using two magnets instead of 1

[andres macias]

Hi,

just got my hugh piggot book and i´m looking at building a 4 footer as my 1st project and i stumbled with neodymium magnets of the following dimension : 1" x 1" x 1/2" in a shop really close to my home. could i use two of them and equal the 2" x 1" x 1/2" that the book suggests that i should use in the rotor?

thanks for all your help.

[andyman5002]

I don't see why not but I have not read this book and I am still a noob. But i don't think two magnets put together is as strong as one the correct size. So you will loose a bit of power. If there cheap then I would go for them. Keep us posted on your project and get some pics

Andy

[Flux]

That is about right, you will loose a little bit with 2 magnets but if they are a higher grade then you may even come out better.

Make sure they are good grade neo and you should be ok.

Remember they need to be the same polarity towards the stator and will repel strongly, you may have a little fun keeping them in place.

Flux

[tecker]

 No problem you can skew them to add some duration to the charge cycle (leaving the one closest to the center tdc)

[ghurd]

"you may have a little fun keeping them in place"

I would say 'Virtually impossible to keep them in place'

G-

[zapmk]

Not impossible to keep them in place, Stack a mag on top to get them togather,

use some good epoxy to glue them down wait till the epoxy drys.

What is Virtually impossible is to get the top mag back off.  

 -Zapmk

[andres macias]

how do i "skew" them? seems like i would be better off getting the 2" x 1" x 1/2". from the online store.

[tecker]

I was under under thimpression you had the 1" you wanted to use

[Flux]

Don't skew anything, this is not a motor conversion.

Zapmk's idea is a good one. Use about 1/8" thick cardboard or something non magnetic between the magnets you are trying to glue and the clamp magnet and it will come off a lot easier.

You will need a very good glue to hold the 2 magnets, polyester resin or superglue will not do it. Use good epoxy or a structural glue and you will probably be better gluing 2 magnets at a time rather than try all 16 at once If you can get any glass or polyester tape, wrap several turns round the edge of each magnet pair before you cast the magnets in resin and that will give you extra strength.

If the magnets are really cheap, give it a try. If the cost saving is not great save yourself a lot of trouble and get the proper magnets.

Flux

[tecker]

If  you put both magnets top dead center the duration of charge is fine no doubt .If the top magnet is a few degree ahead of the bottom magnet a longer duration of charge will result .I know this is a technique associted with decoging but the same buck condition is  there when under load .

His first machine is probably not the time for deviation from the plans available .

[andres macias]

thanks its great to have so many info. i still dont have any magnets. but the ones i can get here in mexico were not 2" by 1" by 1/2" and i got a little antsy and wanted to start doing this right away. but with all the tips i guess i will mail order the correct magnets and not try to take shortcuts.

when ever i make some progress i will post a few pics. yesterday with wind hurricane john left behind the planned site had steady 18 mph winds at 5' height.

[tecker]

Sounds good looks like your off on the right foot

[finnsawyer]

Why don't you just use more of the one inch square magnets?  Let's take a look at this.  I assume the design calls for nine coils/twelve magnets.  So, how to begin?  Let's start with the amount of copper (resistance).  The total amount of copper per coil is proportional to the area of copper wire.  I will also assume that the total winding thickness is one half inch.  So, for the 2 by 1 inch magnets one gets:

       3x2 - 2x1 = 4 square inches of copper.

For the 1 inch by 1 inch magnets one gets: 2x2 - 1x1 = 3 square inches of copper.  So, you can increase the number of coils by one third and still have the same resistance.  So, go to 12 coils/16 magnets of the same thickness and the same number of turns of the same wire.  With the greater number of coils we now need a larger rotor, one third larger, in fact.  The larger rotor means the magnets pass the coils faster for the same rpm.  We have increased the total flux by one third and the time rate of change of the flux by one third.  To get the change in voltage relative to using the one inch square magnets in the 9/12 configuration we multiply 4/3 times 4/3 to get 16/9, about an 80% increase in voltage with the same resistance.  This isn't too bad, as going to the 2 by 1 inch magnets isn't going to double the voltage anyway due to geometric limitations.  When the coils are centered over the magnets so that the flux is greatest, the voltage waveform from a coil is actually going through zero.  The voltage maximum occurs when a coil is centered exactly between two magnets.  Unfortunately, the copper wires will not be exactly parallel to the magnet sides, which means the voltage will be somewhat lower than optimum.  Also, the part of the 2 inch magnet closer to the center of the rotor will be moving slower than the top half, which also acts to lower the voltage.

If you are still concerned about having the proper cut-in rpm you could sacrifice resistance by increasing the number of windings by 5% and the rotor diameter by 5%, for a 10% increase in voltage.