Yet more coil questions

[JustinF]

From the following questions you will quickly figure out that I, too, am a newbie at this.  I just love this stuff and love learning!

A generator I started working on has 10 poles.  Single phase is the plan for now.  The magnets are 1" round and are separated by 1" as well.  The rotor is about 8 inches or so.  I did wind a coil with 18 guage wire with 75 turns on it.  That just makes it into the space that will allow 10 coils around the circle.

Using Ed's formula for figuring out the volts for the coil it came to about 1.1 volts at about 200 RPM for one coil.  With testing, I could barely get .3 volts at 200 RPM.  I checked and made double sure the meter was on AC and still get the same read out.

Is this all I am to expect, or am I missing something.  The air space was about as close as I could get it without rubbing.

Any ideas for this really green guy?

Justin

[TheCasualTraveler]

More info please,

Are the magnets neo's or ceramic?

How thick are the magnets?

Are the inside dimensions of the coil close to the outside dimension of the magnets?

What is the thickness and material of the rotor/ magnet disc?

Did you alternate the magnets N S N S etc?

Were all ten magnets on the disc?

[Flux]

I haven't a clue what Ed's formula is, but I seriously doubt that it applies to a single rotor magnetic circuit.

As mentioned you give no magnet thickness even if we assume it is neo. Similarly you give no coil thickness.

I am not surprised at your findings with tiny magnets and no closed magnetic circuit.

If the coils are more than 1/4" thick you are probably not linking much of the flux.

Any useful formula for calculating voltage must contain information on the flux density or total flux of the magnet. Unless you have direct means of measuring this ( which indeed you have) you must be able to calculate or estimate the flux density. That take into account the magnet type surface area, thickness and air gap.

From your figures if they are to be believed you have a flux density under 100mT, seems very low so I assume your coils are thicker than the magnets and you are not linking much of it or you may have ceramic magnets.

Flux

[finnsawyer]

The most obvious possible explanation is that you did not place a second fixed iron disk behind the coil.  As Flux says, to get the maximum flux and therefore the maximum voltage you must complete the magnetic circuit to the best extent possible.  Your coil is presumably about 2 inches in diameter.  Are you aware that you will get zero voltage when a magnet is centered over the coil and maximum voltage either negative or positive when two adjacent opposite polarity magnets straddle the coil equally?  Hope this helps.

[JustinF]

Sorry Andy.  I realize I did leave out a lot of info...here we go:

The magnets are N50 neo's.  They are 1" by 1/2" thick round magnets.  The inside dimension of the coils are almost exactly the outside dimension of the magnet, 1 inch.  All magnets are on the rotor and are arranged N-S-N-S.

Here's where the problem might be then.  The whole thing is made of wood.  I thought that would be good to learn from.  So the only thing that is not wood are the bolts and screws holding the whole assembly together.  Would I lose that much with wood instead of metal?

Thanks for replying.

Justin

[JustinF]

Thanks for replying Flux.  Here's some more info in addition to what I replied to Andy regarding the coils.

The coils are only 1/4" thick which I thought to be sufficient for even these small 1/2" thick magnets.

Any ideas on how to calculate flux density for this.  You know, since you brought it up

Justin

[JustinF]

Thanks for the info GeoM.  I wasn't completely sure about how important it would be to complete the magnetic field.  I still have a considerable amount to learn yet.  

Would adding a second rotor help complete the field-and also increase the power somewhat?

Justin

[TheCasualTraveler]

""Would adding a second rotor help complete the field-and also increase the power somewhat?""

     Yes it would, but first you need to mount those magnets on a steel disc then add a second disc to complete the circuit. Windstuff Ed sells discs that including shipping cost less than what I can get a shop to cut them for.

     Your never going to get a lot of power out of that thing but if you want something that works for learning that can trickle charge a battery try more turns in your coils. I realize that you will need smaller gauge wire. You can also make the coils a little thicker and use the same wire. If the magnets are 1/2 thick I think you can go about 1/2 in the coils.

""The whole thing is made of wood.  I thought that would be good to learn from.""

     Well then, I think the wood served it's purpose.

[Flux]

 "The whole thing is made of wood. "

That is your problem. I assumed that the magnets were at least on a steel disc even if it was a single rotor.

With coils 1/4" thick and 1/2" thick magnets you would see a big improvement with the magnets on a steel disc.

Even more is to be gained from using a second spinning disc to complete the flux path.

Before the days of neo you would never be able to make any sort of alternator at all without working the magnets in a closed magnetic circuit. It becomes possible with neo but you are degrading a high grade magnet material to something little better than the old carbon steel, it just doesn't make sense.

Yes I did roughly calculate your useful flux at something like 80mT whereas most of these machines are working somewhere just over 600mT.

You gave me number of poles, speed, number of turns and volts so I just worked the emf equation backwards to get flux density.

Flux

[JustinF]

"Yes I did roughly calculate your useful flux at something like 80mT whereas most of these machines are working somewhere just over 600mT."

Are you saying that I may be able to get close to 600mT with steel disks or just generalizing regarding most of the machines discussed on this site?

Really, really, really appreciate all the help so far Flux.  Like I've stated earlier, I had no idea of a magnetic circuit or its importance.  What a difference that would make!  I will certainly look into that.

Thanks also for working the equation.

Justin

[JustinF]

Andy,

Thanks a bunch.  You all have been very helpful.  I think I will complete the whole thing as is and then keeping with a posture of learning add the steel disks later on and I'll be able to fully see through the meter what is invisible to the naked eye.

This is a lot of fun.  Thanks again.

Justin

[finnsawyer]

You'll be able to see better with a low cost oscilloscope.  Enjoy!

[finnsawyer]

How large can Ed make the disks?  Twenty inches in diameter by half inch thick?

[TheCasualTraveler]

I think he said he can do 20 something inches. 1/2" thick he would need to order the steel special and shipping would be different. But ask him.

I was talking about 8" and 10" discs as far as price is concerned.

[binarycortex]

So wait, I'm new and understand most of the concepts with alternators, but this is the first on any sites that tell how to make a pm alternator that has mentioned a magnetic circuit. Let me see if I understand this and feel free to correct me if I am wrong. Also a link to a how to or tutorial on completing a magnetic circuit would be good. Ok, so a wooden rotor with magnets attached would let the magnetic field travel right through it, whereas a steel rotor with the magnets attached would focus all of the field in the direction opposite the steel rotor? Am I close? And if that is the case, would you then add a second steel rotor behind the coils or just mount the coils to a steel rotor? Thanks for your patients with us noobs.

Ian (a.k.a. binarycortex)