Magnetizing a small Alnico 5 pole piece

[gameforge]

I'm mainly looking for some practical guidance here. I'd like to build an electromagnet coil that's capable of magnetizing fresh Alnico V (unmagnetized) pole pieces for use in electric guitar pickups.

The maximum pole piece size I need to accommodate is 0.195" in diameter and 0.710" in length. For practical purposes, I'd like to build a coil that's 1" long. http://www.arnoldmagnetics.com/products/alnico/index.htm" states the following about magnetizing Alnico V:

Magnetic saturation at the end-use location requires the application of a magnetizing force 4 to 5 times greater than the coercive force of the material. For Alnico 5, a magnetizing force of 3000 oersteds (240 KA/m) is recommended...The magnetizing force need be applied only momentarily. Thus, impulse magnetizers employing a capacitor discharge are commonly used. Direct current magnetizers are also effective.

Referring to several online sources, I discover that 240 KA/m is 240,000 ampere-turns per meter from an electromagnet coil - for a coil that's only 1" long, this converts to 6,096 ampere-turns for the whole coil.

So if I wrap my coil around a bobbin, will the pole piece (inside of the bobbin...) still receive the requisite 240 KAt/m? I would assume there's a "best" material to make the bobbin out of, and an "ideal" thickness of this material to use... how would I determine this? Here's my math on my coil so far, and just for simplicity I'm assuming 0.2" of inner diameter for the coil. In a perfect world that would only leave 0.005" between the coil and the core, I'm not sure if that's realistic... certainly not if there's a bobbin - but I got this from my own spreadsheet so I can recalculate everything quickly, if it's correct.

I think 20 AWG is a good place to start playing with numbers. For 1" length, this should give me about 31 turns per layer. If I make it 5 layers deep, my coil "wall" thickness should be about 0.16", with (hopefully) 155 total turns. The average turn will be 1.147" in length (circumference of the center of the center layer), requiring a grand total of 14.811' of magnet wire.

At a predicted impedance of ~0.01 ohms per foot, I should have a low coil impedance of only ~0.15 ohms, and (using a web calculator I found and the formula it gave...) an inductance of 54.7 microhenries. For 6,096 ampere-turns in a 155 turn coil, I need to draw about 40 amperes across it - with a total overall impedance of 0.15 ohms, this indicates that I need a supply voltage of about 6 volts.

I'm very fuzzy on exactly how long the above passage means by "momentarily"... does that mean 10ms? 5s? If it's truly short (10ms), then according to my calculated coil inductance and corresponding "power-up" time of 0.4s, I need a capacitor that can deliver 40 amperes at 6 volts for about 10.4ms. The value I came up with for this (using slightly more precise numbers on a spreadsheet) was 82,708 uF. At 6 volts, this is very available.

Does all this sound about right? I would obviously be checking the actual impedance and inductance of my coil after I make it, but am I headed in the right direction here?

I'm concerned about heat... to draw 40 amperes across nearly 15 feet of 20 AWG seems well beyond every rating I can find for RMS wire ampacity, but for a ~10ms burst I just don't know. I am fuzzy but semi-confident on my electrical understanding, pretty so-so on (electro)magnetism and very fuzzy on thermal "science"... am I going to melt the insulation off of the coil if I run that many amperes across 15 ft. of 20 AWG for 10ms?

More importantly, will my pickup pole piece have a permanent magnetic polarity if I put this together?

I lost count of how many questions I asked, and I'm sorry to make a long post like this, as I said I'm mainly just looking for some practical advice on the subject.

Any thoughts or advice would be tremendously appreciated.

 

[trini]

The first thing to keep in mind is that the smaller your wire, the lower your A/t requirement. however, it is the CHANGE in magnetic field which will essentially 'shock' the particles in your alnico into position, so while it is true that you only need a short single pulse to generally line up your particles, you're going to want to use a system which pulses half wave current to achieve optimal orientation of particles, and thus optimum field strength.

Yes, the choice of bobbin material(core material) does indeed make a difference. this is due to a property known as permeability. now typically speaking, you want to design a highly permeable core which will guide as many field lines as possible from your coil to your magnet. i suggest you look for a few notes on magnetic circuits, here's a useful link:

http://services.eng.uts.edu.au/cempe/subjects_JGZ/ems/ems_ch7_nt.pdf

thats all i have time to post for now, but yes so long as the material passes saturation it will retain a large proportion of the b field.

 

[Bob S]

One way to get a high current current pulse into a coil is shown in the attached thumbnail. In this case, a 25-milliFarad capacitor, charged to 1000 volts, is discharged into a 295-microHenry coil. The thumbnail shows the 1000-volt half cycle creating an 8,000 amp pulse for ~5 milliseconds. The components can be adjusted to vary the peak amps and the pulse length.
In magnetizing electrically-conducting objects, the pulse penetration into the center will be slowed by eddy currents, so lengthening the pulse length might be required for larger diameter objects.
Bob S