Wider Genny?

[TomG]

In the "newbies" section here, because there's probably an obvious answer to this, but:

If size isn't too much of a concern (like in VAWTs where the genny is on the ground), wouldn't it help to make the genny wider? Use the same amount of copper, same amount of magnets, just spread it thinner.

If we doubled the diameter of a genny, we could use half the amount of copper in each stator coil, but use twice as many coils. Then the stator would be half as thick, and the surface area of the coils would have doubled, so the cooling should be much better.

The magnets would need spacing out, but there'd be no need to add any more. And the rotors can be moved closer together to close the airgap.

It would be heavier due to the bigger rotor plates, but if it's on the ground, that's not too much of a problem.

And because we've doubled the circumference, the magnets will be going twice as fast, so the coils would "see" twice the RPMs.

So, apart from the bigger parts for the rotors and stator epoxy, what's the catch?

[electrondady1]

http://www.fieldlines.com/story/2006/6/11/1896/64818

hope that works as a link

 if you space the mags farther apart there will be no advantage in going wider.

[electrondady1]

ok there is a lot of useless stuff in there but ed's and flux's comment were to the point .

the thing is , if you simply space the mags wider , on a larger dia. it will not increase the  output .

there would be the same frequency  and the coils would need to be wider (resistance)

[TomG]

the thing is , if you simply space the mags wider , on a larger dia. it will not increase the  output .

I wasn't suggesting it would increase the power output  That would be a bit magic, without any extra copper or magnet! Although, if it ran cooler...

there would be the same frequency  and the coils would need to be wider (resistance)

I'm not sure why the coils would need to be wider? It could happily have the new, thinner coils wired up as twice the number of phases. As a thought experiment, we could pull each coil out individually to its own rectifier - you'd just see an occasional burst of current (one way then the other) from each coil, but added together they'd be continuous.

I guess the furthest extreme this could be taken to (just as an illustration) would be a super-wide stator, where each "coil" is a single U-shaped wire, just going in and coming back out again 1 or 2 magnet's width further along. Then each of those wires could have its own rectifier. Still using the same amount of magnets as we started out with, we'll just very occasionally get an up-then-down spike as the magnet passes each wire.

That genny would start generating at 0.001RPM!

So: same output, but cooler stator, and lower RPMs required (at higher torque)

[finnsawyer]

The answers given here are correct if you stick to three phase alternators.  With a larger rotor you would need more magnets and coils.  With the alternator design that I propose in one of my diaries, which has single phase output, you would need more coils and the magnets would need to be farther apart.  The design uses 18 coils with 12 magnets, for instance, rather than 9 coils with 12 magnets.  The best analysis that I was able to do indicates that my design would be able to put out twice the power with the same number and type of magnets as the three phase design.  This is possible because the design requires a larger rotor due to the coil spacings that are required.  But you should be aware that one can not increase the rotor diameter without limit.  It seems that 24 inches is about where people stop.

[TomG]

OK, I'm trying to explain myself a bit better, but sadly I can't draw. I'm struggling to understand why more copper or magnet would be needed when we're not trying to generate any more power...





In the second, larger genny, the coils (black and green) are half the thickness of the coils in the first, smaller machine. There are twice as many of them, but each one has half as much copper in it. The magnets are exactly the same, just spaced out wider.

If we wire up the black coils exactly as we did in the first genny, we will only produce half the power, because we're only using half the copper. But we can wire the green coils up to an entirely separate rectifier, to get us back to the same power as we had in the first machine.

What have I missed?

[Flux]

Within reason your larger discs will let you have more winding space so you could use thicker wire. There may be a small gain in output and you will have a larger surface area for cooling.

If you depart much from the normal spacings you had better use jerry connection as you will run into a lot of trouble with star connection.

It may be that Finsawyer's series connection does the same as the jerry connection and would simplify the rectifier issues but you will be in the land of experiment.

Jerry has fairly well proved that his connection does reduce the heating over star when you depart from convention proportions.

Whatever you do to obtain the same output from a larger surface area will reduce the heating as long as you don't reduce efficiency by doing it.

Flux

[vawtman]

Hi Tom

 Im working on a axial flux motor.So it wouldnt be direct drive.A small pulley on the motor and whatever size on the vawt.Those serpentine belt used in cars would work nice i think.Nice and long with tensioners.

 Probably gonna start with a exercise bike flywheel so my mags will be going roughly 10 times faster.Plus you would have a sturdy structure.Simple brake rotors and fancy statorLOL

 I think with wide rotors and a skinny stator warping becomes a factor.

 Has you said the stuff is on the ground to play with.Match the load

[finnsawyer]

In the standard three phase arrangement a coil will straddle two magnets.  This results in a larger voltage due to the flux changing from passage over two magnets.  The maximum voltage occurs when the coil is exactly centered between two adjacent magnets.  On the other a coil has to pass from a magnet of one polarity (color) to the following magnet of the same polarity to form one complete waveform of voltage.  In your proposed arrangement this does not occur.  As a coil passes over a red magnet it will produce, say, a positive pulse followed by a negative pulse.  Then it goes through a brief time where no voltage is generated followed by a negative pulse and a positive pulse as the coil passes over a blue magnet (opposite polarity).  This is similar to the situation with my design, which also has this reversal of waveform polarities and the times of no output.  However, my design is optimized so that at all times there is a usable output voltage when the coils are connected in series.  It is not obvious that your idea can be so optimized.  That is, you need to show that the coils can be connected in such a way that you can get a usable three phase output, although I would settle for a single phase output.

[finnsawyer]

With this reversal of waveform polarities it should be easy to see that if one brings the magnets closer together by making the rotor smaller the waveforms will move closer together until they start to overlap at which point the voltage, being the sum of the now overlapping waveforms, gets larger in amplitude (more negative or positive).  The maximum voltage will occur when the two negative pulse or positive pulse maximum values exactly coincide.  This condition is the one that is found in a properly designed three phase alternator with the coils usually touching with the magnets spaced accordingly such as you find on this site.