Crappy multimeter or what?

[wiley]

hi there folks,

another newbie.
I've read a lot and I think I've got the basics down. Or so I thought, cuz I can't seem to make it work.

http://www.youtube.com/watch?v=Dd_xjjTtuq4

It's a little hard to see, so let me describe what I did.

I glued 12 tiny magnets (1/2" dia by 1/8" thick mags) on the edge of a 4" diameter cutout wheel. I used a template to make sure they were equally spaced apart. The polarity is good, n/s/n/s, etc (I've checked many times to make sure). I glued the wheel to a shaft collar, tightened the set screw and put the shaft in the drill chuck.
The coil has approx. 200 turns of #30 wire and it's the shape of a wedge magnet. The hole is slightly bigger than the magnet.
I hooked up the coil to the multimeter and set it to millivolts DC. Don't have an AC setting that will measure that low. I hold the coil near the rotor and start spinning and.. I get the most voltage at startup?? Spinning the rotor faster decreases it rather than increasing it. I don't understand. Holding the other face of the coil doesn't make a difference. Please just watch the video to see what I mean.

I "think" I'm doing it right, am I not? I'm wondering if maybe it's my crappy multimeter that's the problem?? (cheap $3 multimeter from harbor freight)

What I wanted to do was make a dual rotor axial PMA, just on a small scale first, but I can't even get a single test coil to work!! Geez!

This setup is somewhat similar to that hamster powered PMA.(http://www.otherpower.com/hamster.html) He got 1.2 AC volts when I tested one coil. Ok, so he uses 14 slightly larger (3/4" vs my 1/2") magnets instead of 12 and the magnets are on a steel ring whereas mine are on a cutout wheel, plus his test coil had 400 windings instead of 200 like mine has. But still, shouldn't I be getting a little more voltage than what I'm getting?

Anyways, I'm getting a little discouraged here. I've been reading up on this PMA stuff for quite a while now and thought I had the basics down and figured it was time to get busy and practice. And then I can't get one single coil to work. Argh... help please 

[ghurd]

The meter is set to DC, and the coil is making AC.
The volts are there, just the average DC volts of an AC wave is 0V.

Connect the test coil to a bridge rectifier, or make one from Schottky diodes.
It will lose about a volt in the diodes, but the output will be DC.  And the DC will be pulsating, so the DC will read lower than the peak value.

Put a LED directly on the coil wires.  See if it flickers. Thats a visual indicator the AC voltage is more than 1.5V, depending on the LED color.
Doesn't really prove anything scientific, but its cool to see.  

The 750V AC scale reads down to 1VAC.  Try that.

You can see the coil is making at least a tiny bit of voltage because the meter is flat at 0V until you move the coil near the magnets.
The meter is just set in the wrong place, thats all.
Those $3 meters are the ones I almost always use.

You did strip the insulation from the wire where the meter is connected?
Sometimes it takes a bit more cleaning if the readings are out of whack.
G-

[wiley]

I set the multimeter to the 200V AC setting and I got a .6V reading. I got nothing when I set it to the 750V AC setting.

I tried a yellow and a green LED and both didn't flicker.

I changed some things, like in the video I used 12 magnets that are 1/2" diameter and 1/8" thick. After I got the .6V reading when I set it to the 200V AC setting, I added 12 more magnets, so now its like having 1/4" thick magnets. This increased voltage from .6V to .8V. After that I placed a stack of magnets behind the coil and it went up to 1V. But I had to hold on to the magnet so it was stationary so it repelled half the magnets for every revolution of the rotor. So the voltage could've been even more if the magnets behind the coil were turning as well, I think.

Forgot to mention that I cleaned the wires using a tiny torch to burn the coating and then using sand paper.

[ghurd]

Happy that it works a little?
I am a bit surprised it is that low, but there are a lot of things working against you on this one.

The 1st problem is a single rotor.  The flux path is undefined, but bad compared to dual rotors.

Next  problem is the neos are not on iron.  That makes a huge difference.
With no iron, is is twice as bad, though I have no idea what double undefined works out to.
Skippy had iron behind the neos.

If it was a single rotor with the neos on iron, the only coil turns that count for much are those closer than the thickness of the face of the neo.
Meaning only those turns that were within 1/8" from the magnet's surface counted for anything, and maybe not even all of those.
Skippy had 2 coils of 400 turns, though I doubt the coils were thin enough for most turns to be very effective.

And 1/8" thick is almost 0" to work with.
Figure it is half of almost 0", and it is understandable the voltage is very low.
Plus the cutoff wheel had a considerable bit of wobble, and being hand held, I doubt there were vary many coils inside the average distance of 1/8".

Can you stick the magnets on a metal disk?  (no need for glue I don't think, but definitely safety glasses!)
Maybe one of those little diamond toothed disks, or even a circular saw blade with 100X the caution?

It won't make much voltage without an effective flux path, and enough coils inside the region where the flux density where it counts.

It worked a little, which is better than a lot of my first tries!
G-

[Bruce S]

Wiley;
 I think you have it right, but agree with Ghurd on this one. I too use the very same HF meter. I just bought 2 more as they are on sale again.
Almost can;t have too many meters 

If that is a HF cutoff wheel for their grinders, you well certainly need something better with more iron in it.
You could grab a 13oz coffee can top, using a can opener, cut the top out and cut it down to correct size to put on the wheel.
The cutoff wheel is good for working on old iron railings but not for this stuff.
Good start though!! keep at it !!
Hardest part for me was winding to wires to look as good as the one you have.

Cheers;
Bruce S

[Rover]

The nice thing about HF meters , when bought for 2-4 $, is that you don't worry about them. Blow the fuse by directly connecting the + - to the batery while in dc current mode (thinking your were dv voltage ) , no biggie... you're out 2-4$ or the cost of the fuse .. honestly I have a quite a few HF meters lying around with blown fuses, its actually easier for me to pick up a new meter than buy a fuse, I have an HF store on the way home.

Do I also have decent meters.. you bet ... I don't use them in the power shed... reserved for my electronics work

Some day I'll either buy a bunch of 10A  fuses, or sell em as is as voltmeters at a garage sale .   

[ghurd]

honestly I have a quite a few HF meters lying around with blown fuses, its actually easier for me to pick up a new meter than buy a fuse, 

Many versions of the $3 meter have an spare fuse inside waiting to be used. 
G-

[Rover]

Hmm .. didn't see any in me yellow ones ... lol.. I'll go look . (Even at a garage sale I can sell it fr 2$ as a volteter... )

[wiley]

@ghurd: Yes, I am glad it worked a little. And it worked because you told me to put it in the AC setting. I don't know why, but for some reason I thought it should've worked in the DC setting and that the output would just read +0.5V and then -0.5V and then +0.5V again, you know.. negative then positive then negative again because its AC. But I guess it doesn't work that way.

I really wanted to use a metal disk for the rotor but the cutoff wheel was the only thing I could find at that time that had a hole the same diameter as the shaft. I'll use metal next time.

@Bruce: I like your coffee can top idea. I'll try that if I can't find a metal disk. Btw, winding that teeny tiny coil was a real pita    

[ghurd]

For the magnet disk, could try the back end of a larger car speaker.  They usually have a raised part in the center that sticks up through the magnet.
I have an old pair I got at a flea market for $1 with maybe 3.5" dia backs, marked "Ford".
Those 'Boom Thud Boom Thud' speakers have larger backs.  I have one thats about 5.75" dia.

Jerry figured it out.  Here's one he did-
http://fieldlines.com/board/index.php/topic,128784.0.html

G-

[wiley]

Thanks G. Nice way to recycle old speakers.

Btw, has anyone here ever used ceiling fan blades on a wind turbine?

[ghurd]

Yes.
They are about as bad as it gets,
plus they will snap off in any kind of decent wind.

Better off going with PVC blades in the beginning, IMHO.
G-

[wiley]

Oh, ok. I was actually going with PVC blades but remembered I have an old ceiling fan that doesn't work anymore and was wondering if I could use the blades of it. Oh well, PVC blades it is then

[willib]

@ghurd: Yes, I am glad it worked a little. And it worked because you told me to put it in the AC setting. I don't know why, but for some reason I thought it should've worked in the DC setting and that the output would just read +0.5V and then -0.5V and then +0.5V again, you know.. negative then positive then negative again because its AC. But I guess it doesn't work that way.

It does work that way , but the meter averages it out , and the average of all the positive and minuses , is zero Volts.
If you had an older meter with a mechanical needle gauge , you would see it move positive and negative , when it first starts moving, but even that would average to near zero when the rotation speed increases.

[ghurd]

Nice way to recycle old speakers.

I found an old pic of one of my big 'boom thud' speaker plates, and a few seriously hefty HD neos.
G-

[wiley]

Update: I got some metal disks. Two 4" diameter 1/8" thick disks, same diameter as the cutout wheels I first used.
I got a measurement of 3.6V with the drill at full speed (800 RPMs). Quite an improvement, no? 
The coil has somewhere around 200 turns, I lost count a few times    of #30 wire.

Now I can do some calculations to find out how many turns each of the 9 coils would need. I was thinking of three or four 2' blades, unless someone gives me a good reason not to. Also I know I said I would go with PVC blades but I may go and try to carve some out of wood, not sure yet. (yes, I know that's more challenging) Also, was thinking of a cut in speed of 100 RPMs. Unless, again, someone gives me a good reason not to 

Adios amigos. I can see how this can be addicting. Just what I need, another addiction 

[wiley]

Ghurd, you have a computer repair shop or something? Where did you get all these mags from?

[ghurd]

100 RPM cut in with 4' PVC blades is too slow.  It will stall with a suitable PMA.
I can't tell you what it should be with any accuracy, but probably more like 250~275RPM?

Not sure if you plan to use the 4" disks and those little neos, but there is not going to be very much power in something that size.
Certainly no need for a 4' dia prop on that PMA.

Those are just a drop in the bucket.
I went through a phase where I bought large HD lots on ebay.  BTW- A very stupid thing to do.

hehe.  He's a few more, which maybe a few hundred HDs were sacrificed for that stack.
The small rotor has 32 HDs worth of neos on it, and I think I added 16 more HDs worth of neo to it later, for a total of 48 HDs in one little PMA.
Huge dimension pic, but only 60kb.


I learned it is better to just buy neos the size you need instead of buying HDs.
G-

[wiley]

I can't tell you what it should be with any accuracy, but probably more like 250~275RPM?

Hmm.. Well reason I was thinking of 100 RPM was because the average wind speed we get is pretty low (I'm in NE TX). I found an average annual wind speed map for TX at 80m. (http://www.windpoweringamerica.gov/images/windmaps/tx_80m.jpg) For my area it's 5.5  Still think 250-275 is right for my PMA?

I know there's not much power in a PMA this size, but it's my first one. Small scale first ya know 

[ghurd]

There is no power available below 6MPH.
You can get a couple dozen or hundred ma at maybe 4~5MPH, but it takes so many turns of wire the resistance is huge.
High resistance means at 10MPH when you should be making some usable power, the output is only a few hundred ma.

Even if you could do something super complicated, those few ma at 4~5MPH are nothing compared to the overall power, which almost all comes from the high winds.
Here, there is no wind or very light winds about 4 or 5 days a week.  Pretty much all the wind power comes in on the other 2 or 3 days.

I tried the 5MPH thing for a long time.
Then I noticed the wind is almost always below 6 or more than 8MPH.
I decided on the small machines it is better to aim for 6.5 or 7MPH cut in.  The gains are worth the losses.

The trick is making blades big enough to supply the power the PMA can make,
at an RPM where the wind is 6.5MPH and the PMA is at cut in.
Sounds simple enough?  Its not for me.
G-

[Bruce S]

I can't tell you what it should be with any accuracy, but probably more like 250~275RPM?

Hmm.. Well reason I was thinking of 100 RPM was because the average wind speed we get is pretty low (I'm in NE TX). I found an average annual wind speed map for TX at 80m. (http://www.windpoweringamerica.gov/images/windmaps/tx_80m.jpg) For my area it's 5.5  Still think 250-275 is right for my PMA?

I know there's not much power in a PMA this size, but it's my first one. Small scale first ya know 

Wiley;
Good to see the unit jumped 600% in output!! Did you use the coffee can trick?
For the blades and the wind, you should look into shorter fast blades.
There are people on here who can help with the size and such for the blades that are light years ahead of me.
OF a search of fast blades will be bring up something.
I'll help search later gotta go rebuild a VM box. smoke got out

Cheers;
Bruce S

[wiley]

Thanks ghurd, I'll go for the 6.5-7 MPH cut in.

@bruce: Sorry, haven't used the coffee can trick. I had the disks cut out for me. There's this waterjet cutting company nearby and I finally had a reason to go there. I got lucky though, my disks was way below their job minimum, but they cut em anyway  Nice people!

[wiley]

Did some calculations. But I did it a little different, so I'd like to share it with you guys to see if my calculations are right before I do anything.

I read that to get started, you should first make the turbine but without the stator. Then make a test coil and place it in between the rotors and spin it at your desired cut in speed and measure voltage.

Well, here's what I did different. My cut in speed will be 250RPMs, but I have no way to spin it at that speed. The only known speed I have is the drill's maximum speed which is 800RPMs. And at that speed I get 3.6V.

Now if the voltage is 3.6 at 800 RPMs, then will it be 1.8V (half) at 400 RPMs? If so, then the following table should be correct..

RPMs   Voltage
-----------------
800   3.6
400   1.8
200    .9
100    .45
 50    .225

If the above is correct, then at cut in speed of 250RPMs it should produce 1.125V (.9V + .225V). Which isn't enough for my application.

Chris Olson made an example in a thread in the FAQ section. According to his example, then for a 12 pole 9 coil 12 Volt PMA, I need:

14 / 1.4 = 10 AC volts at cut in.
10 / 1.732 = 5.77 volts per phase, in star.
5.77 / 3 = 1.92 volts per coil.

So I need 1.92V per coil. But the test coil only produces 1.125V, which means the coil needs more turns than the 200 it has now. Below is how I calculated how many more turns it needs.

1.125V devided by 200 turns = .005625 volt per turn.
1.92 - 1.125 = .795V
.795 devided by .005625 = 141 more turns.

So the coils need 200 + 141 = 341 turns. Wow.. that's alot of turns.

Note that the 1.92V per coil is when winding for full charging voltage(14V). If I used 12V in the calculations, I'd come out to 293 turns.  And 318 turns for 13V.

Chris Olson mentioned the following:

"It's important to note that full charging voltage on a 12 volt is 14 volts, 24 is 28 volts, and 48 is 56 volts.  If you wind for full charging voltage and you hammer the batteries down to 12, 24 or 48 respectively, the turbine will run in stall at cut-in trying to bring the batteries back up.  It's not as big of a deal with 12 volt as it is for 48.  So if you're building a 48 you might want to use the low end (48-50 volts) as the cut-in DC voltage so you don't end up with turbines like I got that are overwound and stall badly unless the batteries are fully charged."

I'm not sure what he means by "hammer the batteries down to 12", but I don't want the turbine stalling at cut in, so perhaps I should wind for 13V and use 318 turns.

Well, that's about it. I hope I got it right because it took me long enough to figure it out. I don't know anything about this electrical stuff 

Oh, I took some more pics.

[Fused]

In my own experience my drill was rated 400 rpm and 1100 rpm. I went by this with minimal success, When I hooked a  tach to test the drill rpm its self I found my actual rpm was much slower than what was stated on the drill. My drill is cordless so battery condition plays an important role in actual rpm output. Yours may be electric (120v ac) but I don't know how accurate it will be either.

Trying to get it exact will only be frustrating. Matching the blades to the alternator with a close estimation of cut in is the way I go.

Good luck and have fun.

Fused

[willib]

wiley if you move your magnets a quarter of an inch inward you will have the proper spacing for 12 pole - 9 coil three phase alternator with 1/2 inch mags. which is 3.5 inches in diameter.
also , why not use round coils , since your mags are round.
i made my coils 1 inch in diameter , back when i was using 1/2 inch dia mags.
also if you use more of the inner part of the coil that will give you more room to use more wire.
If i recall correctly my coils had an inner diameter of 5/16 ", so you can see with a coil of those dimensions there is more room to get more windings in there AND your coil will be thinner .

My new mags will get here tomorrow, they are 1.5 inches in diameter by 1/2 inch and i will use 3 inch diameter coils.

[ghurd]

Its hard to argue with a real test,
but that seems like an awful lot of turns to me.
I have a hard time thinking it would take much more than 100 turns if everything was just right?

I could be wrong, but something just doesn't feel right somehow.

Is the distance between the magnets less than 7/16"?

And, if it helps, that 250RPM is still way too slow.
Even on a 3' dia with PVC blades, you should be thinking more like 350~400RPM.
I doubt it will need 3' dia blades, so even over 400RPM could be a decent place to start.
G-

[wiley]

@Fused: Good point! I was wondering the same thing. But the thing is I don't know of any other way to turn the shaft at an known speed.(or at a speed close to the known speed)


@Willib: 3.5 inches? Ok, thanks for that info. How did you come up with that number? Is there like a formula for calculating the proper spacing for magnets?

As for my coil shape... well, I've done a LOT of reading lately and to be honest with you I'm not sure what size/shape my coils should be anymore. Some make em round, some make em rectangular, and others make em wedge shaped (like mine) regardless of the shape of the magnet. Some think the hole in the coil should be the same diameter as the magnet, some think it should be slightly larger and others (like you) think it should be slightly smaller. So... it's a little confusing, ya know


@ghurd: It IS a lot of turns. Maybe its because of the small size of the PMA? Or maybe my PMA is just VERY inefficient? You didn't spot any mistakes in my calculations? I definitely wouldn't be surprised if my calculations are wrong!!

The distance between magnets is about 7/16".

Btw, 400 RPM cut in speed? That's quite a jump from 250. Isn't that a little high? Probably doesn't mean anything but in all my reading I've done here I have yet to read about a PMA with a cut in speed as high as 400. But if 400 is right, then I can just stick with 200 turns, can't I? It's making 1.8V at 400 RPMs. Or should I add a few more turns and go for 1.92V?

[DanG]

Code: [Select]

400 RPM cut in speed? That's quite a jump from 250
Limiting factor is blade length and the resulting tip-speed, or, what a two or four foot mill can get away with that a ten or twelve can't.

[JW]

Hi All,

 I fixed all the images on this page. if your trying to post a hosted picture with a url of anykind, dont for get to hit the "image" button, before you insert the URL. I only had to add the html tag to make the pictures display.

JW

[wiley]

Actually, I didn't display the pics in this thread on purpose for those on dial up. If they wanna see the pics, they'll click the link 

[JW]

Actually, I didn't display the pics in this thread on purpose for those on dial up. If they wanna see the pics, they'll click the link 

I cross check for compatibility with dial up, by the time they/anyone reads the first comment or two, the pictures display. We have good dial-up support on this end, dont worry about it. 

JW

[JW]

Here's an example,

Ghurd's picture

Huge dimension pic, but only 60kb.


This picture only takes 45 seconds to download on dial-up, that's not bad at all... But yes, if your picture was huge "1mb" the forum will not handle it, and you would get an error, the post would not process, infact the largest picture uploaded or even linked to the forum can only be a max of 150kb or less. Because of this the forum will not handle any picture that's too big, hence the max download time on dial-up is always less than 2 minuite's per picture no matter what. This also allows the site to handle a large number of pictures without loading our database that much, if photos are uploaded to the forum ,vs hosted elsewhere.. Both are limited at the same file size 150kb per picture.

JW

[willib]

Is there like a formula for calculating the proper spacing for magnets ?

Yes it goes like this.
I used to lay out the magnet rotors on a cad program , very tedious and time consuming.
so i said to myself there HAS to be a relationship between magnet diameter and rotor size that will produce a good working alternator.
so i worked the numbers for a little while and i found it.
it is amazingly simple to use, and now i just have a sheet of paper with the numbers on it and i can design an alternator for any size diameter magnets , using the 3/4 rule , three coils for every 4 poles. And using coils that are twice the diameter of the magnets.
using my next alternator as an example, i wanted to find out the available diameters that i could use to see if i had anything on hand that i could use.
so the magnets are 1.5 inches in diameter x 1/2 inch thick. with a 1/4 inch hole .
I ended up ordering 24 so i will have 12 poles, and looking at my sheet the number for a nine coil 12 pole machine is 7 , so 1.5 times 7 is 10.5 inches .
and i know that i can get brake rotors that are 10.5 inches in diameter.
for your example , to make a 9 coil 12 pole machine with 1/2 inch dia mags , take the magnet diameter , 1/2 inch and multiply it by seven to get 3.5 inches in diameter.