My first alternator

[jhardin]

I am just learning about this stuff and it has been a lot of fun, but I have no experience or education related to this topic so forgive my ignorance.

I am trying to determine if I am getting the expected voltage from my 3-phase axial flux alternator.  Here are the details:  two 6" diameter circles of 3/4" plywood for the rotor and the stator, 8 magnets (neodymium, 1" dia, .25" thick), and 6 copper coils (20 gauge) with with 50 windings each. The windings have a 1" inner diameter to match the magnets and are placed at every 60 degrees.  The magnets are placed at every 45 degrees on the rotor.

Casually spinning the rotor with my hand (~60 RPM) I get ~.14 volts AC from a single coil, about double that for each phase, and ~.8 volts DC when all 6 coils are connected in a star configuration and rectified (all three start wires for each phase connected together, each of these coils' end wires connected to the start wire of the second coil in the phase, leaving the remaining three end wires for output).  If I really spin it hard, I can get over 1.2-1.5 volts (AC and DC) and get an LED to flicker.  

Maybe my expectations were too high, but I was hoping to beat the hamster on otherpower.com.  From this description, does it sound like I am getting the right amount of voltage or did I likely make a mistake?

Thanks.

Josh

[fungus]

A few thoughts;

Plywood discs for the rotor arent very good to conduct flux; you'd be much better off with a steel disc or at least with a steel backing behind the magnets to get more power. 50 turns per coil isnt very much for something like this; might be better to got to something like 23awg and cram as many turns in as possible to get as much voltage as you can get.

[spinningmagnets]

Hi Josh, I am also a newbie. The way I understand it, If you want the most cost-effective, best "bang-for-your-buck" 3-phase, you follow the 3-coil/4-magnet rule. (it sounds like you have) The way I understand it (and theres no guarantee this is 100% accurate) The alternating faces of the magnets should be evenly spaced out NSNSNSNS (sounds like you did that).

Each "N" magnet face is paired with the "S" face to its left, and also to the "S" face to its right. In a 6-coil PMA there will be two coils that have an N and S magnet faces directly over the two legs of two of the 6 coils. One magnet is pulling, the other is pushing. (in an oval coil, the two long sides are the legs, and the two short connecting sides are just a "neccesary evil").

You may want to check the shape of your coils to ensure the centers of the two opposing magnets are centered over the two legs of one of the coils. At the precise moment when that orientation occurs, a pulse is created.

At that moment, two other coils will have a magnet pair just leaving their two legs, and two of the coils will have a magnet pair just about to arrive.

The flux of a single magnet should normally be about equal on both the N and S sides. If you put a steel backing plate on one side, it will "pull in" the flux on that side, and strengthen the flux on the side that the stator is on. A smaller "air-gap" will put the coils closer to the strongest part of the magnets flux, increasing output, but it will also cause it to take more force to initially get it spinning.

If you add a second rotor on the other side of the stator with the magnet faces opposite of the first rotor, the two steel-backed rotors will have a very strong magnetic flux in the middle, where the stator is.

You may have free or cheap round magnets, but round magnets over oval coils means only about half the copper is getting a "flux push/pull" and the other half is adding resistance without adding any voltage. Wedge shaped magnets and coils are slightly more efficient because 2/3rds of the coil is sitting in a magnetic field, while only about 1/3rd is unused.

At least I think so...Since you have actually built one PMA, you have done more than me!

[Flux]

You will never do any good while you insist on putting your magnets on plywood discs.

Magnets need to work in magnetic circuits and you can only do this by mounting them on steel discs. I wish this plywood rotor would go away once and for all, it's crazy.

Flux

[TomW]

Flux;

As usual, excellent advice.

I think of mounting alternator magnets on plywood right up there with using fishing line for wire. Sure it "looks" like the real thing but will have much less than satisfactory results.

Well, half the fun is messing with it, I guess. There seem to be a few good basic rules that need to be followed at least at some level.

Strong magnets on thick steel rotors seems essential for an effective alternator design.

I also think that Dans old "Wood AX" should have a big red stamp on the page stating "This is an early, experimental design that you should not copy to build a useful alternator". One of those sure it works but not very powerful and we had a lot of fun doing it designs.

Cheers.

TomW

 

[wooferhound]

The reply from 'spinningmagnets" was not real clear about mounting the rotors.

all of the magnets on one rotor should be attracted to the magnets on the other rotor.

[finnsawyer]

"You may want to check the shape of your coils to ensure the centers of the two opposing magnets are centered over the two legs of one of the coils. At the precise moment when that orientation occurs, a pulse is created."

I'm surprised no one jumped on you for this statement.  In point of fact if everything is lined up perfectly (yeah, sure), the only thing you can say about a "precise time" is that the voltage of a coil will be going through a zero crossing at the precise time the coil is centered over a magnet.  As the coil moves off of the magnet in either direction a voltage pulse of either negative or positive polarity starts to build usually reaching a peak value when the coil exactly straddles two adjacent magnets.

[jhardin]

I wanted to get the technique correct first on wood before moving to steel.  I am realizing now that 20awg wire is probably too thick for a small alternator and your response confirms this.  Thanks.

[jhardin]

Thanks for the advice all.  I planned on gettting the most out of a six inch wooden version, learning from my errors while improving efficiency, and then move up to steel.  Besides upgrading to steel rotors, it would seem that using thinner wire is the best bet for increasing the voltage on a such a small alternator.  

Thanks

Josh

[jhardin]

All other things being equal, how much of an increase in voltage could I expect just by switching to steel for the rotor?  

[jhardin]

Thanks for not assuming anything =) To hear the basics is useful for me or anyone else building their first alternator and you included the fundamentals that can be easily missed when just starting out.  

I am interested in your comparison of the shapes of coils and magnets, but if the magnet is supposed to fit in the center of the coil regardless, then how will I increase the coverage by using a wedge as you suggested?

Thanks

Josh

[ghurd]

Mr.Fungus was being polite.

Plywood rotors may as well be covered with cat fur as magnets.

I am not sure if "monte350" files are still intact, but they would be worth a look.

They should show a plywood rotor being a waste of money, be it spent on magnets or copper.

G-

[finnsawyer]

I'm not sure what the 'coverage' deal has to with it, since the coils will presumably be wound to fit nicely over the magnets whatever their shape.  Strangely, the shape of the magnets and their associated coils should not matter.  If say round magnets and wedge shaped magnets have the same flux then the average voltage generated when the coils (same number of turns) move from being centered on one magnet to the next will be the same in the two cases as long as it takes the same time.  This comes from Faraday's Law.  The change in flux is the same and the time is the same.  Same average voltage.  The peak voltage values could be different, though.  It may be possible using the wedge shaped magnets to pack more flux into a given area and thereby get better performance (higher voltage).  You should also be aware that for the same area magnets you will need less wire for the same number of turns for the round magnets.  This translates into lower resistance, which means more power for a given voltage or better performance.  If this is confusing, simply chock it up to the fact that any design will have trade offs.