New Book THE FERRITE GENERATOR

[Yianie123.]

There is supposed to be a new book being written by Jonathan Schreiber and Hugh Piggots.  I have been waiting a while.  Does anyone have any updates?

[MagnetJuice]

Hi Yianie,

I contacted Jonathan about 6 months ago. He said that the book was running behind and that he will let me know when the book was out.

So far, no news.

Ed

[Yianie123.]

Thank you...

[Bruce S]

I'd be interested to know too!

Thanks!!
Bruce S

[Yianie123.]

I have been waiting for several months for this book.  Does  anyone have any idea what the status is?  Would really like to design my build, although due to circumstances would like to modify the design for a VAWT.  Or if anyone has any references on building a super low RPM, PMG, that will be ideal.  Thank you everyone!!!!

[Adriaan Kragten]

I have built my first PM-generator about 45 years ago. At that time, neodymium magnets were not yet available and so I have used Ferroxdure magnets which were supplied by Phillips at that moment. Ferroxdure has a much lower remanence Br than neodymium and so you can only get a sufficient flux density in the coils of the stator if the magnetic flux is concentrated. This is realised by chosing a rather high pole number of the armature and by positioning of the magnets radially. The first and the second PM-generator were made from a car dynamo. The third PM-generator was made from an asynchronious motor. All generators had a 12-pole armature and a 3-phase winding. The procedure is described more in detail in chapter 3 of my public report KD 341 which can be copied for free from my website: www.kdwindturbines.nl at the menu KD reports.

The point with this kind of generators is that you need a stainless steel shaft on which a mild steel bush is glued by epoxy or anairobe glue. The magnet grooves are made up to a depth less than 1 mm from the stainless steel shaft, otherwise to much magnetic flux is lost by magnetic short circuit in between adjacent armature poles. The stainless steel of the shaft prevents magnetic short-circuit through the shaft. The magnet grooves must make an angle with the shaft axis such that there is just one stator pitch groove overlap in between the right and the left side of the armature. This eliminates fluctuation of the cogging torque.

The magnetic flux coming out of a north pole is supplied by the magnets in two adjacent grooves because the north poles of these magnets face each other. The concentration factor is the ratio in between the total magnet area of the magnets in two adjacent grooves and the area of one armature pole (if the lenght of the armature is the same as the lenght of the stator). If the grooves are deep, if the magnets are high and if the chord of an armature pole is small, it is possible to get a concentration factor of about 3 for a 12-pole armature. This makes that the stator iron is saturated resulting in a very strong generator, even if Ferroxdure magnets are used.

[Yianie123.]

Thank you for your great detailed response.  To be honest, your explanation is far beyond my understanding.  Do you know of any "How To" book titles or publications that will assist me in building a 1-3kw, very low RPM, PMA?  Thank you.  John

[JW]

LOL  ya Adriaan does that, yes we can help you. So your looking for- Do you know of any "How To" book titles or publications that will assist me in building a 1-3kw, very low RPM, PMA?  Thank you.  John

Look at this link   https://www.otherpower.com  DanB and DanF have made a book for this you should find it there. its for a 1 to 3kw alternator. the book is "Homebrew Wind Power" just scroll down the page a bit on the left side. The link on the page doesn't work but its available on other sites than Amazon. the book title is-  "Homebrew Wind Power" by Dan Bartmann and Dan Fink


https://www.google.com/search?gs_ssp=eJzj4tFP1zc0Ssq1qEoptDRg9BLOyM9NTSpKLVcoz8xLUSjIL08tAgDF4QvU&q=homebrew+wind+power&oq=homebrew+&gs_lcrp=EgZjaHJvbWUqDAgBEC4YJxiABBiKBTIPCAAQIxgnGOMCGIAEGIoFMgwIARAuGCcYgAQYigUyBggCEEUYOTIKCAMQABixAxiABDINCAQQABiDARixAxiABDINCAUQABiDARixAxiABDIMCAYQABhDGIAEGIoFMgoIBxAAGLEDGIAEMgcICBAAGIAEMgoICRAAGLEDGIAE0gEJMTQ0MzBqMGo3qAIAsAIA&sourceid=chrome&ie=UTF-8

Don't worry about the link its safe

JW

[Yianie123.]

Is this for super low rpm generators?  I'm looking to build a 1kw VAWT.  THANKS TOO ALL WHO RESPONDED!!!

[Yianie123.]

Hoping to use ceramic magnets for the lower cost and no corrosion

[JW]

There was a user Chris Olson that used gear reduction, he may have deleted his account, let me look around some more-

[JW]

https://www.fieldlines.com/index.php?topic=149203.0

https://www.fieldlines.com/index.php?action=profile;u=21517

https://www.fieldlines.com/index.php?action=profile;area=showposts;u=21517

https://www.fieldlines.com/index.php/topic,149570.msg1060043.html#msg1060043

I will search tomorrow for more, but Chris did use a gearbox to accommodate slow RPM.

https://www.fieldlines.com/index.php?action=profile;u=21517;area=showposts;start=3630

this should give you leads, but there is a lot of data the info is there. To see all info you have to be logged in...

https://www.fieldlines.com/index.php?action=profile;area=showposts;u=21517

JW

[Adriaan Kragten]

Thank you for your great detailed response.  To be honest, your explanation is far beyond my understanding.  Do you know of any "How To" book titles or publications that will assist me in building a 1-3kw, very low RPM, PMA?  Thank you.  John

I don't know what you don't understand in my explanation. A picture of a 4-pole PM-generator made from an asynchonous motor is given in figure 1 of KD 341. A 12-pole generator has the same construction but only twelve rows of magnets instead of four.

The generation of the voltage in a PM-generator is explained in two ways in chapter 9 of KD 341. The first explanation describes best the generation of the voltage for a generator with iron in the coils like it is the case for a generator made from an asynchronous motor. The second explanation describes best the generation of the voltage for a generator with no iron in the coils like it is the case for most axial flux generators. If you use Ferroxdure instead of neodymium, you need iron in the stator or the armature otherwise it isn't possible to concentrate the magnetic flux.

If you want to design a PM-generator, at least you have to study literature in which it is described how it works. I have seen several PM-generators designed by people without the required knowledge. Those generators have elementary mistakes and therefore function very badly.

Theoretically it is possible to design a very low rpm PM-generator. However, to get a high peak torque, this generator needs a large armature volume and therefore a lot of magnets which makes the generator very expensive. To generate energy by a wind turbine at reasonable investment cost per generated kWh, one should not use a windmill rotor which supplies the maximum power at a low tip speed ratio and so at a low rotational speed. So you should forget all drag machines or Savonious rotors.

[clockmanFRA]

Hugh Piggotts Ferrite wind turbines.

......

https://www.smashwords.com/books/view/445583

This is a online E Book, as he has still not done a paper back publication.

[joestue]

Hoping to use ceramic magnets for the lower cost and no corrosion

I just looked up the price at magnet 4 less and a 2x1x.5 neodymium magnet is 12.50, a 6x2x1 ferrite magnet is 7.59

When you work out the energy product of those magnets.. the ferrite magnet is only slightly cheaper at those prices, however it will make your generator weigh a lot more, which means it costs more, unless the metal is free.

You might consider making a radial flux air core using a steel drum with the 6x2x1 magnets on the inside of the drum. the copper coils get glued to the outside of the round outer diameter of a large induction motor stator lamination.. if you can find one that's totally round on the outside. most have polygons of some kind.

[joestue]

i suppose if the coils are going to be in air anyways it doesn't matter if you have an iron core, or if you replace it with another set of magnets.

I tend to believe that radial flux machines are intrinsically more efficient because the entire coil is at a given radius, producing torque, and the coils are square, not pie shaped which is the worst shape for the perimeter to area ratio. also because magnets are generally only available in square and rectangles, its hard to build an efficient axial machine unless the pole count is high enough that the square or rectangle magnet is not too far removed from its ideal trapezoid shape.

So if you do go the 6x2x1 ferrite magnet route, then perhaps an inner and an outer drum of magnets is the way to go. the coils then are stretched and compressed into a frame of some sort and then bonded together. The drum only need be 1/4" thick.

It is worth investigating if the 6x2x1" magnets should be spaced apart 3/8" of an inch, and a standard 7/8x3/8x1.875" magnets be slipped into the gap as a halbach array. The purpose of this isn't to make the magnetic field stronger but to space the poles apart slightly so the flux density in the air gap is better proportioned, and you might as well fill the gap anyways.


for the coils, an overlapping 3 phase coil should net at least 50% more copper into the air gap than the standard coils most people here are using.

the only problem being the end turns are thicker than the coils, which means that if you manufacture the coils such that they can slip over the inner drum of magnets.. the outer drum of magnets has to be split so that it can be opened up to get it over the coils.

basically the coil cross section is going to look like a dogbone.

[Yianie123.]

Thank you all for your responses.  I am very interested in the last comment using larger magnets.  Any pictures anyone can provide of your work will be appreciated.  Thank you again everyone.

[Adriaan Kragten]

About 1 1/2 year ago the price of neodymium magnets was more than twice as high as the current price. So at that time it was more attractive to use Ferroxdure magnets. But the current price of neodymium magnets is rather low. The big advantage of neodymium is that it has a very high remanence Br and so you can get a large magnetic flux in the coils of an axial flux PM-generator, even if you use no iron in the coils. The advantage of axial flux is that the coils can be positioned easily in between two flat iron disks with magnets at the inside (see KD 341 figure 5 and 6).

It is possible to design a radial flux generator with no iron in the coils (see KD 645 and KD 667). However, the path of the magnetic flux coming out of the armature poles is rather long and so the flux density is rather low. Using a stator with an iron stamping for radial flux generator makes that much thinner and so much cheaper magnets can be used. The disadvantage of iron in the coils can be that you can get a large peak on the cogging torque if the generator isn't designed well. However, this peak can be reduced strongly by using many mechanical armature poles for one magnetic armature pole (see KD 718 an KD 747).

[joestue]

About 1 1/2 year ago the price of neodymium magnets was more than twice as high as the current price.

I remember when it was 7$ per cubic inch, now its 12?, seems to me I do remember the price at 14.50.

https://www.magnet4less.com/neodymium-rare-earth-magnets-2-in-x-1-in-x-1-2-in-block-n42 at $8.30 now that's cheaper than ferrite by a good bit.

I bought 6 of those 6x2x1" magnets for 36$ free shipping on ebay, about 6 years ago.

I agree that building a radial air core, double rotor stator is going to be difficult to split the drum, position the coils accurately, and put the outer drum on. Axial is easier because you can change the air gap easily.

I suspect that an axial airgap ferrite machine made from multiple  6x4x1" magnets glued into a single piece say 6x12x1.. and then you cut it into trapezoids on a tile saw and place it around a disk.. is an effective and quick way to build trapezoidal magnets without wasting too much material.

[Adriaan Kragten]

The price difference may depend on the magnet size and the magnet supplier. In my post "Reduction of neodymium magnet prices" of 29 September 2024, I have compared neodymium magnets size 40 * 15 * 5 mm and found that in May 2023 one magnets costs € 2.88 and in Sept. 2024 one magnet costs € 1.27 so a reduction by a factor 0.44. May be that other suppliers don't follow the recent price reduction and so you pay too much.

You should not compare the prices of neodymium and Ferroxdure magnets of the same geometry because the remanence Br of neodymium is at least a factor 3 higher. So you can use much smaller neodymium magnets for the same flux density in the coils. To get a generator with Ferroxdure magnets which has the same maximum torque level as a generator with neodymium magnets, the first one must be a lot bigger. This results also in more other materials than magnets like iron and copper. I doubt if a generator with Ferroxdure magnets is a good choice with the current prices of neodymium magnets. Another advantage of neodymium is that it is almost impossible the reduce the remanence by the counteracting magnetic flux generated by the current in the stator. Even short-circuit of the stator winding won't reduce the remanence. Neodymium magnets will only loose their strenght if they become too hot.

[joestue]

You should not compare the prices of neodymium and Ferroxdure magnets of the same geometry because the remanence Br of neodymium is at least a factor 3 higher. So you can use much smaller neodymium magnets for the same flux density in the coils. To get a generator with Ferroxdure magnets which has the same maximum torque level as a generator with neodymium magnets, the first one must be a lot bigger. This results also in more other materials than magnets like iron and copper. I doubt if a generator with Ferroxdure magnets is a good choice with the current prices of neodymium magnets.

Regarding your last point, i doubt its possible to demagnetize a ferrite air core machine.

I did account for Br being 3 times higher.. you take the BR value, square it, multiply by the volume of the magnet and then divide by price.  At current prices of 7 usd / cubic inch.. neodymium is probably cheaper than ferrite even when its as cheap as 8$ for a 6x2x1 magnet, all things considered.

[Adriaan Kragten]

You may be right for ferrite air core machines but I have heard of a radial flux generator made from an asynchronous motor, so with an iron stamping in the stator coils, which has lost a part of the remanence of the ferrite magnets after strong short-circuit at high rpm. It might be that too high temperature during long short-circuit also has had an influence. It is long ago and I can't remember the brand of the wind turbine.

I have a small wind turbine running with a small axial flux generator of Hefei Top Grand (see KD 595). First is was connected to a dump load with a fixed DC voltage of about 13 V. At this voltage, the turbine is running that fast that it makes too much noise. So now I have short-circuited the generator at the end of the cable to the shed in which the electronics are placed. So there is some voltage drop over the cable. At low wind speeds, it turns very slowly but at high wind speeds, the braking torque is too low and so it speeds up but up to a lower rotational speed than for 13 V. The noise level is now acceptabe. I had expected that this would result in too high a temperature of the winding but the generator can have it and the open U-n curve hasn't changed. So there is also no demagnetization of the (trapezium shaped) neodymium magnets.

[MattM]

Ceramic magnets can lose strength from strong fields hitting it.  I accidentily let a ceramic touch a neo and it lost its strength to almost zero in a mere few seconds of contact.  So maybe backfeed through coils can kill them.

[Yianie123.]

Does anyone know the status of this book? 

[joestue]

Ceramic magnets can lose strength from strong fields hitting it.  I accidentily let a ceramic touch a neo and it lost its strength to almost zero in a mere few seconds of contact.  So maybe backfeed through coils can kill them.

so it looks like the typical y40 grade ferrite will demagnetize itself under zero flux conditions.
the more expensive grade hyt45 can handle a zero field condition, that is, the opposing amp turns by the copper windings equals the starting equivalent amp turns inside the magnet.

so basically, you have to design the generator to not be demagnetized, in the cold during high wind conditions.
given the relative cost of ferrite vs neodymium there many not be any point to use ferrite magnets for wind turbines.

given the low efficiency of air core small axial flux machines.. i don't think you will be able to demagnetize them with the poor copper fill factor of a single layer coil and the air gap intrinsic to a coreless machine.

this is an interesting paper on a ferrite linear generator for wave power harvesting. achieving 84% efficiency at 1 meter per second is pretty impressive, but you have to remember that efficiency and power density scale with the 4th power of the scale of the generator. this is because the cross sectional area of both the core and the cross sectional area both follow the square of the scaling factor, but the volume only follows the cube. so, 84% efficiency sounds great.. but its an 8000 KG machine only producing 76KW of power. or about 100KG per kilowatt..

https://www.researchgate.net/publication/325400757_On_the_Optimal_Pole_Width_for_Direct_Drive_Linear_Wave_Power_Generators_Using_Ferrite_Magnets/link/5b0c01dfa6fdcc8c253524f4