Powerful magnetic field

[fred2014]

Hi

I have a concept for a device to assist in a manufacturing process that would require a hand held electro-magnetic

device. (Not fixed magnet) (I would prefer not to discuss the actual process for confidentiality reasons)

The device needs to be hand held - not a robot arm.

 

The device needs to be comfortable to hold and move about for short periods of time (I'm thinking weighing

less than 1lb in total including power source) (Must be battery powered)

 

I'm looking to develop as high a field strength as possible as the higher the better - also if it can be focussed to some

degree that would help.

 

Whilst I have electronics skills I have no practical experience of electromagnet construction or materials beyond common

electrical knowledge and my knowledge of magnetic fields is similarly limited.

 

Does anyone have any comments on a good way to go about this (presumably there is science beyond simple windings

and cores somewhere?) A prototype device has demonstrated this is worth my following up but I need much higher

field strength and to be able to make it hand held to be really useful.

 

Any comments or ideas on any area of this are welcome at this stage.

 

Thanks

 

[swansont]

Battery power is likely to be a problem. If it's just on/off that you require, you may be better off with a very strong permanent magnet that can be placed within a shield of some sort. When the magnet is in one orientation, you get the strong field. To turn it off, you rotate it or the shield (e.g. soft iron) and the field will preferentially exist within the shielding material. External field is basically nil.

 

examples:

https://www.thorlabs.com/thorproduct.cfm?partnumber=MB175

http://www.leevalley.com/US/Wood/page.aspx?cat=3,43576&p=65258

[Sensei]

Strength of electromagnet depends on current flowing through wire.

The larger current I [A], the stronger electromagnet.

 

Because when current is multiplied by time, it's charge Q=I*t,

and divided by e = 1.602176565*10^-19 C, is quantity of electrons.

The more electrons flowing through wire, the stronger magnetic field around wire.

 

Current flowing through wire depends on resistance of wire:

I=U/R

So the higher voltage, or the smaller resistance, the higher current.

Resistance depends on temperature.

At low (really low) temperatures conductor can lose resistance, and become superconductor.

https://en.wikipedia.org/wiki/Superconducting_magnet

 

Device utilizing superconductor is f.e. MRI, which has superconducting electromagnet, to create really powerful magnetic field:

https://en.wikipedia.org/wiki/Magnetic_resonance_imaging

 

The best material for wire, other than superconductor, will be such that has very small resistance. Or even actively cool it down to decrease resistance.

 

Battery AA NiMH, have typically 1900 mAh to 2900 mAh capacity=charge (A*s=C).

In other words,

if 1A current is flowing through wire, you can run it for ~3 hours.

If 10 A current is used, it will work for 17 minutes.

 

Your device has to work all the time continuously?

You have to take into account for how long batteries will last.

For 450 grams device, and 10 AA batteries, each 23 grams, the maximum will be 170 minutes of work @ 10A.

 

Typical core of electromagnet is f.e. iron.

but using ferromagnetic material for core, causes induced eddy currents.

Optimal core has to have this effect as small as possible.

f.e. Ferrite core

https://en.wikipedia.org/wiki/Ferrite_%28magnet%29

https://en.wikipedia.org/wiki/Ferrite_core

Edited January 24, 2016 by Sensei

[fred2014]

Thank you both for the quick replies.

As mentioned a fixed magnet is not possible.

I understand the benefits of supercooling but in such a device that's a bit impractical!

 

Operation is only required in bursts of 1-2 seconds at a time - short high powered magnetic

field is needed. The field need not extend from the device more than about 1cm

 

Are there any tricks that can be used to :

a) Reduce the size and weight

b) Increase the current flow - or otherwise increasing field strength

 

I was thinking maybe special design of coil or multiple coils

and is there anything better than iron as a core.

(I should also say cost is not a big factor for the design)

 

Thanks again

[swansont]

What about the application *requires* an electromagnet?

[fiveworlds]

If it were in a factory you could make a spring attachment from the ceiling of each room with the power cables inside the lack of a battery would make it light enough to be portable around a workstation and could be plugged in like a lightbulb.

[fred2014]

I think I've made the parameters clear enough.

Trying to change them doesn't help me I'm afraid.

[MountainGuardian]

Without having any guess as to the power level of the field or the purpose of the unit I am working the other direction.

 

How powerfull can a hand held one pound electromagnet be made?........ One could use superconductive materials and there are some pretty amazing batteries out there today.

.

I would question battery operation myself, a far more powerfull unit could be made if it was not battery operated and I find myself a little curious about the battery operated mandate, the thought of some potential terroristic uses comes to mind. Being ex military I have a bit of a suspicious mind...

.

With a little research you will be able to find information on current super conductors, you can easily research powerful batteries. Getting a field strength in the range of 1 cm is incredibly easy, in fact it would be difficult to keep a field of any real level of power from extending much further out than 1cm.

.

A permanent magnet would work for just an ordinary field, but for a modulated field you would have to go with an electromagnet, if you intend to modulate the electromagnetic field you have to realize that even electromagnets take some amount of time to switch back and forth, you would be constrained to relatively low frequency ranges with ordinary materials. This is also the part that I see as having potential as being used in ways that could be disruptive to technology ie terroristic potential.

[Externet]

Concentrate your efforts towards the shape and size of the pole pieces, to fit and focus the application of magnetic field to the intended part area.

 

Sawing off a choke core to create a suitable gap and feeding it with DC should allow evaluation of results.

----> http://image.made-in-china.com/4f0j00kBETYdGJaWzH/Toroidal-Air-Core-Choke.jpg

 

A handheld tool with a momentary 'on' pushbutton is what I envision, with a removable and spare battery pack that can be recharging.

Adapting the electromagnet head to one of these instead of the lamp is a good start :

----> http://ecx.images-amazon.com/images/I/71RVfwKZkHL._SL1500_.jpg

 

For the question "b) Increase the current flow - or otherwise increasing field strength"

To increase field, what matters is Ampereturns : More current, more turns, less resistance as in thicker wire, more volts. But there is a missing factor: focusing the field only to where it is needed can make a difference !

 

¿? ----> http://www.intechopen.com/source/html/47744/media/image12.png

Edited January 25, 2016 by Externet

[Danijel Gorupec]

Fred,

 

I like Swansont's solution. But if you must have an electromagnet...

 

1. For magnetic core you should use 'soft' magnetic material (soft iron, permalloy, mu-metal...). You will not find anything better than iron and its alloys. The material must be 'soft' because I understand you want to be able to switch your magnet off.

 

2. As I understand, your core does not have to be laminated. It would need to be laminated if you employ fast magnet switching (few tens of Hz or more).

 

3. The magnetic core will face the object you handle. Magnetic field lines will close through the handled object. Make sure the handled object is made from sufficiently thick ferromagnetic material so that it allows for strong magnetic flux... The surface area between your magnetic core and the object should be large. The overall length of magnetic field lines should be as short as possible. You magnetic core (as well as the object you handle) must have iron cross-section as large as possible. All this will reduce magnetic resistance... Take a look at so-called "lifting electromagnets" and notice their shape.

 

4. More iron you have in your magnetic core more powerful magnet you can make. Iron is limited to about 1.5T (Tesla) and everything above it is just a dream for a hand-held device generating permanent magnetic field. In practice, you might have trouble to achieve more than about 1T of flux density.

 

5. Iron is heavy. This limits the 'power' of your hand-held magnet.

 

6. Gap (you mentioned 1 cm) is crucial for your success. I suggest to get rid of the gap! At least keep it in sub-millimetre range... You will not be able to generate strong magnetic field over 1cm gap - this would require large number of ampere-turns and you cannot afford it with your battery power supply.

 

7. The energy needed to permanently keep the strong electromagnetic field is theoretically zero. In practice you need to spend at least some energy because your winding will have non-zero resistance. Use wire gauge and number of wire-turns to match your battery voltage and preferred discharge rate.

 

8. I suggest an easily-exchangeable battery - like in some battery supplied power tools. You might share the battery with some power tools.

 

9. If an exchangeable battery is not an option you might make an induction charger that will charge your magnet battery wirelessly.

 

 

 

Far above the Moon
Planet Earth is blue
And there's nothing (more) I can do (for you).

 

***D. Bowie (spoiled by me)***

[fred2014]

Thank you all for the responses.

[Enthalpy]

If 10ms duration were possible, capacitor discharge would achieve a higher induction. For a few seconds, only magntic poles and coils. Then both the material limits the induction (2.1T for pure Fe, at the worst places, so 1.8T in air would be an achievement, and 2.3T for Fe50-Co50) and the power. It's essentially a matter of gap length : zero length takes nearly zero watt, but the desired length would often demand MW.

 

Permanent magnets would have been the solution of choice. I can't really imagine an application that excludes them, especially if you accept pole shoes, which have always some remanence and are polarized my Earth's field.

 

Focussing a static magnetic field outside the poles is impossible. One can only bring the flux to the poles' faces and hope it won't spread too much. This is a difficult part of the design, often done with coils close to the gap or aroud the gap - but if only the induction counts and leaks are accepted, then conical poles work.

 

I'm afraid more can't be said without knowing the project better, but designing a magnetic circuit is difficult, takes time to learn, and you'll fail the first 10¹ to 10² times.

[John Cuthber]

I think I've made the parameters clear enough.

 

I don't.

What are you trying to do?

[fred2014]

If 10ms duration were possible, capacitor discharge would achieve a higher induction. For a few seconds, only magntic poles and coils. Then both the material limits the induction (2.1T for pure Fe, at the worst places, so 1.8T in air would be an achievement, and 2.3T for Fe50-Co50) and the power. It's essentially a matter of gap length : zero length takes nearly zero watt, but the desired length would often demand MW.

Thank you Enthalpy.

Well spotted.

 

I've been looking into the possibility of sequenced capacitive discharge. (Using a microcontroller to sequence things)

I'm working on a design to experiment with this now. If I can keep the output regulated sufficiently this may be one way to go.

I'm also looking into more exotic materials and complex coil construction.

If anyone knows of any good design software that can help with this sort of work I'd appreciate a heads up.

[Enthalpy]

You get a square pulse from several capacitors bridged by inductors, in an LC ladder made of discrete elements that mimick a propagation line. That was used to power radar transmitters. More efficient, no need for synchronization nor software.

 

More exotic material : if you have developed one that Mankind still ignores, fine. If not, forget the hope of finding one in a catalogue. The best induction at saturation is 2.3T for Fe-Co, and that's it - enough people have researched the topic that the outcome is known.

 

Coil construction too is very well known, both with and without a permeable core. If the only goal is strong induction, it consists essentially in packing the conductor as much and as closely to the gap as possible, that is, as a fat ring. Then you have variants if the forces are destructive or if cooling is difficult.

 

Well, again, magnetic design is fully specific to each problem. Once a bozo came to me who wanted to concentrate a magnetostatic field at a distance - is aim was probably a weapon to knock down people at a time the TMS (transcranial magnetic stimulation) began to work at contact distance.

 

My general impression is that you're weaker on electromagntism than on general electronics, and that will be a very hard nut to crack. EM is difficult for real - university courses are only the very beginning of the art. Unfortunately, the more EM designs one has made, the better one realizes that most EM design goals are unrealistic.

 

Have you decided what induction in what volume you want, what magnetic energy it means, and what copper losses it implies? The known optimum design, a fat ring, puts limits on them, which use to be a game stopper for hand-held designs. As an example, I achieved 7T in 50mm*50mm*50mm for 10ms, and this meant almost 10MW copper losses and 1m³ capacitors.

[fred2014]

thanks again...

Indeed magnetism is complex - by any standards. The fact that people are using FEA to calculate fields tells me enough.

 

Yes you're right my experience is more electronic than magnetic (I started in military radar many years ago) - I have basic

transmission theory knowledge but it's unused - and no design experience of magnetic fields at all. I think I'm

understanding maxwell better now - but hopefully I can avoid getting too involved in the math and rely on experiment to start with

then refine things later. I'm looking at various modelling packages - the one at edinburgh for ground penetrating radar looks like it may offer me some

insight into principles of interest. This is really a seat of the pants project - I'm pushing everything to see what I can get rather than designing for

specific values. The function has been tested with very large impractical equipment - the task now is to make it practical. That may mean throwing out any

pre-concieved notions if I can find genuine ways around them. (Invention is kind of like that...)

 

Fortunately I don't believe in death rays or perpetual motion - although I do have knowledge of a long range accoustic "gun"!

(look into non-lethal weaponry)

 

Copper losses are obviously an interest - especially with battery power - (a 7T field is impressive) - one thing I do have down to look into is a graph

of cooling - at what temp do losses start to reduce and whether copper is the best medium - gold maybe? (as mentioned

above - cost isn't an issue). It's been brought to my attention that significant cooling may be a possibility in the volumes

I'm working with.

 

I take your point about ferro cores. Coil construction - I'm not so sure. I have a couple of ideas - maybe nonesense but maybe not.

 

There may also be some info at nasa I can use - I seem to remember they ran one of their "competitions" to design a small

portable mri machine - there may be some papers worth reading on that - although probably fixed magnets again for the machine

but someone has probably tried to be clever - they often do in those competitions. Out of the box is where I live right now and

sometimes crazy can be usefull to spark things off.

 

Are you able to discuss your 7T project at all?

[Enthalpy]

My 7T experiment : sure. It was a part of a project in a club, and anyway, we did it the same way as everyone.

 

1m³ electrolytic capacitors charged to 350V, their rated voltage. Thick copper bars between them, connected at their middles as a 3D tree.

 

One 2.5kA thyristor (could be a Gto now) used at 20kA, its rated nonrepetitive peak current for 10ms half-wave.

 

Normal thick wires but hold by good staples on the table - or they fly away. Normal connecting blocks, but double and triple-check they're tightened firmly - or they volatilize and you're deaf for a day. Fun: the cables are cold, and 10ms later they're lukewarm.

 

I made the coil with copper foil, as broad as the coil, winded as a spiral in alternance with a plastic film. On a PVC tube kernel and hold in a hole in thick plywood. No cooling for 10ms, but afterwards the coil is warmer.

 

It's the standard way to make strong magnets, SmCo and NdFeB. These need more than 2.3T so a magnetic core isn't an option, and since teslas in air demand megawatts, but the magnets react quickly, everyone uses capacitive discharge. Just ask at a company that makes magnets - possibly one that makes servomotors.

 

----------

 

For longer duration, the decent option is superconductivity. Some 8T are a difficult realistic target, and 20T an exotic achievement.

 

A few people, maybe three research groups worldwide, produce >20T over many seconds. This needs copper because superconductors get resistice before, plus water to cool it, and many MW (optionally from a homopolar machine or similar). It also demands a very strong construction.

 

A stronger induction has been achieved by capacitive discharge - 50T then, do I remember 200T now? The coil is extremely reinforced (understand: fibres alternate with copper) but it deforms and survives only a few shots. A compulsator could supposedly replace the capacitors and its construction be fused with the coil.

 

The flux compressors, invented by Sakharov, achieve supposedly a bigger induction using explosives, metal and an initial field, but I don't know the figures. They serve in weapons as EMP sources.

[Sensei]

1m³ electrolytic capacitors charged to 350V, their rated voltage.

The first time I hear somebody measuring capacitors by volume they take..

I am/was using electrolytic 400/450 V capacitors (2.2 uF or so) to produce high voltage, in Cockcroft-Walton generator. Hundred of them in series.

And they have size approximate 1 cm x 1 cm x 3 cm including board and wires and little space between them.

In 1m3 could be packed 100^3 / 1x1x3 = 333k such capacitors. Their price per unit is (or "was", when I bought them), $0.125.

So whole 1m3 of capacitors would cost $42k.

I have seen people filling such capacitor arrays by oil (or mineral oil) to have good heat conduction (but damage of exploding capacitor in such environment is devastating)

 

So what is the real quantity of your capacitor array?

 

Anyway +1 Enthalpy.

Edited February 2, 2016 by Sensei

[fred2014]

hmm...

 

Ok thanks.

The original question seems to be drawing blanks right now.

 

If anyone has anything to suggest for my original post I'll pop back from time to time.

Thanks to all posters for the input.

[Enthalpy]

Well, there isn't much more to say with the little information provided...

If 1T is enough, or at most 2T, use magnetic cores. Apparently you've decided not to.

With magnetic cores, use permanent magnets. For some reason you don't want to.

The next reasonable choice is superconductivity - again, you eliminate it.

 

What remains is capacitive discharge, but figures will probably rule it out, especially the energy.

 

Most desires are impossible in magnetic design. At some point, after checking a few figures like power and energy, one very often has to rethink the wishes. "Focus" a magnetostatic field is impossible under any circumstance, and multi-Tesla induction uses to be extremely impractical.

 

If the goal is to induce a voltage, try many short pulses instead of one long if possible. For transcranial magnetic stimulation it would change everything:

http://www.scienceforums.net/topic/70203-transcranial-magnetic-stimulation/

including reduced forces on the coils, which can have a better shape then.

 

----------

 

1m³: these were 3500µF 350V, all in parallel, nicely connected by screws in copper bars. They had cost a shiny penny, sure.