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magnets: to stack or not to stack

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brandnewb:
So I have learned here that stacking some types of magnets
(in my scenario N45 neodymium 60x10x5mm ) can increase the field up to a point.

My earlier test demonstrated to me that we are talking about a 70% increase.

But that test was one of the laziest I have ever done so truth be told there is not a whole lot we can depend on there.

Now I am asking for advice on how to properly test the field increase when stacking those magnets.

My idea is to do not bother with iron powder and simply stack the devoid of iron powder test magnet holders you might have seen and then just stack.

I measure from 10mm in between a magnet pair that is not on the outside of the test pieces.

Does this hold up to scrutiny or should I use other means to make decisions?

MattM:
Each magnet still operates independently, their effects however can overlap.  Your distance between magnet and coil decide the strength of the flux.  Instead of stacking you are able to get the second magnet closer by using two rotors so that both magnets are closer to the coil they affect.

electrondady1:
i remember testing  ceramic magnets in repulsion . Holding a single magnet close to another over a plastic ruler , with  like poles facing each other and  measuring the distance between when one magnet could push the other back. stacking a second mag and then recording the distance  was increased but not doubled. i have read that stacking neo  mags  increases flux density but  only by 10 or 15 % were as doubling the number of poles does double the output .

brandnewb:
agreed, doubling the number of poles does indeed double the field. That is what I have been doing not once but 2 times already ;)

Now as I have met the practical limits of diameters I thought what other ways are there to bump up the field.

BTW in the meantime I have found a company that can custom make any shape magnet at any strength within the laws of physics so this whole thread is really only about me trying to make the best of the magnets I already have. It would be a waste to discard them.

Please also not that I am not so religiously clinging on to the readings on my tesla meter. It is a cheap and very unreliable apparatus that for me only does one thing. Tell me the difference between 2 readings in one session.
You see as soon as I turn off and turn on the device again the readings are much different than earlier sessions.

So here is my baseline reading;

It is a non stacked array of magnets axially configured at a 600mm inner radius at a 3.75 degree (96 magnets) interval.

And then it's counter part on top of it and the readings make me soooo confused. I think it is because somehow now the field has found a shorter path to adjesent magnets.



Now both sides of the array double stacked


I know this is not much to go by but if in the most basic sense of making measurements I am not completely missing the mark here then I will consider adding another 192 magnets. Just to make sure this turbine is getting a fighting chance to do anything useful.

Adriaan Kragten:
Stacking two 10 mm thick magnets on each other has the same effect as using one 20 mm magnet with the same outer geometry and the same remanence Br. The flux density which you get in the air gap depends on the ratio in between the total magnet thickness in a magnetic loop and the total thickness of the air gap in that loop. Calculation of the flux density in the air gap is similar to the calculation of the current in a resistor. The resistance of a magnet is about the same as for air and so for the resistances, we can compare the magnet thickness with the thickness of the air gap. Assume that the total magnet thickness is t1 and that the total air gap thickness is t2. Assume that the remanence is Br and that the flux density in the air gap is B. For B you get the formula:

B = Br * t1 / (t1 + t2)      (in T).

So if the air gap is the same as the total magnet thickness (t1 = t2) you get a flux density which is 0.5 Br. If the total magnet thickness is twice the air gap (t1 = 2 t2) you get B = 0.6667 * Br. If the total magnet thickness is half the air gap (t1 = 0.5 t2) you get B = 0.3333 T. So this demonstrates that doubling the magnet thickness from the situation t1 = t2 gives only an increase of the flux density in the air gap from 0.5 T up to 0.6667 T. Therefore it is useless to use extremely thick magnets. In practice taking the total magnet thickness equal to the total thickness of the air gap is a good choice.

The given formula is only right if the stator iron is not saturated. The magnetic resistance of non saturated iron is very low compared to the resistance of air or the resistance of magnets and can therefore be neglected. So after calculation of B, one has the check if the stator iron is far from saturation!

For the calculation of B one needs the value of Br for the chosen magnet quality. This value is normally supplied by the magnet supplier. It varies in between about 1.19 T for neodymium magnets N35 and about 1.45 T for N52. A list with the values of Br depending on the magnet quality is given on the website: www.supermagnete.de/dut/data_table.php.

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