alternator ?'s

[Greg Moore]

More questions:

If a second magnet rotor were added to Hugh's single rotor alt design, with all other factors remaining the same, what change in performance could be expected? Cutin speed? Voltage? Prop requirement? Stalling?

Are there any disadvantages to spacing a given size and number of magnets farther apart on a larger magnet disk? I imagine the effect would be similar to an increase in rpm as the coils would pass through the magnetic flux at a higher speed. Is this assumption correct?

What is the ideal coil size relative to the magnet? Should the 'hole' in the winding match the magnet size? Should the clipping from rounded winding corners be compensated for with larger coils?

Thanks everyone!

[Victor]

Are there any disadvantages to spacing a given size and number of magnets farther apart on a larger magnet disk? I imagine the effect would be similar to an increase in rpm as the coils would pass through the magnetic flux at a higher speed. Is this assumption correct?

 The speed you are refering to is in feet per second. The "speed" in induction formulas is in lines of flux per second so making the diameter larger without adding flux or more turns would be of no help.

Victor

[Flux]

I assume you are referring to the small 4 ft design when you say single rotor.

If you add a second rotor you change everything. Roughly you will halve the cut in speed or double the voltage for the same speed. Up to a point it will need a bigger prop to avoid stall, but you will have the same winding resistance so you can't expect to take very large currents from it.  In general if you double the magnet volume you get 4 times the power for a given speed but the larger prop needed may not be as fast.

At a rough guess I would consider an 8 pole dual rotor using those magnets to be suitable for a 6 or 7 ft prop, but you would most likely need to change the winding for best results.  Remember that it is a proven design and has been optimised for that case. If you make changes you need to think of all the consequencies, for instance the stator thickness would no longer be optimum.

Regarding spacing magnets farther apart on a disc, yes you can get more output if you do it right, but having gone that far you can do even better by adding more magnets so it becomes a trade off between total output and output for given cost.

Your last question about coil size has been discussed over and over again and largely depends on the type of winding but in general the hole does work out to be about the size of the magnet, it's not that critical that you need worry about rounding corners. Some prefer wedge shape coils others go for rectangular coils one gives maximum volts the other least resistance.  If you space magnets farther apart you have longer turns and rectangular coils with less resistance will usually win out.

Flux

[scoraigwind]

If a second magnet rotor were added to Hugh's single rotor alt design, with all other factors remaining the same, what change in performance could be expected? Cutin speed? Voltage? Prop requirement? Stalling?

As has been stated already, you would double th eflux.  This allows you to use a larger rotor but you'd also have to reduce the number of turns for a given voltage.  You would not halve the number of turns because you would be going slower, but you'd reduce it a lot.  You'd get a lot more power.  Maybe four times, but watch out for overheating.  The idea of the single rotor is to keep the construction simple.  And it allows very good cooling of the stator.

As for the details, I can help if you want.  I do quite often help out with different designs. If you contact me off list and then also keep me informed how it works out then I am very happy to do this.

Are there any disadvantages to spacing a given size and number of magnets farther apart on a larger magnet disk? I imagine the effect would be similar to an increase in rpm as the coils would pass through the magnetic flux at a higher speed. Is this assumption correct?

The number of magnets will still be the same and so the magnets passing a coil each second will be the same. You will slightly improve the total amount of flux because there will be less leakage of flux from pole to pole. You will also have room for bigger coils with more turns.  But you will need a bigger steel disk and it will take up more space.

What is the ideal coil size relative to the magnet? Should the 'hole' in the winding match the magnet size? Should the clipping from rounded winding corners be compensated for with larger coils?

I like to make the inside of the coil the same as the magnet size, and I believe this is optimal, but there are arguements why it should be made bigger or smaller.  There is still some flux outside the magne facet (it spreads) but longer coil turns have higher resistance.

[Jerry]

Hi Hugh Happy new year.

What about the outside of the coil? What size rrecamondation do you have for this?

In particular with respect spacing between magnets and available space?

                       JK TAS Jerry

[joebob]

I am thinking outloud, and feel free to update me.

Voltage = #winds x (chg in flux)(chg in time).

Flux is coil area x mag field.

On a larger disk, the magnet will pass by faster, so the (chg in flux)(chg in time) will be higher.

So, V is higher, but will last for a shorter time.

That means that the power will probably be the same, but a key is that to get a good sine wave, the magnets should be spaced so that the end of the +V spike (maybe bump is a better word)happens at the same time as the beginning of the -V bump.

Joebob

[troy]

Howdy out there in electron land,

The rule of thumb that I've seen a bunch of times on the board is that the hole in the coil should be about the same size as the magnet.  Being the curious type, I wanted to see what happened to the waveform as I changed coil core size in relationship to the magnet size.

To generate the wave forms, I made three test coils.  The hole in test coil A was half the width of the magnet.  The hole for coil B was about the same size and test coil C had a hole 25% bigger than the magnet.  The vertical legs of the coil (the effective part) all had three turns of 18 AWG magnet wire for all three coils.

Since I have only a two channel o'scope, I first compared coil A to coil B, with the following results:

Then I compared coil B and coil C with the following results:

I have superimposed the two waveforms for easy comparison.  In the A vs B pic, the very short AC wave with the pronounced dip in the middle is the output from coil A, the little skinny coil.  It's obvious that you really start losing efficiency when the magnet can cover both legs of the coil at the same time, causing a voltage cancellation.

In the B vs C wave form comparison, C definitely outperformed B, with virtually no dip as the magnet passes over the center of the coil. This demonstrates that the flux doesn't just shoot straight out of the magnet, but rather, spreads out somewhat, so we still get a little voltage cancellation in coil B as the FLUX of the magnet can cover both legs at the same time, briefly.  

Like everything though, it's a compromise.  If you make really big holes in your coils, you'll have really great "efficiency" and pretty wave forms, but less total output than if you pack more copper in with a smaller hole size.  However, this is definitely a case of diminishing returns.  If you fill the air core hole up with wire completely, the very middle part of that will generate basically no useful electricity and just add to total resistance, reducing your generating capacity.

I intentionally used very few turns in the coils, to eliminate the averaging of the wave form from a full coil.  A full solid coil with no central gap might still show a decent wave form as the outer layers are compensating for the cancellation of the very center.

So yeah, I'm back to making the holes in the coils just a little smaller than the magnet.

Voltage and amperage measurements with a Fluke DMM produced a similar outcome, with the small "A" coil producing about half of the watts of the the "B" coil, despite having the same amount of copper in the effective part of the legs.

Best Regards,

troy

[Ungrounded Lightning Rod]

For battery charging service you'd like something approximating a square wave.  So a coil slightly larger than the magnet (flattening out that dip your observed) should be best.