Electromagnet: How to build the right one?

[deanEdinburgh]

Hi there! I hope someone can help me ... I'm trying to build an electromagnet to use in a system inspired by Bob Beck and electro-medicine.

I have a friend building me a circuit to generate a pulsed wave to drive an electromagnet. It's my job to build the electromagnet, but after several weeks of scratching my head on the maths for this, I still can't figure out how many times I should wrap the copper wire around the core? (and what materials I should use for the core and the copper wire).

I would like the magnet to be 10cm wide, with a maximum field strength of 6000 Gauss.

I understand using a iron core, with copper wire is best, but how many turns and of what thickness wire should I have? If the current is too high, thinner wire might burn out.

I understand Neodymium core magnets would be helpful in increasing the magnets strength and lowering the amount of turns of copper. However, cost is an issue for me, so I don't mind doing the extra work with an iron core if that can still reach the target field strength.

Also, how much current is required to reach the target field strength level, if the unit is being pulsed at 10hz?

I am a medical student and am building this magnet so that I can test the Bob Beck thesis out to see if pulsed electromagnetic fields can work to regenerate tissue and joint damage. Any help you can offer would be greatly appreciated!

 

[marcusl]

I assume you want a region of uniform field for your experiment. You might be surprised to learn that the magnet you describe is big (think one or more hundreds of pounds of iron and copper wire) and non-trivial. Saturation of the core is an issue for getting 6 kG outside the pole faces, so a reasonably careful calculation of winding and core magnetization is needed. That means access to a finite-element E&M analysis package, and enough knowledge to model your designs and interpret the results. You will need to understand enough E&M to understand B, H and M fields, and the various magnetic permeabilities (initial, incremental and saturated). The winding inductance will be large, so producing pulses with fast rise times without arcing and destroying your coil will be a considerable challenge. Enough circuit theory to understand inductances and time-varying currents will come in handy here. Cooling the magnet is another issue. Finally, add in welding and machining skills.

You might be better off purchasing a ready-made magnet. The following magnet nearly reaches 6 kG within a 4x4" experimental volume
http://www.lakeshore.com/products/electromagnets/models/pages/overview.aspx

Here is another one in the range of what you are looking for.
http://www.svslabs.com/Products/ProductDetails1.aspx?ProductID=25&CategoryID=1

Another approach is to team up with someone (maybe a master's student) in the physics or engineering schools. Better yet, you might be able to borrow an old magnet from a physics lab or maybe an old NMR chemistry lab.

Edit: To answer your question about cores, stay away from permanent magnets. You want soft iron, especially since you want to pulse the field.
Edit 2: That reminds me that core losses at 10 Hz due to both hysteresis and eddy currents can be significant, so your power supply and cooling system need extra capacity.

 

[deanEdinburgh]

To answer your question about cores, stay away from permanent magnets. You want soft iron, especially since you want to pulse the field.

Thanks for your time!

I found the following author who suggested an 'air core' with no permanent magnet used.

Once the charge is dumped to the coil, the current flowing through the coil generates a moving magnetic flux as governed by the following equation. We must look at the coil as a DC electromagnet, because in essence that is what we have created. Thus:
Bpeak = Inductance (L in Henries) of the Coil*Peak Amps*10^8 / (N*A)
Where Bpeak is Peak DC Flux Density in Gauss, L is the inductance of the coil in Henries, Peak Amps is the measured RMS Peak Amps flowing through the coil, N is the number of turns in the coil, and A is the cross-sectional area of the air-core section of the coil.
SOTA's MPG coil is rated at 2.5 milli-Henries, has 270 turns, and has a cross-sectional area of 3.22 cm^2. We measure 150 Peak DC Amperes on each pulse through our coil. Substituting we get:
Bpeak=0.0025 Henries*150 Amps*10^8 / (270 Turns*3.22cm^2) =43,133 Gauss*
[*This 43,133 Gauss measurement is at the intercoil winding flux. Magnetic flux intensity follows the inverse square rule - the further you are away from the magnetic core, the faster the Gauss intensity drops off. At face of Magnetic Coil, the Gauss drops to ~6,000 Gauss.]
As you can see, all the components come together to generate the required Gauss output.
In order to drive the necessary amps through the coil, the coil must have a low impedance or DC resistance. The use of 18 AWG or thicker wire ensures this.

One suggestion from another place is a 2.5 mH 16AWG Air Core Inductor Coil. I found these
http://www.madisoundspeakerstore.co...n=all&search_str=16AWG+Air+Core+Inductor+Coil
which are about 2-3 inches wide and weigh about 2-3 pounds.

Would something like
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=255-270
offer a stronger field?

 

[marcusl]

The coils you show will have very small regions of field homogeneity, or large field variations over 10 cm distances. How uniform does the field need to be? Over what volume?

 

[deanEdinburgh]

I don't think the field has to be necessarily uniform, I would expect it would be stronger nearer the center of the electromagnet/ inductor coil.
A wider base would offer a wider area of coverage. The magnet would be held close or on the skin of the patient, and this could extend up to 40cm into the body. Of course there would be some drop-off of the field, hence holding the coil nearer the site of injury would be required.

 

[benny61]

Why not make multiple magnets and spread evenly around where a patient would be treated. This lower the impact of an un-even field. Maybe I'm wrong, just thought I'd throw my 2 cents in.

 

[deanEdinburgh]

From what I've seen, the typical approach is to use a single point of magnetism, as this allows a 'ripple' effect to spread through the surrounding cells, much like a stone thrown into a lake. If there are multiple points, this causes overlap and conflicting ripples. However, I haven't seen any data that show these multiple points would be good/bad, it's mostly theory.
The main issue would be generating enough power to drive the magnets to reach the 6000 Gauss rating. With two magnets, you'd have 12000 Gauss to generate !

 

[0xDEADBEEF]

I am not thinking very highly of this type of experiments, because you have almost no control over the field and the field gradient at the target or the duration or a useful theory why it should do anything at all.

Anyhow some remarks. You are trying to discharge a capacitor through a coil. So for a split second you produce a large field. I would ask my friends what the series resistance of the capacitor is that they want to discharge and try to match that with the coil's resistance. AWG18 has 21 ohms per kilometer this should give you the maximum length of wire to work with. I would start with a lot of turns and reduce it if I find the field is too small, because then the inductance may already be limiting. Anyhow if you want to be more exact calculate the LCR oscillator circuit to see what kind of peak current you can expect and work from there.

 

[deanEdinburgh]

I beg to differ! My theory for designing this experiment is based on the NASA research from (Goodwin 2003) - which had weak magnetic fields around cells in petri dishes, by virtue of an 'air coil' - this caused cells to grow at 2.5 times to 4 times faster than control groups. In practice this could mean accelerated healing of bone injuries and wounds.

If someone could help me get this magnet specification sorted out - it would mean I could advance the cause of medicine and science - as this is part of my masters thesis studies!

The questions I would like to ask are -
Does the use of copper wire vs copper foil make any difference to the magnetic field ...
Does 2.5mH work to produce a stronger field than 3.3mH ...
Is AWG 18 producing a stronger field than AWG 16 ...
I suppose a wider air core would offer a shorter range of field, so a narrower unit would be able to penetrate deeper.

The series resistance of the charging capacitor hasn't been fixed yet, this unit is still in the design phase, so no spec has been confirmed yet.

I wanted to start with the magnet first, and then work out what was needed to drive it. The bigger the magnet and stronger the field was the aim! So far, 10cm wide seems to be the limit of what's practically feasible to obtain 6000 Gauss on the surface.

Here is a link to a photograph of a unit built by NASA, its seems to be about 20cm, with two coils in a Helmholtz configuration. http://research.jsc.nasa.gov/PDF/SLiSci-12.pdf This will deliver a homogeneous field, but with only about 30-40 Gauss in the middle of the coils, which makes it difficult to use on a patient.