magnets: to stack or not to stack

[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.

[brandnewb]

Yes Sir. Thank you Adriaan for pitching in.

What would you have done if;
* money was no object.
* make use of the magnets one already has.
* make sure this turbine gets a fighting chance.

Stack or would you not bother?

[Adriaan Kragten]

Yes Sir. Thank you Adriaan for pitching in.

What would you have done if;
* money was object
* make use of the magnets one already has
* make sure this turbine gets a fighting chance.

Stack or would you not bother?

It depends on the ratio in between the total magnet thickness and the total air gap thickness which you have now. If the existing air gap is large with respect to the magnet thickness, increase of the magnet thickness is logic. However, if the total magnet thickness is about the same as the total air gap thickness, you can better keep it as it is. If you double the amount of magnets needed by doubling of the pitch circle diameter and doubling of the number of coils, the power at a certain rotational speed will increase by a factor four. This is because the frequency increases by a factor two and so the voltage in one coil increases by a factor two. The number of coils increases by a factor two and so the total voltage increases by a factor four. So increase of the pitch circle diameter has a much stronger positive influence on the power than increase of the total magnet thickness.

The generated voltage in a coil depends also from the width of the magnets. What I see in your photo's is that the magnets are very slender and that there is a big distance in between adjacent magnets. A voltage in only generated if a coil leg is in between the magnets. So for very slender magnets at a large distance, there is a large part of the time for which no voltage is generated resulting in a low average voltage and so in a low power. So you might also think about doubling of the magnet width by using two magnets besides each other. This will almost double the average voltage and so it will increase the power much stronger than doubling the magnet thickness.

[brandnewb]

Ahhh Now I think about it..  I carefully align the coil width with the magnet width. Now if I were to side by side 2 magnets to become one pair (I am really not looking forward to the amount of pull or push I can expect there ) then the second phase of the coil is magnetically excited sooner than it should have been so we will not end up with a nice 120 phase shift.

BUT!!! here comes the BIG but. Hey does that really matter since we might end up rectifying it all anyway?

[brandnewb]

ahhh yes, you could well be correct. Which should of course been a no brainer as you seldomly are not.

But anyway let me test 2 of those magnet side by side and see if I am confident I can contain them.


But truth be told I am not confident and frankly a bit scared of what I am about to try.

[brandnewb]

Ok So I did not ever try. I just dare not to.

Ands even though your suggestion was regarding stacking. Sure in a horizontal manner rather than  vertical.

Nahy Please let us respect my limbs and not taunt faith too much.

I would be cool with stacking vertically. Horizontally will just introduce too many dangers I am not ready for yet.

so here in my revised version of my question to you;

What would you have done if;
* money was no object.
* make use of the magnets one already has.
* make sure this turbine gets a fighting chance.
* stacking only vertically.

Stack or would you not bother?

[Adriaan Kragten]

Ahhh Now I think about it..  I carefully align the coil width with the magnet width. Now if I were to side by side 2 magnets to become one pair (I am really not looking forward to the amount of pull or push I can expect there ) then the second phase of the coil is magnetically excited sooner than it should have been so we will not end up with a nice 120 phase shift.

BUT!!! here comes the BIG but. Hey does that really matter since we might end up rectifying it all anyway?

If you would measure the voltage fluctuation of one phase on a scope, you will see that the voltage fluctuation is far away from sinusoidal. For these slender magnets, you will get a high positive block shaped voltage when a leg is opposite to a north pole, then a long part of no voltage at all and then a high negative block shaped voltage when the same leg is opposite to a south pole. There will always be a 120° phase shift in between the patterns of the three phases if the coils are laid correctly. If the three phases are rectified with a 3-phase rectifier there will still be a strong fluctuation of the DC voltage if you have these strong fluctuations in the AC voltage of each phase.

If the voltage fluctuation in one phase is sinusoidal, you get a DC voltage with only a small fluctuation (see report KD 340). It is possible to get a constant DC voltage out of three alternating block voltages but only for two conditions. One is when one blok has a length of half the length of the part for which the voltage is zero. The DC-voltage is equal to the peak voltage of one phase for this condition. The other is when one block has the double length of the part for which the voltage is zero. The DC-voltage is twice the peak voltage of one phase for this condition. All other options result in a DC voltage which is built up from blocks of different height and which may even have parts for which the DC voltage is zero if the part of the AC voltage which is zero is very long. This is the case for your construction and so don't expect a constant DC-voltage.

[brandnewb]

SO I take that to be a yes? Go stack?

Roger SIr!!. I will order right away

[Adriaan Kragten]

SO I take that to be a yes? Go stack?

Roger SIr!!. I will order right away

How can you conclude to stack the magnets after my explanation? It is much better to make the poles wider instead of thicker.

[brandnewb]

sorry Adriaan,

I mean no combat here. I would hate to lose your support. You know I view you as the utmost authority.

But with my primitive brain I think like this;

I have given all the data that I could have given this short time frame.
I have not read that you explicitly tell me to NOT stack.

Then I just default into my gut feeling based on what my data tells me.

Look you and I might benefit to get a beer at one time or another. life is too short ;(

I would love to show you  my Air wheel blades

[brandnewb]

I tried in the most polite way I know of to ask for concrete non long winded advice.

Often when I ask simple yes or no questions I get long winded theoretical responses that only scollars can deal with.

I have tried to massage the most prolific in this behavior to take me into account in where I (and I am sure many others that do not dare to speak their minds) am not really helped by hard scientific language that intimidates.

To avoid any confusion or distrust

Here are my Air Wheels that I am working on;

[Mary B]

I tried in the most polite way I know of to ask for concrete non long winded advice.

Often when I ask simple yes or no questions I get long winded theoretical responses that only scollars can deal with.

I have tried to massage the most prolific in this behavior to take me into account in where I (and I am sure many others that do not dare to speak their minds) am not really helped by hard scientific language that intimidates.

To avoid any confusion or distrust

Here are my Air Wheels that I am working on;
(Attachment Link)

It is a technical subject so a yes/no answer isn't possible...

Stack magnets now your gap is wider so you gave up some(most?) of what you gained... going wider puts more magnetic material across each coil so a wider pulse is developed. Wider pulse = higher voltage...

Do you have an oscilloscope? If not one of these is very low cost and will show you the true voltage and waveform you are getting so you can compare stacking up to stacking side by side. 

https://www.amazon.com/dp/B0CBKKN6RM?ref=ppx_yo2ov_dt_b_product_details&th=1

$31 so not a bank breaker! I have 2, I loan one out and if they break it oh well. My 200mhz  $400 Rigol scope doesn't leave my radio test bench!

[Adriaan Kragten]

I say it again, if it is useful to increase the magnet thickness depends on the total thickness of the magnets and the total thicknes of the air gaps in a magnetic loop. In my previous post I have given the formula to calculate the flux density in the air gap and I have given three examples to show the effect of the magnet thickness. What more can I do to make things clear? I don't know the thickness of your magnets and the thickness of your air gaps and so you have to make this very simple calculation yourself. It is your generator and you should make the decisions about the geometry and not expect that someone on this forum takes the decisions for you. But you should not design PM-generators if you don't have the basic knowledge about the magnet and the coil configuration and the way how a voltage is generated in a coil.

Some knowledge is given in chapter 9 of my public report KD 341. A lot of information can be found on this forum if you search well. Figure 5 out of KD 341 gives about the optimum geometry for rectangular magnets and a 1-layer winding for an 8-pole generator. If you want more poles, you can follow the same procedure.

Your ideas about the windmill rotor become stranger and stranger. How can you expect that a drag machine with flapping cups can have a reasonable lifetime? I remember that long ago someone in Amsterdam had built a VAWT with eight umbrellas which open and close. One heavey wind gust and the whole thing was blown away.

[electrondady1]

I like building vertical mills. The next step for me is to double the diameter to 4 feet . I don't want to build a 4 foot diameter alternator so I will try running a small alternator from the circumference of  a 4 foot diameter drive rotor at the base of the windmill , i may build 4 small alternators  around the circumference to act as additional support for the windmill.

[brandnewb]

I say it again, if it is useful to increase the magnet thickness depends on the total thickness of the magnets and the total thicknes of the air gaps in a magnetic loop. In my previous post I have given the formula to calculate the flux density in the air gap and I have given three examples to show the effect of the magnet thickness. What more can I do to make things clear? I don't know the thickness of your magnets and the thickness of your air gaps and so you have to make this very simple calculation yourself. It is your generator and you should make the decisions about the geometry and not expect that someone on this forum takes the decisions for you. But you should not design PM-generators if you don't have the basic knowledge about the magnet and the coil configuration and the way how a voltage is generated in a coil.

Some knowledge is given in chapter 9 of my public report KD 341. A lot of information can be found on this forum if you search well. Figure 5 out of KD 341 gives about the optimum geometry for rectangular magnets and a 1-layer winding for an 8-pole generator. If you want more poles, you can follow the same procedure.

Your ideas about the windmill rotor become stranger and stranger. How can you expect that a drag machine with flapping cups can have a reasonable lifetime? I remember that long ago someone in Amsterdam had built a VAWT with eight umbrellas which open and close. One heavey wind gust and the whole thing was blown away.

Ok I told my self that I should back off for a little while to let things settle back to normal. But I just do not want to forget having a funded response on this.

I am honored that you call my ideas strange. That is what advancement is based on if we would ask the people that advanced science Albert Einstein comes to mind to name one.

Now that some experiment in Amsterdam was poorly executed says nothing about the viability of the concept in it's core.

Also I'd like to remind people that this will be the first time I publicly tell here in this forum (sure I have mentioned it elsewhere already but there the audience after 2 years is understandably running a bit dry)

That I intend this as a seasonal solution for my power generation. You see that during summer we have gotten this insane surplus of energy via the solar panels.
Now my fellow country men know that netmetering will come to an end sooner or later. And since I have no more space to put solar panels then the only logical step forward, that I can think of, is to go wind.

Let this spin on our lawn during our darkest hours while we are not using the lawn and reap in the much needed energy (hahha if of course it will ever do anything fruitful)
As it will be a free standing structure there will be no basis for the municipality to interfere.

And then when spring comes I take it all down again and store it for next autumn while I do some maintenance on it.

I hope this helps to translate that not always are things purely theoretical looking for absolute top performance. Sometimes one needs just a simple hands on practical solution that is also safe to get one by.

I would be honered if you would come marvel at my work if I indeed do pull it off. I will make sure I have your beverage of choice served at its optimum serving temperature.

respect

[Mary B]

There have been huge advances in fabrics for sails in the last few years, kevlar, carbon fiber... a LOT stronger! So a failed try a few years ago may work now especially with a lot of maintenance.

[brandnewb]

I would like to take the moment to explain my self. Otherwise thoughts might linger while there really is no reason for it.

So I started a bit more than 2 years ago experimenting with wind. I am not really sure any more if the corona pandemic had already struck or that there was rumours that it would.

As my house was already fully covered with solar panels and there was no more space left that has optimum solar radiance. Wind yes wind started to make sense.

So I became a member of this other forum. At first I started as a newb because I still to this very day am one.

I dissected a cheap 5 bladed horizontal wind turbine and asked as many questions as I could.

Then I realized that one can't buy turbines that do what they need to do (at least in my specific scenario) so I started a thread there about building a very cool one.

Now of course this was all a mix of crowd sourcing ideas on how to actually do it and also for me at least a way of keeping the pressure up. I have been hyping up ever since and I mean if one shouts loud from the roof tops then one better finished ones sentence. Else be ridiculed.

At one time in this process I was advised to go here with my questions which I tried. But let\s just say that getting on here was a non usual experience to keep the phrasing adequate.
Now once I did finally get on here I knew I was in the presence of experts so my tone here is way less show boating then the tone I have used to keep me motivated elsewhere.

I am in absolute awe of all of you.

Also I have tried to explain JW that I am a little unusual in behaviour. I can come across as fragmented.
Some call it weird others call it just another way of dealing with things.

Here my tone is cool because I know I am surrounded by experts. Elsewhere I have tried to shout so I can use the expectations I set as a clear drive to push through else risk blamage.

Now all this does not mean that I am a push over only in need of external influences. Nah strong headed perhaps with a hint of fool hardyness is perhaps the best term.

Now all in all please let me know if one is interested in my progress even if I have no questions. I would really like to show it here.
Boy would it help fortify my resolve in getting things done.

Kind regards.

[Bruce S]

Keep going, there are those of us in the background who like to see the questions and continuing sharing of knowledge.

Kind Regards

Bruce S

[Mary B]

I agree! Keep asking questions! Experts can learn too! Especially with 3d printing, laser cutting of parts is readily available now...

You need to get that cheap oscilloscope I mentioned a few posts back. You will see your peak voltage and waveform as you play with magnet shaping/stacking!

[brandnewb]

Do you have an oscilloscope?

Yes I have one rather similar to the one you linked to. I have it tucked away in a dusty corner somewhere so I am not sure if the resulution/frequency range is high enough but I guess it will be. After all we are talking about rather low frequencies here.

I will start using it more. This reminder is just what I needed to undust it.

My 200mhz  $400 Rigol scope doesn't leave my radio test bench!

Thats ok Mary I will bring my complete wind turbine to your test bench then hahaha. Can you imagine hurling that thing across the world

[Mary B]

Wind turbines are very low frequency so a cheap scope is fine. The scope will give you a visual... the taller the waveform and the wider the better. Tall but narrow will yield a lower peak voltage when rectified.

[Adriaan Kragten]

A drag machine with cups which open and close will not only be very sensible for wear but it will also be very noisy. So if the rotor rotates, you will hear a loud bang every time a cup opens. Your neighbours won't like this. This is the problem with all VAWT's. Every solution for a certain problem creates new problems. There is a VAWT for which the blade going against the wind has almost no drag and that is the rotating blade design as described in my report KD 417. As this is mainly a lift machine, it will have a rather high maximum Cp. But this idea needs a complicated mechanisme to rotate the blades with half the rotational speed of the rotor and it needs a vane for optimal orientation.

[brandnewb]

Yes Adriaan, thanks for the heads up.

I did think of this issue already and thought to use a sponge like material on the back and in between of rigid foam panels to minimize any noise.

To avoid them staying open, closed or stuck in an outward position due to centrifugal (or is it centripetal) force I intend to use tethered weights that actually pull the panels back in the correct position due to the rotational force.

Luckily I do not expect that force to be great with a diameter of over 7 meter./

I have some photo's of the panels I intend but it is such an early prototype that I am almost too ashamed to show them
If one does not demand to see them early then I will wait a while until they are a bit less early stage.

[MattM]

Rigid panels are better because as your panels open and close it requires drag energy to close that is working against your goals.

[brandnewb]

Ok so it took a bit of bravery mixed with a dash of foolhardiness. 2 broken magnets and some shrapnel that I will have to remove from my dry wall at one time or another. Luckily I wear safety glasses when running experiments. The shrapnel part is heavily exaggerated btw but why not add a little drama for the readers entertainment

But finally I think I understand the suggestion of stacking side by side (horizontal in an axial configuration). I'd like to make sure I do.

Now at first I thought one meant keeping the poles aligned. That would be a complete nuclear accident waiting to happen as the magnets would strongly repel and I do not have to explain here how difficult it would be to contain that.

Now the other way, and the way one probably meant to advice me on, is to side by side them while opposite. So they attract.

Hahha. Now this does not mean things can not get out of hand as my dry wall can confess if one were to ask it privately

Anyway story telling mode aside.

What will happen to the coils, I think, is that the flux will have a quick field reversal every 3.75 degrees of magnet movement.

So a quick up down with a short period where it flows over to the next cycle.

I have not taken out my oscilloscope yet but the frequency of field reversal seems to be involved.

Although I am going to introduce a formula I would still like things to stay at a beginner level.

I have been trying for the last hours to find my misplaced bookmark on a simplified version of lenz's law (I think)

E= b * v * I

I think it was as it was too long ago ;(

I can only advice one to not organise their documenting like I do. You will struggle to find things back when you need them most.

Anyway I did not find a frequency component in that formula. So now I am puzzled as I thought it was the frequency of field reversal that was important. But this formula seems to imply that it is the v(velocity) that is a part of the equation.

You see? It is easy for some of us to get a little lost in the fast woods.
But I like exploring them anyways



ahhh I think I remember now someone already mention it to me.
The velosity dictates the frequency. so it all boils down to the same thiing in the end.

[Mary B]

Nope, stacked same pole, yes it is a pain in the... glue one magnet down with JB Weld and give it 24 hours to setup, then move next magnet in and glue and clamp in place. Helps to make a template to place the magnets that contain magnets sliding sideways.

Gives a wider pole on such a large machine so a stronger pulse from each coil. IF you follow the 4:3 rule...

[brandnewb]

Only a mad person would embark on something like this SO I guess you understand that I will never.


Not try it. I finally found the courage to risk magnets, limbs and dry wall as in the meantime I have gotten this hunch that the field will be extended more. But I will try and calibrate my meter to make it behave a little better and get some kevlar body armor before I actually go and do this.

But I will most likely just decide to not do this. This goes against having something safe spin in ones lawn.

But in the interest of knowing I am now very interested.

I did find what I was looking for though.

Building a Generator: size of induced current
it was;
e = B x I x v
https://www.youtube.com/watch?v=KUrMt6ic53o

[mab]

...
But finally I think I understand the suggestion of stacking side by side (horizontal in an axial configuration). I'd like to make sure I do.

Now at first I thought one meant keeping the poles aligned. That would be a complete nuclear accident waiting to happen as the magnets would strongly repel and I do not have to explain here how difficult it would be to contain that.

Now the other way, and the way one probably meant to advice me on, is to side by side them while opposite. So they attract.
...

I could be wrong, but my reading of it was that you do want them stacked side by side the same way up - i.e. repelling - as this would double the duration of current output for each half cycle of a.c. output, without changing frequency : rpm ratio.

Putting them side by side one up one down will likely reduce the flux through the coil as much of the flux will take the shortcut to its neighbour, instead of crossing the coils to the other rotor.

But yes, getting the magnets in postion and keeping them there may well be... challenging  .

[brandnewb]

JB weld I could not find here over on the wrong side of the pond. Same goes for kevlar body armour.

I guess we are not as liberal as I thought we were here in Europe.

No what I will instead do is calibrate the meter and go again into a deep dive. And carefully document all the differences between the different options I have (single magnet, single magnet with powder, vertically stacked, vertically stacked with powder)

Horizontal stacking might be featured as a fun experiment so I might just include it in my upcoming deep dive. but i just can't take the risk of actually using it in something in service.