnets have countersunk holes in them... does putting a bolt through the hole weaken the available magnetic field at all? Just a thought. Second - I don't know exactly what wire to use. Here's what I got - I made 100 turns of this wire I got, with 2 strands in hand. The resistence in the coil is 2.4 ohms (that's alot, isn't it?). When I turn the wheel by hand, it starts off at around maybe 80 rpm (let's say). I generate 2.5 volts open circuit, 2 volts closed circuit, and a max of 0.8 amps closed circuit. If I hold the coil in my hand, it vibrates like crazy. I'm aiming for 12 volt charging voltage for batteries, and at as low an rpm as possible. Question: how significantly will output increase if I replace that 1/4 inch stack of thin steel sheets with a 1/4 to 1/2 inch solid ring of iron for the backing on the magnets? It won't be easy, but I can have that made if it is necessary. Another question: I don't know what gauge wire I have, but what is the recommented guage? I was looking at 14-16 guage, but I wanted your opinion. I understand the tradeoffs involved with sizing the wire and number of turns of copper, but I don't know where to start and copper is too expensive to waste. Last question: Is my output of 1.5 to 2 watts for that coil normal, or under what I should expect? I'm planning to make this a 3 phase alternator with 24 magnets and 18 coils. It just seems disappointingly low. I have 2 theories - that it could be the wire, or it is the less than ideal steel magnet plate I'm using. Is there anything I'm overlooking? I think I have all the basics, but I'd like to light more than a flashlight bulb.

Low power output, 24 pole 3 phase alternator | 11 comments (11 topical, 0 editorial)

Re: Low power output, 24 pole 3 phase alternator w (3.00 / 0) (#1) by tecker on Wed Aug 20th, 2008 at 12:39:26 PM MST (User Info)

Pictures will be the way to approach the problems focus on the geometry ,air gap, coils and the like.

Re: Low power output, 24 pole 3 phase alternator w (3.00 / 0) (#2) by rfirth (rfirth1@(remove this part)lsu.edu) on Wed Aug 20th, 2008 at 01:09:17 PM MST (User Info)

Yes, I'll be getting pictures soon. It might take a few days before I can get them though, because I'm currently about 1000 miles away from it... Robert Firth [ Parent ]

Re: Low power output, 24 pole 3 phase alternator w (3.00 / 0) (#3) by Flux on Wed Aug 20th, 2008 at 01:13:58 PM MST (User Info)

If you have 2.5v per coil , with 6 coils per phase you will have 6 x 2.5v or 15v. Star connected you will have 15 x 1.7 = 25.5v ac. The dc mean will be x 1.4 or about 36v. This doesn't seem so bad. No using thicker iron will make no difference your error is to have no flux return path for the magnets. You would need a high grade laminated iron backing or a spinning flux return disc to get most benefit. The resistance is high but until you decide on the volts and speed you want you can't do much about choosing wire size. Your short circuit currents are virtually meaningless, dont waste time worrying about them. Flux

Re: Low power output, 24 pole 3 phase alternator w (3.00 / 0) (#4) by rfirth (rfirth1@(remove this part)lsu.edu) on Wed Aug 20th, 2008 at 02:16:22 PM MST (User Info)

Thanks! When you put it that way, it all sounds better. I was beginning to get worried. "No using thicker iron will make no difference your error is to have no flux return path for the magnets. You would need a high grade laminated iron backing or a spinning flux return disc to get most benefit." Wait, I thought my pile of steel sheets provided a flux return path for the magnets? The magnets are sitting on a bunch of thin steel sheets, 18" in diameter. The steel ring is just as wide as the magnets (2") and 1/4" thick. Doesn't that work, or am I fooling myself? I've heard that if a paperclip sticks to the back, it isn't thick enough. Paperclips and lots of other things stick to the back of my steel magnet backing. I've got an steel backing on the magnets. Are we talking about an iron backing behind the coils as well? And laminated? I think I can visualize how that would help. I'm also guessing that iron is better than steel for this purpose. I'm looking for as much power as possible (aren't we all?), 12+ volts at around or below 60 rpm. I've got a savonius with 3.5 square meters of surface area. I'm willing to use gearing, but I'd like to avoid that. I see you calculated that I could get 36 volts dc, star connected. Am I right in assuming that if delta wired I'd get 21 volts dc, and more current? I'm just trying to get the formulas right. I promise I'll have pictures soon! I very much appreciate your help. Robert Firth [ Parent ]

Re: Low power output, 24 pole 3 phase alternator w (3.00 / 0) (#5) by Flux on Wed Aug 20th, 2008 at 02:27:15 PM MST (User Info)

You only have a closed magnetic circuit on one side. Yes you do need to close the other side of the coils and when you do this then you may well find that your original steel backing is not thick enough. With a closed circuit each side iron in the region of 1/4" or more will be needed. You will need to laminate any fixed iron over the coils or spin a disc with the magnets to avoid eddy currents. Yes delta will get you down to 21V at the same speed. Flux [ Parent ]

Re: Low power output, 24 pole 3 phase alternator w (3.00 / 0) (#6) by rfirth (rfirth1@(remove this part)lsu.edu) on Wed Aug 20th, 2008 at 02:55:58 PM MST (User Info)

Wow, thanks. That makes perfect sense. Now I know what to do. One last thing - the laminate on the iron - is that any insulating material? Does the laminate work well, or is spinning the disk a better option? It seems to me that keeping that disk stationary makes the design simpler, if I didn't have to worry about eddy currents. I just don't understand how laminating the iron prevents eddy currents. I see you call it a "high grade laminated iron backing", and high grade usually means expensive. Thank you for your time. I was disappointed before, but now I'm excited. This is going to work. Robert Firth [ Parent ]

Re: Low power output, (3.00 / 0) (#7) by Norm (peppysue@suite224.net) on Wed Aug 20th, 2008 at 03:08:04 PM MST (User Info)

I also made a test coil from some copper  wire I stripped from a dishwasher motor I found  down the road the other day.  I suppose you would notice after you scraped the insulation from the wire whether it was aluminum or copper...? the enamel coating on aluminum makes it look like copper.....I'm assuming that you would know right off....still...you know what they say about assume....  If not for you then for some newbie.... aluminum is not quite as conductive as copper. ( :>) Norm

Re: Low power output, (3.00 / 0) (#8) by rfirth (rfirth1@(remove this part)lsu.edu) on Wed Aug 20th, 2008 at 04:25:35 PM MST (User Info)

I've tried to strip off more enamel, and it still seems to be copper colored. It could be that I'm not getting all the enamel off, and the remaining enamel is making it copper colored, or it could just be copper. At this point I'm not 100% convinced either way. It looks like it could be impure copper. I'll do more testing and get back to you. Thanks for the warning! Robert Firth [ Parent ]

Re: Low power output, (3.00 / 0) (#10) by TomW on Wed Aug 20th, 2008 at 05:47:59 PM MST (User Info)

Robert; DanB used to suggest burning the enamel off with a torch. Get the copper cherry hot and scrub off the rest with emery paper. He also thought this softened the wire and prevented stress cracks from vibration, I believe. Just a thought. Tom "Education consists mainly of what we have unlearned."--Mark Twain [ Parent ]

Re: Low power output, 24 pole 3 phase alternator (3.00 / 0) (#9) by wooferhound (tim((NoSpamAt))wooferhound.com) on Wed Aug 20th, 2008 at 05:39:48 PM MST (User Info) http://wooferhound.com

at 2.4 ohms per coil then times 6 coils per phase equals 14.4 ohms. I think it will be really hard to get usable power out of that. W o o f -={(

Re: Low power output, 24 pole 3 phase alternator (3.00 / 0) (#11) by rfirth (rfirth1@(remove this part)lsu.edu) on Thu Aug 21st, 2008 at 08:04:41 AM MST (User Info)

That's what I thought as well, although it sounds like I could get away with fewer turns of wire and still get the required voltage. Nonetheless, I'll be getting better wire. Am I right to be aiming for less than 1 ohm for an entire phase (across all 6 coils)? That should be reachable if I can get the coils to fit. Seems if I could get 14 times less resistence with heavier wire, use a few less turns, and get my iron magnetic flux plates worked out, that output should be increase substaintially. Robert Firth [ Parent ]

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