How to Cool Electromagnetic Coils? 7

[redtreetoad]

Hi,
I'm trying to create a magnetic coil using AC that emits magnetic radiation . This design is somewhat opposite to most magnetic circuits that attempt to keep the magnetic radiation within (e.g. a transformer). The closest analogy I can think of is the use deflection coils used in a TV, or perhaps a speaker coil.

So far I've tried air core, iron core, powdered and ferrite cores, but all of my attempts end up heating the coil & core to where you can't even touch it because its too hot. The coil must be able to be handled without burns. For various reasons, I can't use a cooling fan, but have had some luck winding magnet wire around a circular aluminum heat sink. I was thinking of using a light oil or antifreeze to surround the heatsink in a case, but am afraid this might be dangerous if it popped open.

Some design requirements :
Core diameter: can be 0.75” thru 3” (bigger is better)
Must be run from batteries (any type is OK)
Max. Flux Density at the coil surface: 150 Gauss
Typical Flux Density at the coil surface: 120 Gauss
Max. Temperature at the coil surface: 90 degrees
Frequency: 2- 20 Hz
Max. Voltage: 60 VAC RMS

Any help/ideas would be greatly appreciated!

Thanks so much,
/// Mike

 

[MacGyverS2000]

150 Gauss from a battery?! That had better be a big battery or a very short pulse...

If you want the max radiation, place your object inside of the coil turns, not just near the coil. If it's getting too hot to handle, you're running too much current for the wire size... increase the voltage (why 60V?).


Dan
Owner

http://www.Hi-TecDesigns.com

 

[redtreetoad]

Dan,
Thanks for your reply.

The batteries are necessary for particular medical reasons (mains isolation and portability in the field) and could be a second box/module if need be. I'm using 34 AWG magnet wire. I tried 28 gauge wire previously, but couldn't get enough turns on the forms I was using. I now have new forms so perhaps I should revisit this...

The 60V max was really a self imposed limit. Right now I'm using 2 Gel Cell 12V 1 AH batteries and the coil voltage doesn’t exceed 22VAC.

Also I have some 12VDC -> 60V DC converters (haven't tried them yet) that I purchased off of EBay for a song. I have been told that V/Hz is a rough proportion to use for consistent flux density.

Roughly what kind of flux density should I expect (without over-heating)? Are there any formulae or references that you would recommend?

Thanks,
/// Mike

 

[itsmoked]

Increasing the voltage?? That reduces the amp-turns.

Redtree: Surrounding that whole um... contraption will reduce its cooling. That is the connection to the air. Perhaps a thin tubular form with fins radiating in towards the center and a small fan forcing air thru the center.

Something like this.

http://www.aavidthermalloy.com/products/forced/index.shtml

They will make anything you want.

 

[nbucska]

Superconductive magnet will solve heat problem and increase
field strength except portability .

Doyou need 50% duty cycle? or short pulses would be enough?


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[SolderingGunslinger]

The focus magnets used on Inductive Output Tubes (IOT, a type of Klystron) are often liquid cooled. Consider winding the coil with a tubing instead of a solid conductor. A coolant can be circulated through the coil and then through a small radiator. Depending upon the duty cycle, no radiator may be needed and convection through an expansion tank might supply sufficient cooling for your magnet.

There is portable and then there is portable. I can remember a quad video tape recorder which Ampex sold as being "Portable"...it took three men and a boy to move the darned thing, but yes, it was "Portable". The military considered the liquid cooled .30 machine gun (Or for the British the liquid cooled .303 Maxim gun) portable as well.

I remain,

The Old Soldering Gunslinger

 

[MacGyverS2000]

Cute little site I just came across while playing with Google...

http://www.geocities.com/CapeCanaveral/2404/design2.html

Dan
Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

Nice.

 

[Warpspeed]

Just a thought, would solid silver wire help ?

Silver is not as expensive as you might expect at around seven dollars US per ounce these days, and it has the highest thermal and electrical conductivity of all metals.

 

[redtreetoad]

itsmoked: Thanks for the link; I didn't know that forced convection heatsinks existed (at least the type on the page you suggested); very interesting. Also - I didn't understand your comment "Increasing the voltage?? That reduces the amp-turns." At a particular Z & frequency wouldn't an increase in voltage increase the amperage as well?

nbucska: Super cooling would be ideal and yes I'll try reducing the duty cycle. I can't use short pulses as the frequency and waveform archetype is crucial. I have spoken to the folks at

http://www.ultraconductors.com

and their room temperature superconductors won't be available (retail or otherwise) for another couple of years. They are polymers, but still, the potential applications are astounding...

SolderingGunslinger: Thanks for the info. I have a tiny liquid/oil pump, but was hoping not to have to use it if I could. The added wires and tubing would be a little much. But if I must, I must. What's an expansion tank? Yeah - portable is a broad term. I meant about a foot square by about 4" for the microcontroller/oscillator/amplifier and perhaps a separate unit for the batteries. I remember the first Compaq "Portable" - it weighed around 20 lbs (the unit that with the CRT and the keyboard that came off the front). Of course that used 115 VAC as a power source. Nothing like a machine gun though - wouldn't want to hump a piece of equipment like that around (even to impress my neighbors) .

macgyvers2000: Thanks for the link. Much more practical equations than deriving from Maxwell .

Warpspeed: Yes, silver would be better, but I can't seem to find any silver magnet wire for a reasonable price (especially now that I realize I'm probably going to have to a larger gauge). I actually like the thought of using silver (especially since it's not the same price as it was in the early 1980's!). Incidentally did you know that copper prices are going through the roof? I checked my local paper at breakfast last week, and the price last year was about 20 times cheaper than this year! Must be the switch to fiber optics or something...

 

[Warpspeed]

Check out some of the commercial silver suppliers and manufacturers. They supply bare silver wire to jewelers, as well as rods/wire of silver solder, solid silver electrical contacts, and other silver products often made to special order.

If you are manufacturing these coils, and there is sufficient demand for the wire, they may be prepared to tool up to supply any gauge of wire you may require.

The ordinary magnet wire manufacturers may also be prepared to run a decent sized batch of silver wire for you, if you can source the wire. They just coat it with the normal polyester amide coating, and it is just as easy for them to fit a drum of silver wire, as a drum of copper wire to their coating bath.

I am sure it would all be possible, but getting started may not be easy. Final cost is something else.

 

[itsmoked]

Redtree:

A magnetic field is caused by current flow NOT voltage. Note the lack of a magnetic field emanating from a battery.

Because current generates the field a unit of field strength in electromagnets is Ampere-Turns.

You can crank up the field with lots of turns or less turns and more current.

 

[sreid]

redtreetoad.

What you really need to tell us (besides the 150 gauss number) is how far from the coil and the area/volume for the field. Also the required duration the field has to last.

MWS Wire can supply almost any type of magnet wire required (see below micro square silver wire)

http://www.mwswire.com/microsq.htm

You might be able to use heat pipes to transfer the heat to where the hand isn't.

 

[redtreetoad]

Warpspeed: "the normal polyester amide coating" - wow , sounds like you've done this before. I suppose Home Depot doesn't sell this stuff ... Thanks again.

itsmoked: Thanks. I understand that Ampere-Turns is proportional to magnetic field strngth. I guess what I don't understand is why current isn't proportional to voltage with (a fixed) impedance? In other words the equivalent of Ohms Law in an AC circuit.

sreid: Thanks for the link, I suspected square wire would provide better flux linkage between the windings. I think I'd have to run my homemade coil-winder pretty slow to make sure the wires were wound tightly beside each other. Anyway - finding any vendor like this has been a struggle for me; this a great source. In answer to your questions:

1) "How far from the coil and the area/volume for the field?"
A: It varies, but the nominal coil diameter is 2 inches puting the area at ~6.3 square inches. I'm trying to target tissue at a nominal depth of about .5 inches. When you say the volume, I assume that you mean how much flux density is required at the target and what volume of the tissue is to be er... stimulated. Again this varies, but the nominal volume would be something like 0.75 inches cubed.
2) "[What is] the required duration the field has to last?"
A: Again it varies, but the minimum design constraint is 15 minutes and maximum of about an hour (the maximum could be lowered if necessary).

/// Mike

 

[sreid]

Maybe the low power solution would be to use a Neo RE permanent magnet and spin it using a DC motor powered by a battery to get the AC field. Magnetic Componet Engineering has some on axis flux calculators.

http://www.mceproducts.com/Calculators/flux/flux.shtm

In fact, you could use the permanent magnet as the rotor of the motor. If this looks attractive we could discuss details.

 

[Warpspeed]

I think the confusion here is that the flux in a dc solenoid will be proportional to ampere turns, but that may or may not be the whole story. It depends.

In an ac circuit frequency and voltage come into it too. Note that current is not a part of Faraday's law.

As an example, the flux developed in a transformer core (and the developed secondary voltage) remain almost constant regardless of the load current. So ampere turns in an ac transformer have no real relevance.

Taking a stab at this, I would probably compare the dc coil resistance to the inductive reactance and try to decide if it is going to behave more like a dc solenoid or an ac inductor at the frequencies of interest. All this will be highly dependant on the number of turns and coil geometry finally decided upon.

Because the frequencies are so very low, my guess is that ampere turns will predominate, with enough drive volts and dissipated resistive watts to reach the required current.

An interesting problem to be sure.

 

[SolderingGunslinger]

What's an expansion tank?

It is a tank with extra room to allow for the expansion of the coolant as it gets hot.

The top tank on an automobile's radiator is an expansion tank. The little plastic thing is for the overflow when it expands too much (as often happens here in the Sonoran desert of Arizona.

I remain

The Old Soldering Gunslinger

 

[logbook]

A typical prior art for emitting a magnetic field is the recording head of a tape recorder. The head is of course laminated to reduce eddy currents. The magnetic path is cut by a thin brass shim, the brass being very slightly diamagnetic. This encourages the field to go outside of the iron path.

If possible the object should be placed between iron faces of the magnet. You shouldn’t try to generate a field parallel to the ferromagnetic material because a ferromagnetic material is a magnetic "short circuit".

Air coils are no good because most of the field is everywhere other than where you want it.

150 gauss is nothing in terms of magnetic fields. You can get 2 tesla fields in transformers. Isn’t it 10,000 gauss per tesla?

 

[redtreetoad]

sreid
Thanks again for the link. This is quite the helpful forum. Yes – I would definitely be interested in discussing details of your idea. It sounds very interesting (as long as the DC motor magnetic emissions don’t have a discernable affect on the emissions of the device). Should I give you my e-mail address?

Warpspeed
“Note that current is not a part of Faraday's law” – I stand corrected! Yes and also - the frequencies are so low that the resultant impedance is primarily resistive. The inductive reactance is minimal. I tried using a larger gauge magnet wire but ran out of wire before I could reach a good deal of Ampere-Turns (260 to be precise starting with a 1.5” core). I managed to get up to around 75 Gauss before heating began. By the time I reached 150 Gauss, it was really hot (melted my high-heat glue!).

SolderGunslinger
“It is a tank with extra room to allow for the expansion of the coolant as it gets hot.” That’s what I thought, but just wanted to verify with you what you meant. Thanks.

logbook
Thanks, I looked at a tape recording head, but your explanation about brass shim being slightly diamagnetic connected the dots. I have two prototypes I would like to create, one like the tape recorder head (pinpoint the field leaving the magnetic circuit), and one that radiates a wider area (~3” round). I noticed that a speaker coil I removed had a gap in the core it was wound on too, but that core seemed like some kind of laminated or hardened thin cardboard?
“Isn’t it 10,000 gauss per Tesla?” - Yes, according to my CRC it is. That’s one of the problems I’m finding with magnetic theory/applications, they not only have two sets of measurement systems, but some relationships are “pseudo metric” ?.

 

[Warpspeed]

Some great ideas here chaps.

I suppose it comes down to the required size and geometry of the required magnetic field.