Magnet questions...dimensions and pulling power

[ve7khz]

I have been perusing the magnet suppliers and have some questions regarding the specs I am looking at.  I see pulling power listed and would assume that more is better. Say a 2x1x1 inch has a 136lbs pulling force and is $8, and a 3x1.5x3/4" has 145 lbs pulling force and is $25.  How important is that 9 pounds pulling force? I am wanting to build something 4-5 KW at 48VDC, 17-20' blades. I am looking at the Nirvana turbine on Hugh's site and it looks quite interesting. 16' rotor, 28 magnets per rotor and about 4KW out. He used 46x30x10mm N42 magnets and I was thinking of using 2x1x1".

Your thoughts?

Thanks

Paul

[tecker]

 The  thing the 3"mags will cover is a longer coil leg .I've not bought any 3" mags for the reason you posted ( big price tag ).

[CompDoc]

There is allot more to the decision of 'magnet selection' than pulling-force.  For example, the 3x1.5x3/4 would require more funds in rotor-size and copper, etc., and may cost you more than you initially expected.

It would be easier to discuss more "alike" magnets and the desired gennie.  Lets say your target is a 24volt charging axial-flux system.

We can compare the common 2x1x1/2 with the 2x1x1 for ease of discussion.  The pulling force is a good layman's way of selecting magnets, and the pulling force of the same grade 1-inch thick megs are double that of these commonly used ½-inch megs for the 12/24 volt dual-rotor 9/12 machines.   Keeping the stator at 5/8-inch thick; The rotor-diameter could be smaller for the 1-inch thick megs, saving costs, and the stator would require only half as much of the same gauge copper if you chose the 1-thick megs, saving costs.  

Initial magnet investment/cost-savings is negligible compared to the accumulative power-loss due to double the stator internal resistance, if you had decided to go with the common 2x1x1/2 megs in this example, not to mention the battle to keep the alternator cool... etc.

This discuss could go on and on... and my point is, allot can be said/written on this subject.

.

[Flux]

If you want to fish a bucket out of a lake then pulling pulling power is useful, otherwise it is misleading and is more related to the shape and position of poles than anything else.

It is some sort of comparison for similar shape and size magnets, otherwise forget it.

The dimensions and grade with neo tells you all you need.

[finnsawyer]

Not very important.  You could buy three times as many of the smaller magnets for the same money.  With the 3 inch magnets and 6 inch wide coils you would need a 11.5 (really 14) inch rotor for an eight magnet alternator.  With twenty four of the smaller magnets and proportionally smaller coils (four inch wide coils) you would need a (surprise) 23 (really 24) inch rotor.  The larger rotor means the magnets are moving faster past the coils imparting a larger kick for the same flux.  Since the magnets are smaller you have less flux per magnet, but more overall since each 2 inch magnet is nearly as strong as the 3 inch ones (that pulling power thing, if you wish).  You have 18 coils in the second case versus 6 in the first.  It might appear that you have three times as much resistance, but that is not likely to be the case.  The resistance per coil depends on the size of the wire and the number of turns.  This, in turn yields a certain weight or volume of copper.  In going from the large coils to the small coils you will have reduced the volume of copper per coil to less than half in this example with the same thickness stator.   So, in using the same size wire the resistance with three times as many coils will be increased by a factor of less than 1.5.  You will also have fewer turns per coil, but not as many as you might think, as it takes more copper per turn in the outer windings.  In order to prevent cut-in from occurring at too low an RPM you might then want to reduce the wire size.

The bottom line is that you should follow a existing design and not try to second guess all the experience that has gone before.  Alternator design is as much art as science.

You also should keep in mind that it is less costly to redo the stator rather than the rotor if the alternator doesn't perform as expected.    

[CompDoc]

You are correct, of course; however this newbie is not fishing...

"It is some sort of comparison for similar shape and size magnets, otherwise forget it."

Exactly my point, and reason for the quick example.

[CompDoc]

GeoM is correct, stick with the plans. As you can see it gets confusing the more that this topic is expanded. Some advise; take the time to expand your knowledge on all the pieces of this project that you find needs clarification. http://www.magnetsales.com/Design/DesignG.htm

[Hugh]

The bottom line is that you should follow a existing design and not try to second guess all the experience that has gone before.  Alternator design is as much art as science.

and the world would still be flat

[finnsawyer]

Hugh, you wrote:

"The bottom line is that you should follow a existing design and not try to second guess all the experience that has gone before.  Alternator design is as much art as science.

and the world would still be flat

thanks allways"

This was a response to a Newbie, as you probably were aware.  There is no reason to advise a newbie to try something experimental unless he has indicated a desire to do so, particularly in light of the costs involved.   Now, in your case, I might direct you to my proposed alternator design, since you obviously wish to experiment.  It is my interest to advance the science of wind power on the small scale, but not everyone using this site has the background or desire to do so.  The response should be appropriate to the perceived circumstances.

[Hugh]

No offense intended I'm sure the advice you gave was completely sound, and there was no argument offered.

The phrase caught my eye & I commented on it.

So very sorry

Hugh