Excellent explanation JMFahey
I have found out few things all together. Im building a magnet charger to charge upto 285mm Ferrite magnets and smaller Neo magnets but building the same charger for both the cases. Well trying though.
One what I have found is that the charger made for Neo need a strong and large impulse.
I have simulated the data in comsol and found certain spec for coil which is not large but demands about 700V DC with 84mohm coil and about 8KA and I have got two 5600uf/450V in series designed for magnet charger application. At 700V and 8kA will give approximately 686Joules of energy storage in the capacitors. Now getting a SCR to match to the target impulse current.
If we go with lower voltages we need very large coils as you have said. Have simulated the data in comsol and found that the pole piece gets saturated to 2 Tesla.
We if we use two coils and iron in the center portion as cylinders in both coils and use the magnet in between the flux density will be quite high and very good for neo magnet charges.
Please correct me if I`m wrong in here or anything else to consider.
Sorry but 150-300 Amp thyristor is too small.
Look at Chinese magnetizers data I link below ad they mention 15000 to 40000 A output capability.
Admittedly those are millisecond wide pulses, but even so....
http://www.vatmag.com/products_28/364.html
Look at datasheets, "official" spec is continuous duty, such as handling electric motors, etc. , all have a much higher single pulse rating which applies here, as long as you allow for, say, 30 seconds or more per speaker magnetization, in any case derate at least to half that peak, basic "good Engineering practice".
That is a very good and complete manual for an excellent magnetizer.
But it´s a Lab/University type machine, capable of small magnet charging, designed for testing samples , say a thumb sized rod, a small Alnico or Ferrite cube , "up to" a Magnetron magnet.
Power is 440 Joules and it fits on a bench top.
Commercial loudspeaker magnetizers I linked above go from 4500 Joule (10 times larger than the Lab/Bench type) to mind boggling 90kJ
FWIW I am using around 6kJ and it works fine for up to 156mm*20mm Guitar speaker magnets, think Celestion Vintage 30 or G12H
That said, a "large" machine is basically the same as the RFL440 , only scaled up 10-15X or more.
But exact same Physics behind it.
Functional and simple design, like all of Elliott´s
Does work, of course, almost same as RFL440, and somewhat higher energy storage:560 Joule.
Still in the experimenter´s area size, not industrial.
Again, same Physics apply.
That, a modern update covering Rare Earth magnets.
Which are hard to magnetize: Ferrite is roughly 3X as hard as Alnico ; rare Earth ones 3 to 4 times as hard as Ferrite.
FWIW I can NOT magnetize them, should build a new one when/if required.
My rule of thumb (staright from Philips manuals) is 800kAT/m ; they cover newer ferrites so suggest:
still quite in the ballpark.
Ceramic (Hard Ferrite) 10,000 – 12,000 796 – 955
Science is freely available in the Net and at University Libraries, all kinds of Electro Magnetism books are available, plus a few dedicated ones, I mentioned Scientific and Engineering values many times, now fine details on a Commercial product ... different thing.
My own "Bible" is late 60´s early 70´s Philips Eindhoven books about magnetic materials they manufactured: one is dedicated to magnetic and Piezoelectric materials, go figure; the other is "Permanent Magnets and their applications" , covering their trademarked Alnico: "Alcomax" and Ferrites :"Ferroxdure"
There is a third relevant book, about "Loudspeakers", not covering magnet design but showing commercial examples, which to me are practical "reality checks".
There is a widespread "snobbish" attitude in DIY circles: "Commercial=cheap-bad ; DIY/esoteric= GOOD!"
I follow quite the contrary: Industrial-Commercial must be quite good (at its price range of course) or it simply will not sell. period.
Or it will temporarily sell, based on false promises, and then go the way of the Dodo.
Open market is literally a cutthroat jungle.
IF Science and Engineering serve to get a competitive edge over competition, it is suicidal to freely spread it around in exchange of nothing.
Notice the Chinese will sell you anything and for a (relatively) good price, but they NEVER EVER spread design details, except in vague words used in marketing.
Discharge power supplies can be designed, even simulated in LTSpice as mentioned above .... magnetizing yokes-fixtures not that much (except the very crude very simple "first principles" ones used in a Lab/University setting):
http://www.vatmag.com/products_50.html
That said, a clever and dedicated experimenter such as oldaudioguy will certainly succeed.
But he´ll have to rewind yokes many times (notice RFL440 instructions are vague , as in "20 to 200 Turns", etc. nd burn a few SCRs ... let alone exploding capacitors, dynamite sticks in all except colour.
Important/safety hint: if using Electrolytics (most do), avoid by all means getting a reverse polarity back pulse into them.
Monster inductors (what yokes are) can also store huge energy and be hard to turn off (same as Power Supply chokes do, and for the exact same reason); one of my early "large" machines , capable of 150mm magnets and weighing some 250kg of iron and 80 kG copper, was fed from a 6 diode bridge out of 3x 380V line (our Mains dual purpose standard: 3 x 380V Phase to Phase "Triangle" - 3 x 220V Phase to Ground "Star" so same line feeds Homes (220V single phase) and shops-small Factories:three phase 380V), I got the equivalent of 550-560Vdc, no filtering needed. just 50Hz waveforms overlapping.
Problem was since I still didn´t have a proper switch but was itching to test it (wouldn´t you?) , I used a trick I found in a textbook: I suspended a piece of wire between two posts, and calculated it to blow on its own after a few seconds, it was the old trick o0f using an underrated fuse wire as an auto-off switch.
It DID blow .... that part "worked" ... the problem was that an arc jumped terminal to terminal and kept conduction (my single "switch" was obviously on the DC side) feeding a monster over 300kg inductor which has its own ideas about being switched OFF
I had to RUN to the street mains pole and one by one pull the big fuses feeding not only my own shop, but others sharing the premises (an old semi abandoned Industrial building),drawing BIG sparks from each of them.
Oh well.
PS:
Ok, I mentioned them, all the time because I make ferrite magnet speakers ; have no clue (at last no experience) on rare Earth Magnets.
For the near future, no need either.
Nor experience on electric motors, hard disk head positioning, alternators, generators, not even on fridge type publicity magnets, all of which are legitimate fields, of course..
Yet feel confident that IF I had to design a junkyard type ferrous materials pulling electromagnet would be able to
@oldaudioguy
Thanks for sharing pictures of your magnetizer, nice build, what switching device are you using to engage the capacitor bank, I guess it's the one sitting in the center of that "fridge" but doesn't look like a semiconductor device like a hockey puck style thyristor or IGBT, is it a elector-mechanical operated?
My post quoted above in #62 also had a question why there's such a large air gap between magnetizer and the driver magnet in virtually every magnetizer I have ever seen depicted, while JMFahey wrote back in post #45 the following:
Ok, I can see the practical aspect of it, but in my mind the question still arises; doesn't the magnetizer ought to create an exuberantly excessive magnetic field to ensure an enough large part of the magnetic field permeates thought the driver magnet structure itself due to no magnetic field conducting guides of steel (adjustable plate pieces, rods, jig etc) that could over-bridge and fill in the air void between magnetizer and driver magnet (only bottom side of the driver magnet is usually in direct contact with one pole of the magnetizer, while the other pole, the upper side usually have a rather large air gap).
So I am thinking here that magnetizers would need much less energy for the magnetic field if it was forced through some magnetic field guides closing the path as much as possible so as small part of the magnetic field as possible could short cut the driver magnet directly down to the opposite pole side of the magnetizer.
Thanks for sharing pictures of your magnetizer, nice build, what switching device are you using to engage the capacitor bank, I guess it's the one sitting in the center of that "fridge" but doesn't look like a semiconductor device like a hockey puck style thyristor or IGBT, is it a elector-mechanical operated?
My post quoted above in #62 also had a question why there's such a large air gap between magnetizer and the driver magnet in virtually every magnetizer I have ever seen depicted, while JMFahey wrote back in post #45 the following:
Ok, I can see the practical aspect of it, but in my mind the question still arises; doesn't the magnetizer ought to create an exuberantly excessive magnetic field to ensure an enough large part of the magnetic field permeates thought the driver magnet structure itself due to no magnetic field conducting guides of steel (adjustable plate pieces, rods, jig etc) that could over-bridge and fill in the air void between magnetizer and driver magnet (only bottom side of the driver magnet is usually in direct contact with one pole of the magnetizer, while the other pole, the upper side usually have a rather large air gap).
So I am thinking here that magnetizers would need much less energy for the magnetic field if it was forced through some magnetic field guides closing the path as much as possible so as small part of the magnetic field as possible could short cut the driver magnet directly down to the opposite pole side of the magnetizer.
@Ultima Thule
I am using a contactor/relay for the switching device to dump the charge in the capacitors into the coil. It is a DC contactor that I salvaged from a large battery charging system for forklift batteries. It has tungsten contacts and I believe it is rated to handle around 1200 amps DC continuously so it handles the short pulses for magnetizing easily. I built my magnetizing fixtures to guide the field closer to the magnets as you suggest. The one with the smaller center opening is sized for the old alnico assemblies that JBL and Altec made in the past. It works well. The other one with the larger opening is for ceramic magnets that are larger in diameter but shorter in height. Both fixtures have 1/2 inch steel plates on bottom and sides are 12" steel pipe with 1/2 inch walls. Tops are 1/4 inch steel plate. I do a lot of speaker repair/refurbishing and needed the magnetizer to recharge the magnets that have lost some of their strength(common problem with the old alnico magnets). Off center pole pieces are also a lot easier to re-align when demagnetized first. I have a couple of ideas for fixtures that should be more efficient and should simplify magnetizer power supply design.
Most of the commercial designs you see are for production line use and have to be big to handle continous operation.
Mine is for intermittent use and serves me well so far.
I am using a contactor/relay for the switching device to dump the charge in the capacitors into the coil. It is a DC contactor that I salvaged from a large battery charging system for forklift batteries. It has tungsten contacts and I believe it is rated to handle around 1200 amps DC continuously so it handles the short pulses for magnetizing easily. I built my magnetizing fixtures to guide the field closer to the magnets as you suggest. The one with the smaller center opening is sized for the old alnico assemblies that JBL and Altec made in the past. It works well. The other one with the larger opening is for ceramic magnets that are larger in diameter but shorter in height. Both fixtures have 1/2 inch steel plates on bottom and sides are 12" steel pipe with 1/2 inch walls. Tops are 1/4 inch steel plate. I do a lot of speaker repair/refurbishing and needed the magnetizer to recharge the magnets that have lost some of their strength(common problem with the old alnico magnets). Off center pole pieces are also a lot easier to re-align when demagnetized first. I have a couple of ideas for fixtures that should be more efficient and should simplify magnetizer power supply design.
Most of the commercial designs you see are for production line use and have to be big to handle continous operation.
Mine is for intermittent use and serves me well so far.
Lets consider we have a 20 of 5600uF/450V capacitors aligned two in series and 10 of such in parallel to be used with about 800V DC now what is the way to charge them? I mean any high voltage constant current circuit? since the ESR is minimal as 0.1ohm or less having any resistor either becomes over hot or we need a high voltage and at least decent constant current circuit to charge them at that voltage any ideas?
A basic and practical design of the entire contraption here on diya would be greatly appreciated.
BTW: I could not find the Rod Elliot design om his site. Anyone a link?
There are couple of challenges in this:
Rod Elliots design has output voltage which is quite less less than 100V and when coil is used it should be in very much less resistance like 0.1 or 0.2 ohm.
The whole concept is like this:
Lets start with a magnet of 120 OD and 20mm thickness of a particular grade of Y35 Ferrite magnet now this has remanescent flux density of 0.4Tesla and iHc Oe is about 2.26 to 2.5 T which is the amount of flux density required to saturate the magnets so as it reaches the permanent magnetism in it.
Now the question is that what it takes to reach that flux density is the key. Now there are two approaches one for Ferrite you need multiple pulses to trigger into the coil with various gaps if you remember when you were a kid to magnetize a iron piece you were looping the magnet rub on it consider even on a screw driver to magnetize the tip you do a loop rub in one direction multiple times the same thing applies here for the Ferrite magnets. As it has to flip the magnetic dipoles in one direction.
Now the first thing is the simulation of what coil size and how many turns are required to reach that level now if you look at there are various magnet charges types consider out example with axial type for speaker applications.
Take one example of magnetizing coil one is with iron core in it and another is with air core. So which one is better is based on how much flux density you want to achieve.
Now take the one with Dual coil magnetizer which has iron core in it like in example:
In the above there is some metal core inside which can be 1018 or pure iron where the max flux density possible with those metals is about 2.2T or so so consider one is using it.
Now what happens here is that when two coils are excited the coil starts magnetizing the core and then the speaker magnet gets charged.
Whats the whole point in here is that if you have large gap in between the magnets iHC Oe value saturation would not be met.
Now there comes the air core coil where the saturation is not an issue but you need alot of current to dump in to it.
Here the problem starts: There are many challenges in this
1. You need to have large multi turn air core coil Example 500 turns, and 5000A of current into it.
2. The moment you need 5000A of current even for short time the coil gets heated so you need water cooling for it so as the copper pipe based coil is used. Now for this you also need water cooling system.
3. Now reaching 5000A or even 10kA for short duration of pulses also required large bank of capacitors 10000uF at even 1000V is just starting points / 5kJ is for small magnets if the coil size is smaller.
4. Consider if you have large number of turns in the coil to avoid the high current you are also increasing the resistance of the coil in the process.So there is an optimum figure for it which you need to simulate in Femm or any FEA simulation and realize how much current and how much voltage for a specific number of turns coil, also you need to estimate the coil cooling process which requires high water flow at times here its thermal analysis to be done.
5.Then here is next challenge how do you charge such a high amount of capacitance which can be used as flyback converters but it all depends on how much current does the flyback converter is able to dump if its 0.1A DC out even at 1000V then its considerable long time to charge so it has to be designed to deliver higher current in amps based on number of capacitors one has.
6. Slow capacitors cannot be used for this application, there are capacitors made for this application to be used.
7. Since such a high current should be switched on but not manual mechanical switches but using some capsule thryistors which are able to handle tens of KA of current into the load. Now this one will also get hot once triggered. You need water cooling mount for this as well.
8. Another challenge is that we need to fire this Thyristor with proper current as well which is di/dt not less than 1A in many cases so a driving circuitry to be used for this to trigger.
9. The whole unit starts to wegh in 100s of kilograms which is why magnet chargers are not easy to do.
10. Neodymium magnet charges requires as much as 20Kilo Jouls of charge but like said it all depends on the design of the target magnet sizes if you use a small magnet in large air core coil then it would not be efficient as well.
Rod Elliots design has output voltage which is quite less less than 100V and when coil is used it should be in very much less resistance like 0.1 or 0.2 ohm.
The whole concept is like this:
Lets start with a magnet of 120 OD and 20mm thickness of a particular grade of Y35 Ferrite magnet now this has remanescent flux density of 0.4Tesla and iHc Oe is about 2.26 to 2.5 T which is the amount of flux density required to saturate the magnets so as it reaches the permanent magnetism in it.
Now the question is that what it takes to reach that flux density is the key. Now there are two approaches one for Ferrite you need multiple pulses to trigger into the coil with various gaps if you remember when you were a kid to magnetize a iron piece you were looping the magnet rub on it consider even on a screw driver to magnetize the tip you do a loop rub in one direction multiple times the same thing applies here for the Ferrite magnets. As it has to flip the magnetic dipoles in one direction.
Now the first thing is the simulation of what coil size and how many turns are required to reach that level now if you look at there are various magnet charges types consider out example with axial type for speaker applications.
Take one example of magnetizing coil one is with iron core in it and another is with air core. So which one is better is based on how much flux density you want to achieve.
Now take the one with Dual coil magnetizer which has iron core in it like in example:
In the above there is some metal core inside which can be 1018 or pure iron where the max flux density possible with those metals is about 2.2T or so so consider one is using it.
Now what happens here is that when two coils are excited the coil starts magnetizing the core and then the speaker magnet gets charged.
Whats the whole point in here is that if you have large gap in between the magnets iHC Oe value saturation would not be met.
Now there comes the air core coil where the saturation is not an issue but you need alot of current to dump in to it.
Here the problem starts: There are many challenges in this
1. You need to have large multi turn air core coil Example 500 turns, and 5000A of current into it.
2. The moment you need 5000A of current even for short time the coil gets heated so you need water cooling for it so as the copper pipe based coil is used. Now for this you also need water cooling system.
3. Now reaching 5000A or even 10kA for short duration of pulses also required large bank of capacitors 10000uF at even 1000V is just starting points / 5kJ is for small magnets if the coil size is smaller.
4. Consider if you have large number of turns in the coil to avoid the high current you are also increasing the resistance of the coil in the process.So there is an optimum figure for it which you need to simulate in Femm or any FEA simulation and realize how much current and how much voltage for a specific number of turns coil, also you need to estimate the coil cooling process which requires high water flow at times here its thermal analysis to be done.
5.Then here is next challenge how do you charge such a high amount of capacitance which can be used as flyback converters but it all depends on how much current does the flyback converter is able to dump if its 0.1A DC out even at 1000V then its considerable long time to charge so it has to be designed to deliver higher current in amps based on number of capacitors one has.
6. Slow capacitors cannot be used for this application, there are capacitors made for this application to be used.
7. Since such a high current should be switched on but not manual mechanical switches but using some capsule thryistors which are able to handle tens of KA of current into the load. Now this one will also get hot once triggered. You need water cooling mount for this as well.
8. Another challenge is that we need to fire this Thyristor with proper current as well which is di/dt not less than 1A in many cases so a driving circuitry to be used for this to trigger.
9. The whole unit starts to wegh in 100s of kilograms which is why magnet chargers are not easy to do.
10. Neodymium magnet charges requires as much as 20Kilo Jouls of charge but like said it all depends on the design of the target magnet sizes if you use a small magnet in large air core coil then it would not be efficient as well.
I`m currently stuck at discharge methods, anyone with experience in cross commute the capsule thyristors to have pulse triggering possibility since thyristors are usually known for one time shot and till the total current discharges its on rather its better to have pulse width varied discharging any circuit for this would be helpful
Now I'm looking for an excuse to make one of these beasts... I'd better not, as I have mountains of unfinished projects already!
One thing that strikes me, and I haven't seen it in this thread yet, is what happens at the knee when the target magnet starts to get saturated? The loop would then change from closed with low leakage, to open with a giant air gap, essentially, so that's that critical moment when things start flying, right?
One thing that strikes me, and I haven't seen it in this thread yet, is what happens at the knee when the target magnet starts to get saturated? The loop would then change from closed with low leakage, to open with a giant air gap, essentially, so that's that critical moment when things start flying, right?